Reserve guarded string literals (RFC 3593)

compiler/rustc_lexer/src/lib.rs

@@ -104,6 +104,12 @@ pub enum TokenKind {
     /// for emoji identifier recovery, as those are not meant to be ever accepted.
     InvalidPrefix,
 
+    /// Guarded string literal prefix: `#"` or `##`.
+    ///
+    /// Used for reserving "guarded strings" (RFC 3598) in edition 2024.
+    /// Split into the component tokens on older editions.
+    GuardedStrPrefix,
+
     /// Examples: `12u8`, `1.0e-40`, `b"123"`. Note that `_` is an invalid
     /// suffix, but may be present here on string and float literals. Users of
     /// this type will need to check for and reject that case.
@@ -191,30 +197,41 @@ pub enum DocStyle {
 /// `rustc_ast::ast::LitKind`).
 #[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
 pub enum LiteralKind {
-    /// "12_u8", "0o100", "0b120i99", "1f32".
+    /// `12_u8`, `0o100`, `0b120i99`, `1f32`.
     Int { base: Base, empty_int: bool },
-    /// "12.34f32", "1e3", but not "1f32".
+    /// `12.34f32`, `1e3`, but not `1f32`.
     Float { base: Base, empty_exponent: bool },
-    /// "'a'", "'\\'", "'''", "';"
+    /// `'a'`, `'\\'`, `'''`, `';`
     Char { terminated: bool },
-    /// "b'a'", "b'\\'", "b'''", "b';"
+    /// `b'a'`, `b'\\'`, `b'''`, `b';`
     Byte { terminated: bool },
-    /// ""abc"", ""abc"
+    /// `"abc"`, `"abc`
     Str { terminated: bool },
-    /// "b"abc"", "b"abc"
+    /// `b"abc"`, `b"abc`
     ByteStr { terminated: bool },
     /// `c"abc"`, `c"abc`
     CStr { terminated: bool },
-    /// "r"abc"", "r#"abc"#", "r####"ab"###"c"####", "r#"a". `None` indicates
+    /// `r"abc"`, `r#"abc"#`, `r####"ab"###"c"####`, `r#"a`. `None` indicates
     /// an invalid literal.
     RawStr { n_hashes: Option<u8> },
-    /// "br"abc"", "br#"abc"#", "br####"ab"###"c"####", "br#"a". `None`
+    /// `br"abc"`, `br#"abc"#`, `br####"ab"###"c"####`, `br#"a`. `None`
     /// indicates an invalid literal.
     RawByteStr { n_hashes: Option<u8> },
     /// `cr"abc"`, "cr#"abc"#", `cr#"a`. `None` indicates an invalid literal.
     RawCStr { n_hashes: Option<u8> },
 }
 
+/// `#"abc"#`, `##"a"` (fewer closing), or even `#"a` (unterminated).
+///
+/// Can capture fewer closing hashes than starting hashes,
+/// for more efficient lexing and better backwards diagnostics.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
+pub struct GuardedStr {
+    pub n_hashes: u32,
+    pub terminated: bool,
+    pub token_len: u32,
+}
+
 #[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
 pub enum RawStrError {
     /// Non `#` characters exist between `r` and `"`, e.g. `r##~"abcde"##`
@@ -403,6 +420,12 @@ pub fn advance_token(&mut self) -> Token {
                 TokenKind::Literal { kind: literal_kind, suffix_start }
             }
 
+            // Guarded string literal prefix: `#"` or `##`
+            '#' if matches!(self.first(), '"' | '#') => {
+                self.bump();
+                TokenKind::GuardedStrPrefix
+            }
+
             // One-symbol tokens.
             ';' => Semi,
             ',' => Comma,
@@ -780,6 +803,60 @@ fn double_quoted_string(&mut self) -> bool {
         false
     }
 
+    /// Attempt to lex for a guarded string literal.
+    ///
+    /// Used by `rustc_parse::lexer` to lex for guarded strings
+    /// conditionally based on edition.
+    ///
+    /// Note: this will not reset the `Cursor` when a
+    /// guarded string is not found. It is the caller's
+    /// responsibility to do so.
+    pub fn guarded_double_quoted_string(&mut self) -> Option<GuardedStr> {
+        debug_assert!(self.prev() != '#');
+
+        let mut n_start_hashes: u32 = 0;
+        while self.first() == '#' {
+            n_start_hashes += 1;
+            self.bump();
+        }
+
+        if self.first() != '"' {
+            return None;
+        }
+        self.bump();
+        debug_assert!(self.prev() == '"');
+
+        // Lex the string itself as a normal string literal
+        // so we can recover that for older editions later.
+        let terminated = self.double_quoted_string();
+        if !terminated {
+            let token_len = self.pos_within_token();
+            self.reset_pos_within_token();
+
+            return Some(GuardedStr { n_hashes: n_start_hashes, terminated: false, token_len });
+        }
+
+        // Consume closing '#' symbols.
+        // Note that this will not consume extra trailing `#` characters:
+        // `###"abcde"####` is lexed as a `GuardedStr { n_end_hashes: 3, .. }`
+        // followed by a `#` token.
+        let mut n_end_hashes = 0;
+        while self.first() == '#' && n_end_hashes < n_start_hashes {
+            n_end_hashes += 1;
+            self.bump();
+        }
+
+        // Reserved syntax, always an error, so it doesn't matter if
+        // `n_start_hashes != n_end_hashes`.
+
+        self.eat_literal_suffix();
+
+        let token_len = self.pos_within_token();
+        self.reset_pos_within_token();
+
+        Some(GuardedStr { n_hashes: n_start_hashes, terminated: true, token_len })
+    }
+
     /// Eats the double-quoted string and returns `n_hashes` and an error if encountered.
     fn raw_double_quoted_string(&mut self, prefix_len: u32) -> Result<u8, RawStrError> {
         // Wrap the actual function to handle the error with too many hashes.

