parser: Do not call bump recursively

Token normalization is merged directly into `bump`.
Special "unknown macro variable" diagnostic for unexpected `$`s is removed as preventing legal code from compiling.

src/librustc_expand/mbe/macro_parser.rs

@@ -856,8 +856,6 @@ fn parse_nt(p: &mut Parser<'_>, sp: Span, name: Symbol) -> Nonterminal {
     if name == sym::tt {
         return token::NtTT(p.parse_token_tree());
     }
-    // check at the beginning and the parser checks after each bump
-    p.process_potential_macro_variable();
     match parse_nt_inner(p, sp, name) {
         Ok(nt) => nt,
         Err(mut err) => {

src/librustc_expand/mbe/macro_rules.rs

@@ -267,7 +267,6 @@ fn generic_extension<'cx>(
                     cx.current_expansion.module.mod_path.last().map(|id| id.to_string());
                 p.last_type_ascription = cx.current_expansion.prior_type_ascription;
 
-                p.process_potential_macro_variable();
                 // Let the context choose how to interpret the result.
                 // Weird, but useful for X-macros.
                 return Box::new(ParserAnyMacro {

src/librustc_parse/parser/mod.rs

@@ -404,7 +404,8 @@ pub fn new(
             subparser_name,
         };
 
-        parser.token = parser.next_tok();
+        // Make parser point to the first token.
+        parser.bump();
 
         if let Some(directory) = directory {
             parser.directory = directory;
@@ -418,7 +419,6 @@ pub fn new(
             }
         }
 
-        parser.process_potential_macro_variable();
         parser
     }
 
@@ -430,7 +430,7 @@ fn unnormalized_prev_token(&self) -> &Token {
         self.unnormalized_prev_token.as_ref().unwrap_or(&self.prev_token)
     }
 
-    fn next_tok(&mut self) -> Token {
+    fn next_tok(&mut self, fallback_span: Span) -> Token {
         let mut next = if self.desugar_doc_comments {
             self.token_cursor.next_desugared()
         } else {
@@ -438,7 +438,7 @@ fn next_tok(&mut self) -> Token {
         };
         if next.span.is_dummy() {
             // Tweak the location for better diagnostics, but keep syntactic context intact.
-            next.span = self.unnormalized_token().span.with_ctxt(next.span.ctxt());
+            next.span = fallback_span.with_ctxt(next.span.ctxt());
         }
         next
     }
@@ -896,6 +896,24 @@ fn parse_paren_comma_seq<T>(
         self.parse_delim_comma_seq(token::Paren, f)
     }
 
+    // Interpolated identifier (`$i: ident`) and lifetime (`$l: lifetime`)
+    // tokens are replaced with usual identifier and lifetime tokens,
+    // so the former are never encountered during normal parsing.
+    fn normalize_token(token: &Token) -> Option<Token> {
+        match &token.kind {
+            token::Interpolated(nt) => match **nt {
+                token::NtIdent(ident, is_raw) => {
+                    Some(Token::new(token::Ident(ident.name, is_raw), ident.span))
+                }
+                token::NtLifetime(ident) => {
+                    Some(Token::new(token::Lifetime(ident.name), ident.span))
+                }
+                _ => None,
+            },
+            _ => None,
+        }
+    }
+
     /// Advance the parser by one token.
     pub fn bump(&mut self) {
         if self.prev_token.kind == TokenKind::Eof {
@@ -905,16 +923,17 @@ pub fn bump(&mut self) {
         }
 
         // Update the current and previous tokens.
-        let next_token = self.next_tok();
-        self.prev_token = mem::replace(&mut self.token, next_token);
+        self.prev_token = self.token.take();
         self.unnormalized_prev_token = self.unnormalized_token.take();
+        self.token = self.next_tok(self.unnormalized_prev_token().span);
+        if let Some(normalized_token) = Self::normalize_token(&self.token) {
+            self.unnormalized_token = Some(mem::replace(&mut self.token, normalized_token));
+        }
 
