Fixes #64919. Suggest fix based on operator precendence.

src/librustc_typeck/check/mod.rs

@@ -128,6 +128,7 @@ use syntax::attr;
 use syntax::feature_gate::{GateIssue, emit_feature_err};
 use syntax::source_map::{DUMMY_SP, original_sp};
 use syntax::symbol::{kw, sym};
+use syntax::util::parser::ExprPrecedence;
 
 use std::cell::{Cell, RefCell, Ref, RefMut};
 use std::collections::hash_map::Entry;
@@ -4344,7 +4345,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                                 let max_len = receiver.rfind(".").unwrap();
                                 format!("{}{}", &receiver[..max_len], method_call)
                             } else {
-                                format!("{}{}", receiver, method_call)
+                                if expr.precedence().order() < ExprPrecedence::MethodCall.order() {
+                                    format!("({}){}", receiver, method_call)
+                                } else {
+                                    format!("{}{}", receiver, method_call)
+                                }
                             };
                             Some(if is_struct_pat_shorthand_field {
                                 format!("{}: {}", receiver, sugg)

src/test/ui/conversion-methods.stderr

@@ -41,7 +41,7 @@ LL |     let _prove_piercing_earnest: Vec<usize> = &[1, 2, 3];
    |                                               ^^^^^^^^^^
    |                                               |
    |                                               expected struct `std::vec::Vec`, found reference
-   |                                               help: try using a conversion method: `&[1, 2, 3].to_vec()`
+   |                                               help: try using a conversion method: `(&[1, 2, 3]).to_vec()`
    |
    = note: expected type `std::vec::Vec<usize>`
               found type `&[{integer}; 3]`

src/test/ui/infinite/infinite-autoderef.stderr

@@ -5,7 +5,7 @@ LL |         x = box x;
    |             ^^^^^
    |             |
    |             cyclic type of infinite size
-   |             help: try using a conversion method: `box x.to_string()`
+   |             help: try using a conversion method: `(box x).to_string()`
 
 error[E0055]: reached the recursion limit while auto-dereferencing `Foo`
   --> $DIR/infinite-autoderef.rs:25:5

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`.

src/test/ui/occurs-check-2.stderr

@@ -5,7 +5,7 @@ LL |     f = box g;
    |         ^^^^^
    |         |
    |         cyclic type of infinite size
-   |         help: try using a conversion method: `box g.to_string()`
+   |         help: try using a conversion method: `(box g).to_string()`
 
 error: aborting due to previous error
 

src/test/ui/occurs-check.stderr

@@ -5,7 +5,7 @@ LL |     f = box f;
    |         ^^^^^
    |         |
    |         cyclic type of infinite size
-   |         help: try using a conversion method: `box f.to_string()`
+   |         help: try using a conversion method: `(box f).to_string()`
 
 error: aborting due to previous error
 

src/test/ui/span/coerce-suggestions.stderr

@@ -44,7 +44,7 @@ LL |     f = box f;
    |         ^^^^^
    |         |
    |         cyclic type of infinite size
-   |         help: try using a conversion method: `box f.to_string()`
+   |         help: try using a conversion method: `(box f).to_string()`
 
 error[E0308]: mismatched types
   --> $DIR/coerce-suggestions.rs:21:9