compiler/rustc_lint/messages.ftl

@@ -740,6 +740,9 @@ lint_reserved_prefix = prefix `{$prefix}` is unknown
     .label = unknown prefix
     .suggestion = insert whitespace here to avoid this being parsed as a prefix in Rust 2021
 
+lint_reserved_string = will be parsed as a guarded string in Rust 2024
+    .suggestion = insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+
 lint_shadowed_into_iter =
     this method call resolves to `<&{$target} as IntoIterator>::into_iter` (due to backwards compatibility), but will resolve to `<{$target} as IntoIterator>::into_iter` in Rust {$edition}
     .use_iter_suggestion = use `.iter()` instead of `.into_iter()` to avoid ambiguity

compiler/rustc_lint/src/context/diagnostics.rs

@@ -176,6 +176,9 @@ pub(super) fn decorate_lint(sess: &Session, diagnostic: BuiltinLintDiag, diag: &
             lints::RawPrefix { label: label_span, suggestion: label_span.shrink_to_hi() }
                 .decorate_lint(diag);
         }
+        BuiltinLintDiag::ReservedString(suggestion) => {
+            lints::ReservedString { suggestion }.decorate_lint(diag);
+        }
         BuiltinLintDiag::UnusedBuiltinAttribute { attr_name, macro_name, invoc_span } => {
             lints::UnusedBuiltinAttribute { invoc_span, attr_name, macro_name }.decorate_lint(diag);
         }

compiler/rustc_lint/src/lints.rs

@@ -3053,3 +3053,10 @@ pub(crate) enum MutRefSugg {
 #[derive(LintDiagnostic)]
 #[diag(lint_unqualified_local_imports)]
 pub(crate) struct UnqualifiedLocalImportsDiag {}
+
+#[derive(LintDiagnostic)]
+#[diag(lint_reserved_string)]
+pub(crate) struct ReservedString {
+    #[suggestion(code = " ", applicability = "machine-applicable")]
+    pub suggestion: Span,
+}

compiler/rustc_lint_defs/src/builtin.rs

@@ -92,6 +92,7 @@
         RUST_2021_INCOMPATIBLE_OR_PATTERNS,
         RUST_2021_PREFIXES_INCOMPATIBLE_SYNTAX,
         RUST_2021_PRELUDE_COLLISIONS,
+        RUST_2024_GUARDED_STRING_INCOMPATIBLE_SYNTAX,
         RUST_2024_INCOMPATIBLE_PAT,
         RUST_2024_PRELUDE_COLLISIONS,
         SELF_CONSTRUCTOR_FROM_OUTER_ITEM,
@@ -4996,3 +4997,43 @@
     Warn,
     "detects pointer to integer transmutes in const functions and associated constants",
 }
+
+declare_lint! {
+    /// The `rust_2024_guarded_string_incompatible_syntax` lint detects `#` tokens
+    /// that will be parsed as part of a guarded string literal in Rust 2024.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,edition2021,compile_fail
+    /// #![deny(rust_2024_guarded_string_incompatible_syntax)]
+    ///
+    /// macro_rules! m {
+    ///     (# $x:expr #) => ();
+    ///     (# $x:expr) => ();
+    /// }
+    ///
+    /// m!(#"hey"#);
+    /// m!(#"hello");
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Prior to Rust 2024, `#"hey"#` is three tokens: the first `#`
+    /// followed by the string literal `"hey"` then the final `#`.
+    /// In Rust 2024, the whole sequence is considered a single token.
+    ///
+    /// This lint suggests to add whitespace between the leading `#`
+    /// and the string to keep them separated in Rust 2024.
+    // Allow this lint -- rustdoc doesn't yet support threading edition into this lint's parser.
+    #[allow(rustdoc::invalid_rust_codeblocks)]
+    pub RUST_2024_GUARDED_STRING_INCOMPATIBLE_SYNTAX,
+    Allow,
+    "will be parsed as a guarded string in Rust 2024",
+    @future_incompatible = FutureIncompatibleInfo {
+        reason: FutureIncompatibilityReason::EditionError(Edition::Edition2024),
+        reference: "issue #123735 <https://github.com/rust-lang/rust/issues/123735>",
+    };
+    crate_level_only
+}

