Do not emit coerce_suggestions for expr from destructuring assignment desugaring

[chenyukang]

Fixes #109991

[rustbot]

r? @b-naber

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[compiler-errors]

I don't think this works for other kinds of destructuring patterns, like arrays:

fn main() {
    let x: i32;
    [x] = [&1];
}

Also, does this work to fix the nonsense suggestion for this example?

fn main() {
    let x: &i32;
    [x] = [1];
}

I think the problem here is that for destructuring assignments, the spans are wrong and don't really correspond correctly...

I don't know if fixing this one specific suggestion addresses the larger problem that other suggestions are still broken -- maybe we need a more thorough solution?

[compiler-errors]

Maybe the best compromise here is to detect if the expression is from a destructuring assignment desugaring and just not offer any of the suggestions in emit_coerce_suggestions or emit_type_mismatch_suggestions...

That isn't perfect, but it should also help not suggesting nonsense like:

fn main() {
    let x: String;
    (x,) = (1,);
}

error[[E0308]](https://doc.rust-lang.org/stable/error_codes/E0308.html): mismatched types
 --> src/main.rs:3:6
  |
2 |     let x: String;
  |            ------ expected due to this type
3 |     (x,) = (1,);
  |      ^- help: try using a conversion method: `.to_string()`
  |      |
  |      expected `String`, found integer

[chenyukang]

I don't think this works for other kinds of destructuring patterns, like arrays:

fn main() {
    let x: i32;
    [x] = [&1];
}

Also, does this work to fix the nonsense suggestion for this example?

fn main() {
    let x: &i32;
    [x] = [1];
}

I think the problem here is that for destructuring assignments, the spans are wrong and don't really correspond correctly...

I don't know if fixing this one specific suggestion addresses the larger problem that other suggestions are still broken -- maybe we need a more thorough solution?

Yes, we need a better general solution if we want to suggest like this:

help: consider dereferencing the borrow
   |
17 |     (a, b) = (123, *&mut 123);
   |                    +

we don't have expr for &mut 123 passed into suggest_deref_or_ref.

[compiler-errors]

I don't think it's, in general, possible to suggest what you said above. You can do destructuring assignments where the expressions do not align, like:

let x = (1, 2);
(a, b) = x;

so there's no way to in general pair a with 1 and b with 2 in the type mismatch. it's better to just suppress the suggestions altogether.

[chenyukang]

Updated code to suppress the suggestions altogether.

[compiler-errors]

r=me after blessing tests and squashing the commits

[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[chenyukang]

@bors r=compiler-errors

[bors]

📌 Commit 33f73c2 has been approved by compiler-errors

It is now in the queue for this repository.

[bors]

⌛ Testing commit 33f73c2 with merge 3c5d71a...

[bors]

☀️ Test successful - checks-actions
Approved by: compiler-errors
Pushing 3c5d71a to master...

[rust-timer]

Finished benchmarking commit (3c5d71a): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.6%	[-0.8%, -0.5%]	6
Improvements ✅ (secondary)	-1.0%	[-1.1%, -1.0%]	4
All ❌✅ (primary)	-0.6%	[-0.8%, -0.5%]	6

Max RSS (memory usage)

This benchmark run did not return any relevant results for this metric.

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	2.7%	[1.0%, 4.5%]	11
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.7%	[1.0%, 4.5%]	11

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 662.562s -> 663.05s (0.07%)