Handle str literals written with ' lexed as lifetime

Given `'hello world'` and `'1 str', provide a structured suggestion for a valid string literal:

```
error[E0762]: unterminated character literal
  --> $DIR/lex-bad-str-literal-as-char-3.rs:2:26
   |
LL |     println!('hello world');
   |                          ^^^^
   |
help: if you meant to write a `str` literal, use double quotes
   |
LL |     println!("hello world");
   |              ~           ~
```
```
error[E0762]: unterminated character literal
  --> $DIR/lex-bad-str-literal-as-char-1.rs:2:20
   |
LL |     println!('1 + 1');
   |                    ^^^^
   |
help: if you meant to write a `str` literal, use double quotes
   |
LL |     println!("1 + 1");
   |              ~     ~
```

Fix #119685.

compiler/rustc_lexer/src/cursor.rs

@@ -46,7 +46,7 @@ pub(crate) fn prev(&self) -> char {
     /// If requested position doesn't exist, `EOF_CHAR` is returned.
     /// However, getting `EOF_CHAR` doesn't always mean actual end of file,
     /// it should be checked with `is_eof` method.
-    pub(crate) fn first(&self) -> char {
+    pub fn first(&self) -> char {
         // `.next()` optimizes better than `.nth(0)`
         self.chars.clone().next().unwrap_or(EOF_CHAR)
     }

compiler/rustc_parse/messages.ftl

@@ -835,6 +835,7 @@ parse_unknown_prefix = prefix `{$prefix}` is unknown
     .label = unknown prefix
     .note =  prefixed identifiers and literals are reserved since Rust 2021
     .suggestion_br = use `br` for a raw byte string
+    .suggestion_str = if you meant to write a `str` literal, use double quotes
     .suggestion_whitespace = consider inserting whitespace here
 
 parse_unknown_start_of_token = unknown start of token: {$escaped}

compiler/rustc_parse/src/errors.rs

@@ -1994,6 +1994,17 @@ pub enum UnknownPrefixSugg {
         style = "verbose"
     )]
     Whitespace(#[primary_span] Span),
+    #[multipart_suggestion(
+        parse_suggestion_str,
+        applicability = "maybe-incorrect",
+        style = "verbose"
+    )]
+    MeantStr {
+        #[suggestion_part(code = "\"")]
+        start: Span,
+        #[suggestion_part(code = "\"")]
+        end: Span,
+    },
 }
 
 #[derive(Diagnostic)]

compiler/rustc_parse/src/lexer/mod.rs

@@ -63,6 +63,7 @@ pub(crate) fn parse_token_trees<'psess, 'src>(
         cursor,
         override_span,
         nbsp_is_whitespace: false,
+        last_lifetime: None,
     };
     let (stream, res, unmatched_delims) =
         tokentrees::TokenTreesReader::parse_all_token_trees(string_reader);
@@ -105,6 +106,10 @@ struct StringReader<'psess, 'src> {
     /// in this file, it's safe to treat further occurrences of the non-breaking
     /// space character as whitespace.
     nbsp_is_whitespace: bool,
+
+    /// Track the `Span` for the leading `'` of the last lifetime. Used for
+    /// diagnostics to detect possible typo where `"` was meant.
+    last_lifetime: Option<Span>,
 }
 
 impl<'psess, 'src> StringReader<'psess, 'src> {
@@ -130,6 +135,18 @@ fn next_token(&mut self) -> (Token, bool) {
 
             debug!("next_token: {:?}({:?})", token.kind, self.str_from(start));
 
