"consider borrowing here" suggestion is missing parentheses

[jyn514]

Given the following code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=c03c22d0ff9f0d88bb0cacb6edb95501

fn foo(x: &str) -> bool {
    x.starts_with("hi".to_string() + " you")
}

The current output is:

error[[E0277]](https://doc.rust-lang.org/stable/error-index.html#E0277): expected a `FnMut<(char,)>` closure, found `String`
 --> src/lib.rs:2:19
  |
2 |     x.starts_with("hi".to_string() + " you")
  |       ----------- ^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Pattern<'_>` is not implemented for `String`
  |       |
  |       required by a bound introduced by this call
  |
  = note: the trait bound `String: Pattern<'_>` is not satisfied
  = note: required for `String` to implement `Pattern<'_>`
note: required by a bound in `core::str::<impl str>::starts_with`
help: consider borrowing here
  |
2 |     x.starts_with(&"hi".to_string() + " you")
  |                   +

Ideally the output should look like:

error[[E0277]](https://doc.rust-lang.org/stable/error-index.html#E0277): expected a `FnMut<(char,)>` closure, found `String`
 --> src/lib.rs:2:19
  |
2 |     x.starts_with("hi".to_string() + " you")
  |       ----------- ^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Pattern<'_>` is not implemented for `String`
  |       |
  |       required by a bound introduced by this call
  |
  = note: the trait bound `String: Pattern<'_>` is not satisfied
  = note: required for `String` to implement `Pattern<'_>`
note: required by a bound in `core::str::<impl str>::starts_with`
help: consider borrowing here
  |
2 |     x.starts_with(&("hi".to_string() + " you"))
  |                   ++

[chenyukang]

@rustbot claim

[chenyukang]

@compiler-errors
The code here:

rust/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

Line 1118 in 5ac7e08

span.shrink_to_lo(),

Seems we can only get more context from obligation, with only span we can not know whether the current code snippet is an ident or a more complicated expression.

The current snippet is "hi".to_string() + " you".

An completed solution should make sure we get something like expr in obligation?
A hack I'm thinking is checking whether snippet contains spaces, if so we suggest it like:

2 |     x.starts_with(&("hi".to_string() + " you"))
  |                   ++

[chenyukang]

I guess we have more similar issues for other suggestions come from obligation.

[chenyukang]

This is an similar case, but we handle it well:

fn bar(val: &str) {
    todo!()
}

fn main() {
    bar("hello".to_string() + "world");
}

Output is:

error[E0308]: mismatched types
 --> ./p/n.rs:7:9
  |
7 |     bar("hello".to_string() + "world");
  |     --- ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |     |   |
  |     |   expected `&str`, found struct `String`
  |     |   help: consider borrowing here: `&("hello".to_string() + "world")`
  |     arguments to this function are incorrect
  |
note: function defined here
 --> ./p/n.rs:2:4
  |
2 | fn bar(val: &str) {
  |    ^^^ ---------

error: aborting due to previous error

The codepath is different, we take care parenthesis here, with expr:

rust/compiler/rustc_hir_typeck/src/demand.rs

Line 830 in df04d28

let needs_parens = match expr.kind {