let is not allowed in struct field definitions

Co-authored-by: jyn514 <jyn514@gmail.com>
Co-authored-by: Esteban Kuber <estebank@users.noreply.github.com>

compiler/rustc_parse/src/parser/item.rs

@@ -1788,7 +1788,23 @@ fn parse_field_ident(&mut self, adt_ty: &str, lo: Span) -> PResult<'a, Ident> {
                     }
                 }
             } else {
-                self.expected_ident_found()
+                let mut err = self.expected_ident_found();
+                if let Some((ident, _)) = self.token.ident() && ident.as_str() == "let" {
+                    self.bump(); // `let`
+                    let span = self.prev_token.span.until(self.token.span);
+                    err.span_suggestion(
+                        span,
+                        "remove the let, the `let` keyword is not allowed in struct field definitions",
+                        String::new(),
+                        Applicability::MachineApplicable,
+                    );
+                    err.note("the `let` keyword is not allowed in `struct` fields");
+                    err.note("see <https://doc.rust-lang.org/book/ch05-01-defining-structs.html> for more information");
+                    err.emit();
+                    self.bump();
+                    return Ok(ident);
+                }
+                err
             };
             return Err(err);
         }

src/test/ui/parser/removed-syntax-field-let.stderr

@@ -1,10 +1,16 @@
 error: expected identifier, found keyword `let`
   --> $DIR/removed-syntax-field-let.rs:2:5
    |
-LL | struct S {
-   |        - while parsing this struct
 LL |     let foo: (),
    |     ^^^ expected identifier, found keyword
+   |
+   = note: the `let` keyword is not allowed in `struct` fields
+   = note: see <https://doc.rust-lang.org/book/ch05-01-defining-structs.html> for more information
+help: remove the let, the `let` keyword is not allowed in struct field definitions
+   |
+LL -     let foo: (),
+LL +     foo: (),
+   |
 
 error: aborting due to previous error