Suggest removing let if const let or let const is used

compiler/rustc_parse/src/parser/item.rs

@@ -1124,6 +1124,16 @@ impl<'a> Parser<'a> {
                     Applicability::MaybeIncorrect,
                 )
                 .emit();
+        } else if self.eat_keyword(kw::Let) {
+            let span = self.prev_token.span;
+            self.struct_span_err(const_span.to(span), "`const` and `let` are mutually exclusive")
+                .span_suggestion(
+                    const_span.to(span),
+                    "remove `let`",
+                    "const",
+                    Applicability::MaybeIncorrect,
+                )
+                .emit();
         }
     }
 

compiler/rustc_parse/src/parser/stmt.rs

@@ -247,6 +247,22 @@ impl<'a> Parser<'a> {
     /// Parses a local variable declaration.
     fn parse_local(&mut self, attrs: AttrVec) -> PResult<'a, P<Local>> {
         let lo = self.prev_token.span;
+
+        if self.token.is_keyword(kw::Const) && self.look_ahead(1, |t| t.is_ident()) {
+            self.struct_span_err(
+                lo.to(self.token.span),
+                "`const` and `let` are mutually exclusive",
+            )
+            .span_suggestion(
+                lo.to(self.token.span),
+                "remove `let`",
+                "const",
+                Applicability::MaybeIncorrect,
+            )
+            .emit();
+            self.bump();
+        }
+
         let (pat, colon) = self.parse_pat_before_ty(None, RecoverComma::Yes, "`let` bindings")?;
 
         let (err, ty) = if colon {

src/test/ui/parser/issue-99910-const-let-mutually-exclusive.fixed

@@ -0,0 +1,8 @@
+// run-rustfix
+
+fn main() {
+    const _FOO: i32 = 123;
+    //~^ ERROR const` and `let` are mutually exclusive
+    const _BAR: i32 = 123;
+    //~^ ERROR `const` and `let` are mutually exclusive
+}

src/test/ui/parser/issue-99910-const-let-mutually-exclusive.rs

@@ -0,0 +1,8 @@
+// run-rustfix
+
+fn main() {
+    const let _FOO: i32 = 123;
+    //~^ ERROR const` and `let` are mutually exclusive
+    let const _BAR: i32 = 123;
+    //~^ ERROR `const` and `let` are mutually exclusive
+}

src/test/ui/parser/issue-99910-const-let-mutually-exclusive.stderr

@@ -0,0 +1,14 @@
+error: `const` and `let` are mutually exclusive
+  --> $DIR/issue-99910-const-let-mutually-exclusive.rs:4:5
+   |
+LL |     const let _FOO: i32 = 123;
+   |     ^^^^^^^^^ help: remove `let`: `const`
+
+error: `const` and `let` are mutually exclusive
+  --> $DIR/issue-99910-const-let-mutually-exclusive.rs:6:5
+   |
+LL |     let const _BAR: i32 = 123;
+   |     ^^^^^^^^^ help: remove `let`: `const`
+
+error: aborting due to 2 previous errors
+