compiler/rustc_lint_defs/src/lib.rs

@@ -614,6 +614,8 @@ pub enum BuiltinLintDiag {
     ReservedPrefix(Span, String),
     /// `'r#` in edition < 2021.
     RawPrefix(Span),
+    /// `##` or `#"` is edition < 2024.
+    ReservedString(Span),
     TrailingMacro(bool, Ident),
     BreakWithLabelAndLoop(Span),
     UnicodeTextFlow(Span, String),

compiler/rustc_parse/messages.ftl

@@ -706,6 +706,10 @@ parse_require_colon_after_labeled_expression = labeled expression must be follow
     .label = the label
     .suggestion = add `:` after the label
 
+parse_reserved_string = invalid string literal
+    .note = unprefixed guarded string literals are reserved for future use since Rust 2024
+    .suggestion_whitespace = consider inserting whitespace here
+
 parse_return_types_use_thin_arrow = return types are denoted using `->`
     .suggestion = use `->` instead
 

compiler/rustc_parse/src/errors.rs

@@ -2110,6 +2110,24 @@ pub(crate) enum UnknownPrefixSugg {
     },
 }
 
+#[derive(Diagnostic)]
+#[diag(parse_reserved_string)]
+#[note]
+pub(crate) struct ReservedString {
+    #[primary_span]
+    pub span: Span,
+    #[subdiagnostic]
+    pub sugg: Option<GuardedStringSugg>,
+}
+#[derive(Subdiagnostic)]
+#[suggestion(
+    parse_suggestion_whitespace,
+    code = " ",
+    applicability = "maybe-incorrect",
+    style = "verbose"
+)]
+pub(crate) struct GuardedStringSugg(#[primary_span] pub Span);
+
 #[derive(Diagnostic)]
 #[diag(parse_too_many_hashes)]
 pub(crate) struct TooManyHashes {

compiler/rustc_parse/src/lexer/mod.rs

@@ -10,7 +10,8 @@
 use rustc_lexer::{Base, Cursor, DocStyle, LiteralKind, RawStrError};
 use rustc_session::lint::BuiltinLintDiag;
 use rustc_session::lint::builtin::{
-    RUST_2021_PREFIXES_INCOMPATIBLE_SYNTAX, TEXT_DIRECTION_CODEPOINT_IN_COMMENT,
+    RUST_2021_PREFIXES_INCOMPATIBLE_SYNTAX, RUST_2024_GUARDED_STRING_INCOMPATIBLE_SYNTAX,
+    TEXT_DIRECTION_CODEPOINT_IN_COMMENT,
 };
 use rustc_session::parse::ParseSess;
 use rustc_span::symbol::Symbol;
@@ -251,6 +252,7 @@ fn next_token(&mut self) -> (Token, bool) {
                     let prefix_span = self.mk_sp(start, lit_start);
                     return (Token::new(self.ident(start), prefix_span), preceded_by_whitespace);
                 }
+                rustc_lexer::TokenKind::GuardedStrPrefix => self.maybe_report_guarded_str(start, str_before),
                 rustc_lexer::TokenKind::Literal { kind, suffix_start } => {
                     let suffix_start = start + BytePos(suffix_start);
                     let (kind, symbol) = self.cook_lexer_literal(start, suffix_start, kind);
@@ -781,6 +783,86 @@ fn report_unknown_prefix(&self, start: BytePos) {
         }
     }
 
