Lexer: check in advance_token to avoid regard spare ## as GardedStrPrefix

compiler/rustc_lexer/src/lib.rs

@@ -371,6 +371,7 @@ pub fn is_ident(string: &str) -> bool {
 impl Cursor<'_> {
     /// Parses a token from the input string.
     pub fn advance_token(&mut self) -> Token {
+        let pre_char = self.prev();
         let Some(first_char) = self.bump() else {
             return Token::new(TokenKind::Eof, 0);
         };
@@ -454,7 +455,7 @@ impl Cursor<'_> {
             }
 
             // Guarded string literal prefix: `#"` or `##`
-            '#' if matches!(self.first(), '"' | '#') => {
+            '#' if matches!(self.first(), '"' | '#') && pre_char != '#' => {
                 self.bump();
                 TokenKind::GuardedStrPrefix
             }

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

@@ -65,7 +65,7 @@ fn main() {
     demo1!(###"foo"###); //~ ERROR invalid string literal
     demo2!(#"foo"###);
     //~^ ERROR invalid string literal
-    //~| ERROR reserved multi-hash token is forbidden
+    //~| ERROR no rules expected `#`
 
     // More than 255 hashes
     demon!(####################################################################################################################################################################################################################################################################"foo");

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

@@ -226,18 +226,6 @@ help: consider inserting whitespace here
 LL |     demo2!(# "foo"###);
    |             +
 
-error: reserved multi-hash token is forbidden
-  --> $DIR/reserved-guarded-strings.rs:66:19
-   |
-LL |     demo2!(#"foo"###);
-   |                   ^^
-   |
-   = note: sequences of two or more # 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:71:12
    |
@@ -250,5 +238,20 @@ help: consider inserting whitespace here
 LL |     demon!(# ###################################################################################################################################################################################################################################################################"foo");
    |             +
 
+error: no rules expected `#`
+  --> $DIR/reserved-guarded-strings.rs:66:20
+   |
+LL | macro_rules! demo2 {
+   | ------------------ when calling this macro
+...
+LL |     demo2!(#"foo"###);
+   |                    ^ no rules expected this token in macro call
+   |
+note: while trying to match meta-variable `$b:tt`
+  --> $DIR/reserved-guarded-strings.rs:9:13
+   |
+LL |     ( $a:tt $b:tt ) => { println!("two tokens") };
+   |             ^^^^^
+
 error: aborting due to 21 previous errors
 

tests/ui/rust-2024/reverved-guarded-string-too-many-terminators-issue-140618.rs

@@ -0,0 +1,36 @@
+//@ edition:2024
+
+fn f0(){
+    r#"ok0!"#;
+}
+
+fn f1(){
+    r#"ok1!"##;
+    //~^ ERROR too many `#` when terminating raw string
+}
+
+
+fn f2(){
+    r#"ok2!"###;
+    //~^ ERROR too many `#` when terminating raw string
+}
+
+fn f3(){
+    #"ok3!"#;
+    //~^ ERROR invalid string literal
+}
+
+
+fn f4(){
+    #"ok4!"##;
+    //~^ ERROR invalid string literal
+    //~| ERROR expected one of `.`, `;`, `?`, `}`, or an operator, found `#`
+}
+
+fn f5(){
+    #"ok5!"###;
+    //~^ ERROR invalid string literal
+    //~| ERROR expected one of `.`, `;`, `?`, `}`, or an operator, found `#`
+}
+
+fn main() {}

tests/ui/rust-2024/reverved-guarded-string-too-many-terminators-issue-140618.stderr

@@ -0,0 +1,66 @@
+error: invalid string literal
+  --> $DIR/reverved-guarded-string-too-many-terminators-issue-140618.rs:19:5
+   |
+LL |     #"ok3!"#;
+   |     ^^^^^^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     # "ok3!"#;
+   |      +
+
+error: invalid string literal
+  --> $DIR/reverved-guarded-string-too-many-terminators-issue-140618.rs:25:5
+   |
+LL |     #"ok4!"##;
+   |     ^^^^^^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     # "ok4!"##;
+   |      +
+
+error: invalid string literal
+  --> $DIR/reverved-guarded-string-too-many-terminators-issue-140618.rs:31:5
+   |
+LL |     #"ok5!"###;
+   |     ^^^^^^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     # "ok5!"###;
+   |      +
+
+error: too many `#` when terminating raw string
+  --> $DIR/reverved-guarded-string-too-many-terminators-issue-140618.rs:8:14
+   |
+LL |     r#"ok1!"##;
+   |     ---------^ help: remove the extra `#`
+   |     |
+   |     this raw string started with 1 `#`
+
+error: too many `#` when terminating raw string
+  --> $DIR/reverved-guarded-string-too-many-terminators-issue-140618.rs:14:14
+   |
+LL |     r#"ok2!"###;
+   |     ---------^^ help: remove the extra `#`s
+   |     |
+   |     this raw string started with 1 `#`
+
+error: expected one of `.`, `;`, `?`, `}`, or an operator, found `#`
+  --> $DIR/reverved-guarded-string-too-many-terminators-issue-140618.rs:25:13
+   |
+LL |     #"ok4!"##;
+   |             ^ expected one of `.`, `;`, `?`, `}`, or an operator
+
+error: expected one of `.`, `;`, `?`, `}`, or an operator, found `#`
+  --> $DIR/reverved-guarded-string-too-many-terminators-issue-140618.rs:31:13
+   |
+LL |     #"ok5!"###;
+   |             ^ expected one of `.`, `;`, `?`, `}`, or an operator
+
+error: aborting due to 7 previous errors
+