Fixes #64919. Suggest fix based on operator precendence.

src/librustc_typeck/check/mod.rs

@@ -4344,7 +4344,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                                 let max_len = receiver.rfind(".").unwrap();
                                 format!("{}{}", &receiver[..max_len], method_call)
                             } else {
-                                format!("{}{}", receiver, method_call)
+                                match &expr.kind {
+                                    ExprKind::Binary(_,_,_) => format!("({}){}", receiver, method_call),
+                                    ExprKind::Unary(_,_) => format!("({}){}", receiver, method_call),
+                                    _ => format!("{}{}", receiver, method_call),
+                                }
                             };
                             Some(if is_struct_pat_shorthand_field {
                                 format!("{}: {}", receiver, sugg)

src/test/ui/mismatched_types/abridged.rs

@@ -50,4 +50,13 @@ fn e() -> X<X<String, String>, String> {
     x //~ ERROR mismatched types
 }
 
+fn f() -> String {
+    1+2 //~ ERROR mismatched types
+}
+
+
+fn g() -> String {
+    -2 //~ ERROR mismatched types
+}
+
 fn main() {}

src/test/ui/mismatched_types/abridged.stderr

@@ -66,6 +66,34 @@ LL |     x
    = note: expected type `X<X<_, std::string::String>, _>`
               found type `X<X<_, {integer}>, _>`
 
-error: aborting due to 6 previous errors
+error[E0308]: mismatched types
+  --> $DIR/abridged.rs:54:5
+   |
+LL | fn f() -> String {
+   |           ------ expected `std::string::String` because of return type
+LL |     1+2
+   |     ^^^
+   |     |
+   |     expected struct `std::string::String`, found integer
+   |     help: try using a conversion method: `(1+2).to_string()`
+   |
+   = note: expected type `std::string::String`
+              found type `{integer}`
+
+error[E0308]: mismatched types
+  --> $DIR/abridged.rs:59:5
+   |
+LL | fn g() -> String {
+   |           ------ expected `std::string::String` because of return type
+LL |     -2
+   |     ^^
+   |     |
+   |     expected struct `std::string::String`, found integer
+   |     help: try using a conversion method: `(-2).to_string()`
+   |
+   = note: expected type `std::string::String`
+              found type `{integer}`
+
+error: aborting due to 8 previous errors
 
 For more information about this error, try `rustc --explain E0308`.