+    /// Detect guarded string literal syntax
+    ///
+    /// RFC 3598 reserved this syntax for future use. As of Rust 2024,
+    /// using this syntax produces an error. In earlier editions, however, it
+    /// only results in an (allowed by default) lint, and is treated as
+    /// separate tokens.
+    fn maybe_report_guarded_str(&mut self, start: BytePos, str_before: &'src str) -> TokenKind {
+        let span = self.mk_sp(start, self.pos);
+        let edition2024 = span.edition().at_least_rust_2024();
+
+        let space_pos = start + BytePos(1);
+        let space_span = self.mk_sp(space_pos, space_pos);
+
+        let mut cursor = Cursor::new(str_before);
+
+        let (span, unterminated) = match cursor.guarded_double_quoted_string() {
+            Some(rustc_lexer::GuardedStr { n_hashes, terminated, token_len }) => {
+                let end = start + BytePos(token_len);
+                let span = self.mk_sp(start, end);
+                let str_start = start + BytePos(n_hashes);
+
+                if edition2024 {
+                    self.cursor = cursor;
+                    self.pos = end;
+                }
+
+                let unterminated = if terminated { None } else { Some(str_start) };
+
+                (span, unterminated)
+            }
+            _ => {
+                // We should only get here in the `##+` case.
+                debug_assert_eq!(self.str_from_to(start, start + BytePos(2)), "##");
+
+                (span, None)
+            }
+        };
+        if edition2024 {
+            if let Some(str_start) = unterminated {
+                // Only a fatal error if string is unterminated.
+                self.dcx()
+                    .struct_span_fatal(
+                        self.mk_sp(str_start, self.pos),
+                        "unterminated double quote string",
+                    )
+                    .with_code(E0765)
+                    .emit()
+            }
+
+            let sugg = if span.from_expansion() {
+                None
+            } else {
+                Some(errors::GuardedStringSugg(space_span))
+            };
+
+            // In Edition 2024 and later, emit a hard error.
+            let err = self.dcx().emit_err(errors::ReservedString { span, sugg });
+
+            token::Literal(token::Lit {
+                kind: token::Err(err),
+                symbol: self.symbol_from_to(start, self.pos),
+                suffix: None,
+            })
+        } else {
+            // Before Rust 2024, only emit a lint for migration.
+            self.psess.buffer_lint(
+                RUST_2024_GUARDED_STRING_INCOMPATIBLE_SYNTAX,
+                span,
+                ast::CRATE_NODE_ID,
+                BuiltinLintDiag::ReservedString(space_span),
+            );
+
+            // For backwards compatibility, roll back to after just the first `#`
+            // and return the `Pound` token.
+            self.pos = start + BytePos(1);
+            self.cursor = Cursor::new(&str_before[1..]);
+            token::Pound
+        }
+    }
+
     fn report_too_many_hashes(&self, start: BytePos, num: u32) -> ! {
         self.dcx().emit_fatal(errors::TooManyHashes { span: self.mk_sp(start, self.pos), num });
     }

src/librustdoc/html/highlight.rs

@@ -845,6 +845,7 @@ fn advance(
                 // Number literals.
                 LiteralKind::Float { .. } | LiteralKind::Int { .. } => Class::Number,
             },
+            TokenKind::GuardedStrPrefix => return no_highlight(sink),
             TokenKind::Ident | TokenKind::RawIdent if lookahead == Some(TokenKind::Bang) => {
                 self.in_macro = true;
                 sink(Highlight::EnterSpan { class: Class::Macro(self.new_span(before, text)) });

src/tools/rust-analyzer/crates/parser/src/lexed_str.rs

@@ -187,6 +187,12 @@ fn extend_token(&mut self, kind: &rustc_lexer::TokenKind, token_text: &str) {
                 }
 
                 rustc_lexer::TokenKind::RawIdent => IDENT,
+
+                rustc_lexer::TokenKind::GuardedStrPrefix => {
+                    err = "Invalid string literal (reserved syntax)";
+                    ERROR
+                },
+
                 rustc_lexer::TokenKind::Literal { kind, .. } => {
                     self.extend_literal(token_text.len(), kind);
                     return;

tests/ui/rust-2024/auxiliary/reserved-guarded-strings-macro-2021.rs

@@ -0,0 +1,20 @@
+//@ force-host
+//@ edition:2021
+//@ no-prefer-dynamic
+
+#![crate_type = "proc-macro"]
+
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+use std::str::FromStr;
+
+#[proc_macro]
+pub fn number_of_tokens_in_a_guarded_string_literal(_: TokenStream) -> TokenStream {
+    TokenStream::from_str("#\"abc\"#").unwrap().into_iter().count().to_string().parse().unwrap()
+}
+
+#[proc_macro]
+pub fn number_of_tokens_in_a_guarded_unterminated_string_literal(_: TokenStream) -> TokenStream {
+    TokenStream::from_str("#\"abc\"").unwrap().into_iter().count().to_string().parse().unwrap()
+}

tests/ui/rust-2024/auxiliary/reserved-guarded-strings-macro-2024.rs

@@ -0,0 +1,21 @@
+//@ force-host
+//@ compile-flags: -Zunstable-options
+//@ edition:2024
+//@ no-prefer-dynamic
+
+#![crate_type = "proc-macro"]
+
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+use std::str::FromStr;
+
+#[proc_macro]
+pub fn number_of_tokens_in_a_guarded_string_literal(_: TokenStream) -> TokenStream {
+    TokenStream::from_str("#\"abc\"#").unwrap().into_iter().count().to_string().parse().unwrap()
+}
+
+#[proc_macro]
+pub fn number_of_tokens_in_a_guarded_unterminated_string_literal(_: TokenStream) -> TokenStream {
+    TokenStream::from_str("#\"abc\"").unwrap().into_iter().count().to_string().parse().unwrap()
+}

