fixes #101477: Recover from typo where == is used in place of =

compiler/rustc_error_messages/locales/en-US/parser.ftl

@@ -153,3 +153,6 @@ parser_left_arrow_operator = unexpected token: `<-`
 
 parser_remove_let = expected pattern, found `let`
     .suggestion = remove the unnecessary `let` keyword
+
+parser_use_eq_instead = unexpected `==`
+    .suggestion = try using `=` instead

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -713,6 +713,14 @@ pub(crate) struct RemoveLet {
     pub span: Span,
 }
 
+#[derive(SessionDiagnostic)]
+#[diag(parser::use_eq_instead)]
+pub(crate) struct UseEqInstead {
+    #[primary_span]
+    #[suggestion_short(applicability = "machine-applicable", code = "=")]
+    pub span: Span,
+}
+
 // SnapshotParser is used to create a snapshot of the parser
 // without causing duplicate errors being emitted when the `Parser`
 // is dropped.
@@ -957,6 +965,14 @@ impl<'a> Parser<'a> {
             }
         }
 
+        if self.token.kind == TokenKind::EqEq
+            && self.prev_token.is_ident()
+            && expected.iter().any(|tok| matches!(tok, TokenType::Token(TokenKind::Eq)))
+        {
+            // Likely typo: `=` → `==` in let expr or enum item
+            return Err(self.sess.create_err(UseEqInstead { span: self.token.span }));
+        }
+
         let expect = tokens_to_string(&expected);
         let actual = super::token_descr(&self.token);
         let (msg_exp, (label_sp, label_exp)) = if expected.len() > 1 {

src/test/ui/parser/issue-101477-enum.fixed

@@ -0,0 +1,10 @@
+// run-rustfix
+
+#[allow(dead_code)]
+enum Demo {
+    A = 1,
+    B = 2 //~ ERROR unexpected `==`
+    //~^ expected item, found `==`
+}
+
+fn main() {}

src/test/ui/parser/issue-101477-enum.rs

@@ -0,0 +1,10 @@
+// run-rustfix
+
+#[allow(dead_code)]
+enum Demo {
+    A = 1,
+    B == 2 //~ ERROR unexpected `==`
+    //~^ expected item, found `==`
+}
+
+fn main() {}

src/test/ui/parser/issue-101477-enum.stderr

@@ -0,0 +1,14 @@
+error: unexpected `==`
+  --> $DIR/issue-101477-enum.rs:6:7
+   |
+LL |     B == 2
+   |       ^^ help: try using `=` instead
+
+error: expected item, found `==`
+  --> $DIR/issue-101477-enum.rs:6:7
+   |
+LL |     B == 2
+   |       ^^ expected item
+
+error: aborting due to 2 previous errors
+

src/test/ui/parser/issue-101477-let.fixed

@@ -0,0 +1,6 @@
+// run-rustfix
+
+fn main() {
+    let x = 2; //~ ERROR unexpected `==`
+    println!("x: {}", x)
+}

src/test/ui/parser/issue-101477-let.rs

@@ -0,0 +1,6 @@
+// run-rustfix
+
+fn main() {
+    let x == 2; //~ ERROR unexpected `==`
+    println!("x: {}", x)
+}

src/test/ui/parser/issue-101477-let.stderr

@@ -0,0 +1,8 @@
+error: unexpected `==`
+  --> $DIR/issue-101477-let.rs:4:11
+   |
+LL |     let x == 2;
+   |           ^^ help: try using `=` instead
+
+error: aborting due to previous error
+