More structured suggestions for boxed trait objects instead of impl Trait on non-coerceable tail expressions

[estebank]

When encountering a match or if as a tail expression where the
different arms do not have the same type and the return type of that
fn is an impl Trait, check whether those arms can implement Trait
and if so, suggest using boxed trait objects.

Use structured suggestion for impl T to Box<dyn T>.

Fix #69107

[rust-highfive]

r? @lcnr

(rust_highfive has picked a reviewer for you, use r? to override)

[estebank]

🤯 I am amazed at how little this affected perf. Expect a further iteration cleaning this up.

[estebank]

[rust-timer]

Finished benchmarking try commit (de4f59cb4ed414ffb33a797ae2c0832cb68647a9): comparison url.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. Please note that if the perf results are neutral, you should likely undo the rollup=never given below by specifying rollup- to bors.

Importantly, though, if the results of this run are non-neutral do not roll this PR up -- it will mask other regressions or improvements in the roll up.

@bors rollup=never

[lcnr]

can you move that note below the changed help message?

[estebank]

Suggestions always come last. It's been in my backlog for a while to address that.

[estebank]

We still provide two errors for the case where a bare dyn Trait is the return type, but that "should be enough", at least for now:

fn qux() -> dyn std::fmt::Display {
    if false {
        0i32
    } else {
        1u32 //~ ERROR `if` and `else` have incompatible types
    }
}

error[E0308]: `if` and `else` have incompatible types
 --> fil3.rs:5:9
  |
2 | /     if false {
3 | |         0i32
  | |         ---- expected because of this
4 | |     } else {
5 | |         1u32 //~ ERROR `if` and `else` have incompatible types
  | |         ^^^^ expected `i32`, found `u32`
6 | |     }
  | |_____- `if` and `else` have incompatible types

error[E0746]: return type cannot have an unboxed trait object
 --> fil3.rs:1:13
  |
1 | fn qux() -> dyn std::fmt::Display {
  |             ^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
  |
  = note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
  = note: if all the returned values were of the same type you could use `impl std::fmt::Display` as the return type
  = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
  = note: you can create a new `enum` with a variant for each returned type
help: return a boxed trait object instead
  |
1 | fn qux() -> Box<dyn std::fmt::Display> {
2 |     if false {
3 |         Box::new(0i32)
4 |     } else {
5 |         Box::new(1u32) //~ ERROR `if` and `else` have incompatible types
  |

error: aborting due to 2 previous errors

[varkor]

@bors r=lcnr,varkor

[bors]

📌 Commit 701bb7651762b229bf9d1c91549168dec037d667 has been approved by lcnr,varkor

[estebank]

@bors r=lcnr,varkor

[bors]

📌 Commit c6f2ddf has been approved by lcnr,varkor

[bors]

⌛ Testing commit c6f2ddf with merge 41dc394...

[bors]

☀️ Test successful - checks-actions, checks-azure
Approved by: lcnr,varkor
Pushing 41dc394 to master...