tests/ui/rust-2024/reserved-guarded-strings-lexing.rs

@@ -0,0 +1,80 @@
+//@ edition:2021
+// ignore-tidy-linelength
+
+#![warn(rust_2024_guarded_string_incompatible_syntax)]
+
+macro_rules! demo2 {
+    ( $a:tt $b:tt ) => { println!("two tokens") };
+}
+
+macro_rules! demo3 {
+    ( $a:tt $b:tt $c:tt ) => { println!("three tokens") };
+}
+
+macro_rules! demo4 {
+    ( $a:tt $b:tt $c:tt $d:tt ) => { println!("four tokens") };
+}
+
+macro_rules! demo5 {
+    ( $a:tt $b:tt $c:tt $d:tt $e:tt ) => { println!("five tokens") };
+}
+
+macro_rules! demo7 {
+    ( $a:tt $b:tt $c:tt $d:tt $e:tt $f:tt $g:tt ) => { println!("seven tokens") };
+}
+
+
+fn main() {
+    demo3!(## "foo");
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo4!(### "foo");
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo4!(## "foo"#);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo7!(### "foo"###);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+
+    demo5!(###"foo"#);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo5!(#"foo"###);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo4!("foo"###);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+
+    // Non-ascii identifiers
+    demo2!(Ñ"foo");
+    //~^ ERROR prefix `Ñ` is unknown
+    demo4!(Ñ#""#);
+    //~^ ERROR prefix `Ñ` is unknown
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo3!(🙃#"");
+    //~^ ERROR identifiers cannot contain emoji
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+}

tests/ui/rust-2024/reserved-guarded-strings-lexing.stderr

@@ -0,0 +1,271 @@
+error: prefix `Ñ` is unknown
+  --> $DIR/reserved-guarded-strings-lexing.rs:70:12
+   |
+LL |     demo2!(Ñ"foo");
+   |            ^ unknown prefix
+   |
+   = note: prefixed identifiers and literals are reserved since Rust 2021
+help: consider inserting whitespace here
+   |
+LL |     demo2!(Ñ "foo");
+   |             +
+
+error: prefix `Ñ` is unknown
+  --> $DIR/reserved-guarded-strings-lexing.rs:72:12
+   |
+LL |     demo4!(Ñ#""#);
+   |            ^ unknown prefix
+   |
+   = note: prefixed identifiers and literals are reserved since Rust 2021
+help: consider inserting whitespace here
+   |
+LL |     demo4!(Ñ #""#);
+   |             +
+
+error: identifiers cannot contain emoji: `🙃`
+  --> $DIR/reserved-guarded-strings-lexing.rs:76:12
+   |
+LL |     demo3!(🙃#"");
+   |            ^^
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:28:12
+   |
+LL |     demo3!(## "foo");
+   |            ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+note: the lint level is defined here
+  --> $DIR/reserved-guarded-strings-lexing.rs:4:9
+   |
+LL | #![warn(rust_2024_guarded_string_incompatible_syntax)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo3!(# # "foo");
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:31:12
+   |
+LL |     demo4!(### "foo");
+   |            ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo4!(# ## "foo");
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:31:13
+   |
+LL |     demo4!(### "foo");
+   |             ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo4!(## # "foo");
+   |              +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:36:12
+   |
+LL |     demo4!(## "foo"#);
+   |            ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo4!(# # "foo"#);
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:39:12
+   |
+LL |     demo7!(### "foo"###);
+   |            ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo7!(# ## "foo"###);
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:39:13
+   |
+LL |     demo7!(### "foo"###);
+   |             ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo7!(## # "foo"###);
+   |              +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:39:21
+   |
+LL |     demo7!(### "foo"###);
+   |                     ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo7!(### "foo"# ##);
+   |                      +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:39:22
+   |
+LL |     demo7!(### "foo"###);
+   |                      ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo7!(### "foo"## #);
+   |                       +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:49:12
+   |
+LL |     demo5!(###"foo"#);
+   |            ^^^^^^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo5!(# ##"foo"#);
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:49:13
+   |
+LL |     demo5!(###"foo"#);
+   |             ^^^^^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo5!(## #"foo"#);
+   |              +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:49:14
+   |
+LL |     demo5!(###"foo"#);
+   |              ^^^^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo5!(### "foo"#);
+   |               +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:56:12
+   |
+LL |     demo5!(#"foo"###);
+   |            ^^^^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo5!(# "foo"###);
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:56:18
+   |
+LL |     demo5!(#"foo"###);
+   |                  ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo5!(#"foo"# ##);
+   |                   +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:56:19
+   |
+LL |     demo5!(#"foo"###);
+   |                   ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo5!(#"foo"## #);
+   |                    +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:63:17
+   |
+LL |     demo4!("foo"###);
+   |                 ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo4!("foo"# ##);
+   |                  +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:63:18
+   |
+LL |     demo4!("foo"###);
+   |                  ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo4!("foo"## #);
+   |                   +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:72:13
+   |
+LL |     demo4!(Ñ#""#);
+   |             ^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo4!(Ñ# ""#);
+   |              +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:76:13
+   |
+LL |     demo3!(🙃#"");
+   |              ^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo3!(🙃# "");
+   |               +
+
+error: aborting due to 3 previous errors; 18 warnings emitted
+

