Suggest 1-tuple parentheses, without existing parens

Author: bobrippling

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -2046,16 +2046,16 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                         // build a tuple (issue #86100)
                         (ty::Tuple(_), _) if expected.tuple_fields().count() == 1 => {
                             if let Ok(code) = self.tcx.sess().source_map().span_to_snippet(span) {
-                                if let Some(code) =
-                                    code.strip_prefix('(').and_then(|s| s.strip_suffix(')'))
-                                {
-                                    err.span_suggestion(
-                                        span,
-                                        "use a trailing comma to create a tuple with one element",
-                                        format!("({},)", code),
-                                        Applicability::MaybeIncorrect,
-                                    );
-                                }
+                                let code_stripped = code
+                                    .strip_prefix('(')
+                                    .and_then(|s| s.strip_suffix(')'))
+                                    .unwrap_or(&code);
+                                err.span_suggestion(
+                                    span,
+                                    "use a trailing comma to create a tuple with one element",
+                                    format!("({},)", code_stripped),
+                                    Applicability::MaybeIncorrect,
+                                );
                             }
                         }
                         // If a character was expected and the found expression is a string literal

src/test/ui/consts/const-tup-index-span.stderr

@@ -6,6 +6,10 @@ LL | const TUP: (usize,) = 5usize << 64;
    |
    = note: expected tuple `(usize,)`
                found type `usize`
+help: use a trailing comma to create a tuple with one element
+   |
+LL | const TUP: (usize,) = (5usize << 64,);
+   |                       ~~~~~~~~~~~~~~~
 
 error: aborting due to previous error
 

src/test/ui/typeck/issue-84768.stderr

@@ -12,6 +12,10 @@ LL |     <F as FnOnce(&mut u8)>::call_once(f, 1)
    |
    = note: expected tuple `(&mut u8,)`
                found type `{integer}`
+help: use a trailing comma to create a tuple with one element
+   |
+LL |     <F as FnOnce(&mut u8)>::call_once(f, (1,))
+   |                                          ~~~~
 
 error: aborting due to 2 previous errors