Rollup merge of #91530 - bobrippling:suggest-1-tuple-parens, r=camelid

Suggest 1-tuple parentheses on exprs without existing parens

A follow-on from #86116, split out from #90677.

This alters the suggestion to add a trailing comma to create a 1-tuple - previously we would only apply this if the relevant expression was parenthesised. We now make the suggestion regardless of parentheses, which reduces the fragility of the check (w.r.t formatting).

e.g.
```rust
let a: Option<(i32,)> = Some(3);
```

gets the below suggestion:

```rust
let a: Option<(i32,)> = Some((3,));
//                           ^ ^^
```

This change also improves the suggestion in other ways, such as by only making the suggestion if the types would match after the suggestion is applied and making the suggestion a multipart suggestion.

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

@@ -2044,19 +2044,8 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                         // If a tuple of length one was expected and the found expression has
                         // parentheses around it, perhaps the user meant to write `(expr,)` to
                         // 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,
-                                    );
-                                }
-                            }
+                        (ty::Tuple(_), _) => {
+                            self.emit_tuple_wrap_err(&mut err, span, found, expected)
                         }
                         // If a character was expected and the found expression is a string literal
                         // containing a single character, perhaps the user meant to write `'c'` to
@@ -2119,6 +2108,41 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         diag
     }
 
+    fn emit_tuple_wrap_err(
+        &self,
+        err: &mut DiagnosticBuilder<'tcx>,
+        span: Span,
+        found: Ty<'tcx>,
+        expected: Ty<'tcx>,
+    ) {
+        let [expected_tup_elem] = &expected.tuple_fields().collect::<Vec<_>>()[..]
+            else { return };
+
+        if !same_type_modulo_infer(expected_tup_elem, found) {
+            return;
+        }
+
+        let Ok(code) = self.tcx.sess().source_map().span_to_snippet(span)
+            else { return };
+
+        let msg = "use a trailing comma to create a tuple with one element";
+        if code.starts_with('(') && code.ends_with(')') {
+            let before_close = span.hi() - BytePos::from_u32(1);
+            err.span_suggestion(
+                span.with_hi(before_close).shrink_to_hi(),
+                msg,
+                ",".into(),
+                Applicability::MachineApplicable,
+            );
+        } else {
+            err.multipart_suggestion(
+                msg,
+                vec![(span.shrink_to_lo(), "(".into()), (span.shrink_to_hi(), ",)".into())],
+                Applicability::MachineApplicable,
+            );
+        }
+    }
+
     fn values_str(
         &self,
         values: ValuePairs<'tcx>,