tests/ui/rust-2024/reserved-guarded-strings-migration.fixed

@@ -0,0 +1,99 @@
+//@ check-pass
+//@ run-rustfix
+//@ edition:2021
+
+#![warn(rust_2024_guarded_string_incompatible_syntax)]
+
+macro_rules! demo1 {
+    ( $a:tt ) => { println!("one tokens") };
+}
+
+macro_rules! demo2 {
+    ( $a:tt $b:tt ) => { println!("two tokens") };
+}
+
+macro_rules! demo3 {
+    ( $a:tt $b:tt $c:tt ) => { println!("three tokens") };
+}
+
+macro_rules! demo4 {
+    ( $a:tt $b:tt $c:tt $d:tt ) => { println!("four tokens") };
+}
+
+macro_rules! demo5 {
+    ( $a:tt $b:tt $c:tt $d:tt $e:tt ) => { println!("five tokens") };
+}
+
+macro_rules! demo6 {
+    ( $a:tt $b:tt $c:tt $d:tt $e:tt $f:tt ) => { println!("six tokens") };
+}
+
+
+fn main() {
+    demo1!("");
+    demo2!(# "");
+    demo3!(# ""#);
+    demo2!(# "foo");
+    demo3!(# "foo"#);
+    demo2!("foo"#);
+
+    demo3!(# # "foo");
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo4!(# # # "foo");
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo4!(# # "foo"#);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo6!(# # # "foo"# #);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+
+    demo4!("foo"# # #);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+
+    demo2!(# "");
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo3!(# ""#);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo3!(# # "");
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo2!(# "foo");
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo3!(# # "foo");
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo3!(# "foo"#);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo4!(# # "foo"#);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo5!(# # "foo"# #);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+}

tests/ui/rust-2024/reserved-guarded-strings-migration.rs

@@ -0,0 +1,99 @@
+//@ check-pass
+//@ run-rustfix
+//@ edition:2021
+
+#![warn(rust_2024_guarded_string_incompatible_syntax)]
+
+macro_rules! demo1 {
+    ( $a:tt ) => { println!("one tokens") };
+}
+
+macro_rules! demo2 {
+    ( $a:tt $b:tt ) => { println!("two tokens") };
+}
+
+macro_rules! demo3 {
+    ( $a:tt $b:tt $c:tt ) => { println!("three tokens") };
+}
+
+macro_rules! demo4 {
+    ( $a:tt $b:tt $c:tt $d:tt ) => { println!("four tokens") };
+}
+
+macro_rules! demo5 {
+    ( $a:tt $b:tt $c:tt $d:tt $e:tt ) => { println!("five tokens") };
+}
+
+macro_rules! demo6 {
+    ( $a:tt $b:tt $c:tt $d:tt $e:tt $f:tt ) => { println!("six tokens") };
+}
+
+
+fn main() {
+    demo1!("");
+    demo2!(# "");
+    demo3!(# ""#);
+    demo2!(# "foo");
+    demo3!(# "foo"#);
+    demo2!("foo"#);
+
+    demo3!(## "foo");
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo4!(### "foo");
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo4!(## "foo"#);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo6!(### "foo"##);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+
+    demo4!("foo"###);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+
+    demo2!(#"");
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo3!(#""#);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo3!(##"");
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo2!(#"foo");
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo3!(##"foo");
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo3!(#"foo"#);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo4!(##"foo"#);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo5!(##"foo"##);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+}

tests/ui/rust-2024/reserved-guarded-strings-migration.stderr

@@ -0,0 +1,293 @@
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:40:12
+   |
+LL |     demo3!(## "foo");
+   |            ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+note: the lint level is defined here
+  --> $DIR/reserved-guarded-strings-migration.rs:5:9
+   |
+LL | #![warn(rust_2024_guarded_string_incompatible_syntax)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo3!(# # "foo");
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:43:12
+   |
+LL |     demo4!(### "foo");
+   |            ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo4!(# ## "foo");
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:43:13
+   |
+LL |     demo4!(### "foo");
+   |             ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo4!(## # "foo");
+   |              +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:48:12
+   |
+LL |     demo4!(## "foo"#);
+   |            ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo4!(# # "foo"#);
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:51:12
+   |
+LL |     demo6!(### "foo"##);
+   |            ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo6!(# ## "foo"##);
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:51:13
+   |
+LL |     demo6!(### "foo"##);
+   |             ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo6!(## # "foo"##);
+   |              +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:51:21
+   |
+LL |     demo6!(### "foo"##);
+   |                     ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo6!(### "foo"# #);
+   |                      +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:59:17
+   |
+LL |     demo4!("foo"###);
+   |                 ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo4!("foo"# ##);
+   |                  +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:59:18
+   |
+LL |     demo4!("foo"###);
+   |                  ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo4!("foo"## #);
+   |                   +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:65:12
+   |
+LL |     demo2!(#"");
+   |            ^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo2!(# "");
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:68:12
+   |
+LL |     demo3!(#""#);
+   |            ^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo3!(# ""#);
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:71:12
+   |
+LL |     demo3!(##"");
+   |            ^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo3!(# #"");
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:71:13
+   |
+LL |     demo3!(##"");
+   |             ^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo3!(## "");
+   |              +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:76:12
+   |
+LL |     demo2!(#"foo");
+   |            ^^^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo2!(# "foo");
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:79:12
+   |
+LL |     demo3!(##"foo");
+   |            ^^^^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo3!(# #"foo");
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:79:13
+   |
+LL |     demo3!(##"foo");
+   |             ^^^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo3!(## "foo");
+   |              +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:84:12
+   |
+LL |     demo3!(#"foo"#);
+   |            ^^^^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo3!(# "foo"#);
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:87:12
+   |
+LL |     demo4!(##"foo"#);
+   |            ^^^^^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo4!(# #"foo"#);
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:87:13
+   |
+LL |     demo4!(##"foo"#);
+   |             ^^^^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo4!(## "foo"#);
+   |              +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:92:12
+   |
+LL |     demo5!(##"foo"##);
+   |            ^^^^^^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo5!(# #"foo"##);
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:92:13
+   |
+LL |     demo5!(##"foo"##);
+   |             ^^^^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo5!(## "foo"##);
+   |              +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:92:19
+   |
+LL |     demo5!(##"foo"##);
+   |                   ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo5!(##"foo"# #);
+   |                    +
+
+warning: 22 warnings emitted
+