         // Update fields derived from the previous token.
         self.prev_span = self.unnormalized_prev_token().span;
 
         self.expected_tokens.clear();
-        // Check after each token.
-        self.process_potential_macro_variable();
     }
 
     /// Advances the parser using provided token as a next one. Use this when
@@ -924,9 +943,12 @@ pub fn bump(&mut self) {
     /// Correct token kinds and spans need to be calculated instead.
     fn bump_with(&mut self, next: TokenKind, span: Span) {
         // Update the current and previous tokens.
-        let next_token = Token::new(next, span);
-        self.prev_token = mem::replace(&mut self.token, next_token);
+        self.prev_token = self.token.take();
         self.unnormalized_prev_token = self.unnormalized_token.take();
+        self.token = Token::new(next, span);
+        if let Some(normalized_token) = Self::normalize_token(&self.token) {
+            self.unnormalized_token = Some(mem::replace(&mut self.token, normalized_token));
+        }
 
         // Update fields derived from the previous token.
         self.prev_span = self.unnormalized_prev_token().span.with_hi(span.lo());
@@ -1066,39 +1088,6 @@ fn parse_or_use_outer_attributes(
         }
     }
 
-    pub fn process_potential_macro_variable(&mut self) {
-        let normalized_token = match self.token.kind {
-            token::Dollar
-                if self.token.span.from_expansion() && self.look_ahead(1, |t| t.is_ident()) =>
-            {
-                self.bump();
-                let name = match self.token.kind {
-                    token::Ident(name, _) => name,
-                    _ => unreachable!(),
-                };
-                let span = self.prev_span.to(self.token.span);
-                self.struct_span_err(span, &format!("unknown macro variable `{}`", name))
-                    .span_label(span, "unknown macro variable")
-                    .emit();
-                self.bump();
-                return;
-            }
-            token::Interpolated(ref nt) => {
-                // Interpolated identifier and lifetime tokens are replaced with usual identifier
-                // and lifetime tokens, so the former are never encountered during normal parsing.
-                match **nt {
-                    token::NtIdent(ident, is_raw) => {
-                        Token::new(token::Ident(ident.name, is_raw), ident.span)
-                    }
-                    token::NtLifetime(ident) => Token::new(token::Lifetime(ident.name), ident.span),
-                    _ => return,
-                }
-            }
-            _ => return,
-        };
-        self.unnormalized_token = Some(mem::replace(&mut self.token, normalized_token));
-    }
-
     /// Parses a single token tree from the input.
     pub fn parse_token_tree(&mut self) -> TokenTree {
         match self.token.kind {

src/test/ui/issues/issue-6596-1.rs

@@ -1,7 +1,7 @@
 macro_rules! e {
     ($inp:ident) => (
         $nonexistent
-        //~^ ERROR unknown macro variable `nonexistent`
+        //~^ ERROR expected expression, found `$`
     );
 }
 

src/test/ui/issues/issue-6596-1.stderr

@@ -1,8 +1,8 @@
-error: unknown macro variable `nonexistent`
+error: expected expression, found `$`
   --> $DIR/issue-6596-1.rs:3:9
    |
 LL |         $nonexistent
-   |         ^^^^^^^^^^^^ unknown macro variable
+   |         ^^^^^^^^^^^^ expected expression
 ...
 LL |     e!(foo);
    |     -------- in this macro invocation

src/test/ui/issues/issue-6596-2.rs

@@ -3,7 +3,7 @@
 macro_rules! g {
     ($inp:ident) => (
         { $inp $nonexistent }
-        //~^ ERROR unknown macro variable `nonexistent`
+        //~^ ERROR expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `$`
     );
 }
 

src/test/ui/issues/issue-6596-2.stderr

@@ -1,8 +1,8 @@
-error: unknown macro variable `nonexistent`
+error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `$`
   --> $DIR/issue-6596-2.rs:5:16
    |
 LL |         { $inp $nonexistent }
-   |                ^^^^^^^^^^^^ unknown macro variable
+   |                ^^^^^^^^^^^^ expected one of 8 possible tokens
 ...
 LL |     g!(foo);
    |     -------- in this macro invocation