Add explanatory note to async block type mismatch error

[gurry]

The async block type mismatch error might leave the user wondering as to why it occurred. The new note should give them the needed context.

Changes this diagnostic:

error[E0308]: mismatched types
 --> src/main.rs:5:23
  |
2 |     let a = async { 1 };
  |             ----------- the expected `async` block
3 |     let b = async { 2 };
  |             ----------- the found `async` block
4 |     
5 |     let bad = vec![a, b];
  |                       ^ expected `async` block, found a different `async` block
  |
  = note: expected `async` block `{async block@src/main.rs:2:13: 2:24}`
             found `async` block `{async block@src/main.rs:3:13: 3:24}`

to this:

error[E0308]: mismatched types
 --> src/main.rs:5:23
  |
2 |     let a = async { 1 };
  |             ----------- the expected `async` block
3 |     let b = async { 2 };
  |             ----------- the found `async` block
4 |     
5 |     let bad = vec![a, b];
  |                       ^ expected `async` block, found a different `async` block
  |
  = note: expected `async` block `{async block@src/main.rs:2:13: 2:24}`
             found `async` block `{async block@src/main.rs:3:13: 3:24}`
  = note: no two async blocks, even if identical, have the same type
  = help: consider pinning your async block and and casting it to a trait object

Fixes #125737

[rustbot]

r? @lcnr

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[gurry]

Do suggest any changes in the note's text that could make it clearer or shorter.

[lcnr]

thanks

@bors r+ rollup

[bors]

📌 Commit 251d2d0 has been approved by lcnr

It is now in the queue for this repository.