tests/ui/rust-2024/reserved-guarded-strings-via-macro-2.rs

@@ -0,0 +1,18 @@
+//@ edition:2021
+//@ aux-build:reserved-guarded-strings-macro-2021.rs
+//@ aux-build:reserved-guarded-strings-macro-2024.rs
+
+extern crate reserved_guarded_strings_macro_2021 as m2021;
+extern crate reserved_guarded_strings_macro_2024 as m2024;
+
+fn main() {
+    // Ok:
+    m2021::number_of_tokens_in_a_guarded_string_literal!();
+    m2021::number_of_tokens_in_a_guarded_unterminated_string_literal!();
+
+    // Error, even though *this* crate is 2021:
+    m2024::number_of_tokens_in_a_guarded_string_literal!();
+    //~^ ERROR invalid string literal
+    m2024::number_of_tokens_in_a_guarded_unterminated_string_literal!();
+    //~^ ERROR invalid string literal
+}

tests/ui/rust-2024/reserved-guarded-strings-via-macro-2.stderr

@@ -0,0 +1,20 @@
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings-via-macro-2.rs:14:5
+   |
+LL |     m2024::number_of_tokens_in_a_guarded_string_literal!();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+   = note: this error originates in the macro `m2024::number_of_tokens_in_a_guarded_string_literal` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings-via-macro-2.rs:16:5
+   |
+LL |     m2024::number_of_tokens_in_a_guarded_unterminated_string_literal!();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+   = note: this error originates in the macro `m2024::number_of_tokens_in_a_guarded_unterminated_string_literal` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 2 previous errors
+

tests/ui/rust-2024/reserved-guarded-strings-via-macro.rs

@@ -0,0 +1,12 @@
+//@ run-pass
+//@ compile-flags: -Zunstable-options
+//@ edition:2024
+//@ aux-build:reserved-guarded-strings-macro-2021.rs
+
+extern crate reserved_guarded_strings_macro_2021 as m2021;
+
+fn main() {
+    // Ok, even though *this* crate is 2024:
+    assert_eq!(m2021::number_of_tokens_in_a_guarded_string_literal!(), 3);
+    assert_eq!(m2021::number_of_tokens_in_a_guarded_unterminated_string_literal!(), 2);
+}

tests/ui/rust-2024/reserved-guarded-strings.rs

@@ -0,0 +1,74 @@
+//@ compile-flags: -Zunstable-options
+//@ edition:2024
+// ignore-tidy-linelength
+
+macro_rules! demo1 {
+    ( $a:tt ) => { println!("one tokens") };
+}
+
+macro_rules! demo2 {
+    ( $a:tt $b:tt ) => { println!("two tokens") };
+}
+
+macro_rules! demo3 {
+    ( $a:tt $b:tt $c:tt ) => { println!("three tokens") };
+}
+
+macro_rules! demo4 {
+    ( $a:tt $b:tt $c:tt $d:tt ) => { println!("four tokens") };
+}
+
+macro_rules! demo5 {
+    ( $a:tt $b:tt $c:tt $d:tt $e:tt ) => { println!("five tokens") };
+}
+
+macro_rules! demo6 {
+    ( $a:tt $b:tt $c:tt $d:tt $e:tt $f:tt ) => { println!("six tokens") };
+}
+
+macro_rules! demo7 {
+    ( $a:tt $b:tt $c:tt $d:tt $e:tt $f:tt $g:tt ) => { println!("seven tokens") };
+}
+
+macro_rules! demon {
+    ( $($n:tt)* ) => { println!("unknown number of tokens") };
+}
+
+fn main() {
+    demo1!("");
+    demo2!(# "");
+    demo3!(# ""#);
+    demo2!(# "foo");
+    demo3!(# "foo"#);
+    demo2!("foo"#);
+
+    demo2!(blah"xx"); //~ ERROR prefix `blah` is unknown
+    demo2!(blah#"xx"#);
+    //~^ ERROR prefix `blah` is unknown
+    //~| ERROR invalid string literal
+
+    demo2!(## "foo"); //~ ERROR invalid string literal
+    demo3!("foo"###); //~ ERROR invalid string literal
+    demo3!(### "foo"); //~ ERROR invalid string literal
+    demo3!(## "foo"#); //~ ERROR invalid string literal
+    demo5!(### "foo"###);
+    //~^ ERROR invalid string literal
+    //~| ERROR invalid string literal
+
+    demo1!(#""); //~ ERROR invalid string literal
+    demo1!(#""#); //~ ERROR invalid string literal
+    demo1!(####""); //~ ERROR invalid string literal
+    demo1!(#"foo"); //~ ERROR invalid string literal
+    demo1!(###"foo"); //~ ERROR invalid string literal
+    demo1!(#"foo"#); //~ ERROR invalid string literal
+    demo1!(###"foo"#); //~ ERROR invalid string literal
+    demo1!(###"foo"##); //~ ERROR invalid string literal
+    demo1!(###"foo"###); //~ ERROR invalid string literal
+    demo2!(#"foo"###);
+    //~^ ERROR invalid string literal
+    //~| ERROR invalid string literal
+
+    // More than 255 hashes
+    demon!(####################################################################################################################################################################################################################################################################"foo");
+    //~^ ERROR invalid string literal
+}