+            if let rustc_lexer::TokenKind::Semi
+            | rustc_lexer::TokenKind::LineComment { .. }
+            | rustc_lexer::TokenKind::BlockComment { .. }
+            | rustc_lexer::TokenKind::CloseParen
+            | rustc_lexer::TokenKind::CloseBrace
+            | rustc_lexer::TokenKind::CloseBracket = token.kind
+            {
+                // Heuristic: we assume that it is unlikely we're dealing with an unterminated
+                // string surrounded by single quotes.
+                self.last_lifetime = None;
+            }
+
             // Now "cook" the token, converting the simple `rustc_lexer::TokenKind` enum into a
             // rich `rustc_ast::TokenKind`. This turns strings into interned symbols and runs
             // additional validation.
@@ -247,6 +264,7 @@ fn next_token(&mut self) -> (Token, bool) {
                     // expansion purposes. See #12512 for the gory details of why
                     // this is necessary.
                     let lifetime_name = self.str_from(start);
+                    self.last_lifetime = Some(self.mk_sp(start, start + BytePos(1)));
                     if starts_with_number {
                         let span = self.mk_sp(start, self.pos);
                         self.dcx().struct_err("lifetimes cannot start with a number")
@@ -395,10 +413,21 @@ fn cook_lexer_literal(
         match kind {
             rustc_lexer::LiteralKind::Char { terminated } => {
                 if !terminated {
-                    self.dcx()
+                    let mut err = self
+                        .dcx()
                         .struct_span_fatal(self.mk_sp(start, end), "unterminated character literal")
-                        .with_code(E0762)
+                        .with_code(E0762);
-                        .emit()
+                    if let Some(lt_sp) = self.last_lifetime {
+                        err.multipart_suggestion(
+                            "if you meant to write a `str` literal, use double quotes",
+                            vec![
+                                (lt_sp, "\"".to_string()),
+                                (self.mk_sp(start, start + BytePos(1)), "\"".to_string()),
+                            ],
+                            Applicability::MaybeIncorrect,
+                        );
+                    }
+                    err.emit()
                 }
                 self.cook_unicode(token::Char, Mode::Char, start, end, 1, 1) // ' '
             }
@@ -673,7 +702,16 @@ fn report_unknown_prefix(&self, start: BytePos) {
             let sugg = if prefix == "rb" {
                 Some(errors::UnknownPrefixSugg::UseBr(prefix_span))
             } else if expn_data.is_root() {
-                Some(errors::UnknownPrefixSugg::Whitespace(prefix_span.shrink_to_hi()))
+                if self.cursor.first() == '\''
+                    && let Some(start) = self.last_lifetime
+                {
+                    Some(errors::UnknownPrefixSugg::MeantStr {
+                        start,
+                        end: self.mk_sp(self.pos, self.pos + BytePos(1)),
+                    })
+                } else {
+                    Some(errors::UnknownPrefixSugg::Whitespace(prefix_span.shrink_to_hi()))
+                }
             } else {
                 None
             };

tests/ui/lexer/lex-bad-str-literal-as-char-1.fixed

@@ -0,0 +1,6 @@
+//@ run-rustfix
+fn main() {
+    println!("1 + 1");
+    //~^ ERROR unterminated character literal
+    //~| ERROR lifetimes cannot start with a number
+}

tests/ui/lexer/lex-bad-str-literal-as-char-1.rs

@@ -0,0 +1,6 @@
+//@ run-rustfix
+fn main() {
+    println!('1 + 1');
+    //~^ ERROR unterminated character literal
+    //~| ERROR lifetimes cannot start with a number
+}

tests/ui/lexer/lex-bad-str-literal-as-char-1.stderr

@@ -0,0 +1,20 @@
+error[E0762]: unterminated character literal
+  --> $DIR/lex-bad-str-literal-as-char-1.rs:3:20
+   |
+LL |     println!('1 + 1');
+   |                    ^^^
+   |
+help: if you meant to write a `str` literal, use double quotes
+   |
+LL |     println!("1 + 1");
+   |              ~     ~
+
+error: lifetimes cannot start with a number
+  --> $DIR/lex-bad-str-literal-as-char-1.rs:3:14
+   |
+LL |     println!('1 + 1');
+   |              ^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0762`.

tests/ui/lexer/lex-bad-str-literal-as-char-2.fixed

@@ -0,0 +1,4 @@
+//@ run-rustfix
+fn main() {
+    println!(" 1 + 1"); //~ ERROR character literal may only contain one codepoint
+}

tests/ui/lexer/lex-bad-str-literal-as-char-2.rs

@@ -0,0 +1,4 @@
+//@ run-rustfix
+fn main() {
+    println!(' 1 + 1'); //~ ERROR character literal may only contain one codepoint
+}

tests/ui/lexer/lex-bad-str-literal-as-char-2.stderr

@@ -0,0 +1,13 @@
+error: character literal may only contain one codepoint
+  --> $DIR/lex-bad-str-literal-as-char-2.rs:3:14
+   |
+LL |     println!(' 1 + 1');
+   |              ^^^^^^^^
+   |
+help: if you meant to write a `str` literal, use double quotes
+   |
+LL |     println!(" 1 + 1");
+   |              ~~~~~~~~
+
+error: aborting due to 1 previous error
+

tests/ui/lexer/lex-bad-str-literal-as-char-3.fixed

@@ -0,0 +1,4 @@
+//@ run-rustfix
+fn main() {
+    println!("hello world"); //~ ERROR unterminated character literal
+}

tests/ui/lexer/lex-bad-str-literal-as-char-3.rs

@@ -0,0 +1,4 @@
+//@ run-rustfix
+fn main() {
+    println!('hello world'); //~ ERROR unterminated character literal
+}

tests/ui/lexer/lex-bad-str-literal-as-char-3.stderr

@@ -0,0 +1,14 @@
+error[E0762]: unterminated character literal
+  --> $DIR/lex-bad-str-literal-as-char-3.rs:3:26
+   |
+LL |     println!('hello world');
+   |                          ^^^^
+   |
+help: if you meant to write a `str` literal, use double quotes
+   |
+LL |     println!("hello world");
+   |              ~           ~
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0762`.