tests/ui/rust-2024/reserved-guarded-strings.stderr

@@ -0,0 +1,254 @@
+error: prefix `blah` is unknown
+  --> $DIR/reserved-guarded-strings.rs:45:12
+   |
+LL |     demo2!(blah"xx");
+   |            ^^^^ unknown prefix
+   |
+   = note: prefixed identifiers and literals are reserved since Rust 2021
+help: consider inserting whitespace here
+   |
+LL |     demo2!(blah "xx");
+   |                +
+
+error: prefix `blah` is unknown
+  --> $DIR/reserved-guarded-strings.rs:46:12
+   |
+LL |     demo2!(blah#"xx"#);
+   |            ^^^^ unknown prefix
+   |
+   = note: prefixed identifiers and literals are reserved since Rust 2021
+help: consider inserting whitespace here
+   |
+LL |     demo2!(blah #"xx"#);
+   |                +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:46:16
+   |
+LL |     demo2!(blah#"xx"#);
+   |                ^^^^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo2!(blah# "xx"#);
+   |                 +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:50:12
+   |
+LL |     demo2!(## "foo");
+   |            ^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo2!(# # "foo");
+   |             +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:51:17
+   |
+LL |     demo3!("foo"###);
+   |                 ^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo3!("foo"# ##);
+   |                  +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:52:12
+   |
+LL |     demo3!(### "foo");
+   |            ^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo3!(# ## "foo");
+   |             +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:53:12
+   |
+LL |     demo3!(## "foo"#);
+   |            ^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo3!(# # "foo"#);
+   |             +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:54:12
+   |
+LL |     demo5!(### "foo"###);
+   |            ^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo5!(# ## "foo"###);
+   |             +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:54:21
+   |
+LL |     demo5!(### "foo"###);
+   |                     ^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo5!(### "foo"# ##);
+   |                      +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:58:12
+   |
+LL |     demo1!(#"");
+   |            ^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo1!(# "");
+   |             +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:59:12
+   |
+LL |     demo1!(#""#);
+   |            ^^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo1!(# ""#);
+   |             +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:60:12
+   |
+LL |     demo1!(####"");
+   |            ^^^^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo1!(# ###"");
+   |             +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:61:12
+   |
+LL |     demo1!(#"foo");
+   |            ^^^^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo1!(# "foo");
+   |             +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:62:12
+   |
+LL |     demo1!(###"foo");
+   |            ^^^^^^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo1!(# ##"foo");
+   |             +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:63:12
+   |
+LL |     demo1!(#"foo"#);
+   |            ^^^^^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo1!(# "foo"#);
+   |             +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:64:12
+   |
+LL |     demo1!(###"foo"#);
+   |            ^^^^^^^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo1!(# ##"foo"#);
+   |             +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:65:12
+   |
+LL |     demo1!(###"foo"##);
+   |            ^^^^^^^^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo1!(# ##"foo"##);
+   |             +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:66:12
+   |
+LL |     demo1!(###"foo"###);
+   |            ^^^^^^^^^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo1!(# ##"foo"###);
+   |             +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:67:12
+   |
+LL |     demo2!(#"foo"###);
+   |            ^^^^^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo2!(# "foo"###);
+   |             +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:67:19
+   |
+LL |     demo2!(#"foo"###);
+   |                   ^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo2!(#"foo"## #);
+   |                    +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:72:12
+   |
+LL | ...n!(####################################################################################################################################################################################################################################################################"foo...
+   |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demon!(# ###################################################################################################################################################################################################################################################################"foo");
+   |             +
+
+error: aborting due to 21 previous errors
+