When encountering unexpected closure return type, point at return type/expression #132156

estebank · 2024-10-25T21:28:22Z

error[E0271]: expected `{closure@fallback-closure-wrap.rs:18:40}` to be a closure that returns `()`, but it returns `!`
  --> $DIR/fallback-closure-wrap.rs:19:9
   |
LL |     let error = Closure::wrap(Box::new(move || {
   |                                        -------
LL |         panic!("Can't connect to server.");
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found `!`
   |
   = note: expected unit type `()`
                   found type `!`
   = note: required for the cast from `Box<{closure@$DIR/fallback-closure-wrap.rs:18:40: 18:47}>` to `Box<dyn FnMut()>`

error[E0271]: expected `{closure@dont-ice-for-type-mismatch-in-closure-in-async.rs:6:10}` to be a closure that returns `bool`, but it returns `Option<()>`
  --> $DIR/dont-ice-for-type-mismatch-in-closure-in-async.rs:6:16
   |
LL |     call(|| -> Option<()> {
   |     ---- ------^^^^^^^^^^
   |     |          |
   |     |          expected `bool`, found `Option<()>`
   |     required by a bound introduced by this call
   |
   = note: expected type `bool`
              found enum `Option<()>`
note: required by a bound in `call`
  --> $DIR/dont-ice-for-type-mismatch-in-closure-in-async.rs:3:25
   |
LL | fn call(_: impl Fn() -> bool) {}
   |                         ^^^^ required by this bound in `call`

error[E0271]: expected `{closure@f670.rs:28:13}` to be a closure that returns `Result<(), _>`, but it returns `!`
    --> f670.rs:28:20
     |
28   |     let c = |e| -> ! {
     |             -------^
     |                    |
     |                    expected `Result<(), _>`, found `!`
...
32   |     f().or_else(c);
     |         ------- required by a bound introduced by this call
-Ztrack-diagnostics: created at compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs:1433:28
     |
     = note: expected enum `Result<(), _>`
                found type `!`
note: required by a bound in `Result::<T, E>::or_else`
    --> /home/gh-estebank/rust/library/core/src/result.rs:1406:39
     |
1406 |     pub fn or_else<F, O: FnOnce(E) -> Result<T, F>>(self, op: O) -> Result<T, F> {
     |                                       ^^^^^^^^^^^^ required by this bound in `Result::<T, E>::or_else`

CC #111539.

rustbot · 2024-10-25T21:28:30Z

r? @Nadrieril

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tests/ui/closures/return-type-doesnt-match-bound.stderr

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

Nadrieril · 2024-10-26T14:41:26Z

Huge improvement, this has bitten me before :) Would prefer boring labels over no labels for the diagnostic spans.

compiler-errors · 2024-10-26T16:36:08Z

compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs

+        span: Span,
        diag: &mut Diag<'_>,
        cause: &ObligationCause<'tcx>,


Why are we passing a different span than the span passed in via the obligation cause?

This at least needs an explanation; especially when an argument seems to be redundant in 19 out of 20 calls, and only differs for one, but there's literally no documentation to explain why it's like that. I find that adding new arguments like this with basically no (apparent) rhyme or reason (even if there's a good justification that's not documented) to these very pervasive functions (like, note_type_err is the entrypoint to basically all type error reporting!) to really harm maintainability of these APIs :(

An idea is to rename this to override_span: Option<Span>, and move it to the end of the arg list (or perhaps just behind seconary_span, and add a doc comment explaining that passing None will use the obligation cause's span. Then most of the callsites will just need to pass None rather than redundantly passing both the obligation cause and the obligation cause's span as two args.

Let me know what you think.

Nadrieril · 2024-10-27T19:35:41Z

Looks good to me now, I'll let @compiler-errors decide

compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs

compiler-errors · 2024-10-27T19:45:04Z

After the nit I commented, then there should only be one Some() call, which makes it consistent with the comment, so 👍

compiler-errors · 2024-10-27T22:05:30Z

compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs

+        // `cause.span`. This is used in E0271, when a closure is passed in where the return type
+        // isn't what was expected. We want to point at the closure's return type (or expression),
+        // instead of the expression where the closure is passed as call argument.
+        let span = override_span.unwrap_or(cause.span);


Suggested change

let span = override_span.unwrap_or(cause.span);

let span = override_span.unwrap_or(cause.span());

Turns out that .span and .span() do different things! This is kinda what I meant when I said that these APIs are unmaintainable today.

This should fix the regression in spans, afaict.

#132243 removes the need for that pointless/confusing method.

…e/expression ``` error[E0271]: expected `{closure@fallback-closure-wrap.rs:18:40}` to be a closure that returns `()`, but it returns `!` --> $DIR/fallback-closure-wrap.rs:19:9 | LL | let error = Closure::wrap(Box::new(move || { | ------- LL | panic!("Can't connect to server."); | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found `!` | = note: expected unit type `()` found type `!` = note: required for the cast from `Box<{closure@$DIR/fallback-closure-wrap.rs:18:40: 18:47}>` to `Box<dyn FnMut()>` ``` ``` error[E0271]: expected `{closure@dont-ice-for-type-mismatch-in-closure-in-async.rs:6:10}` to be a closure that returns `bool`, but it returns `Option<()>` --> $DIR/dont-ice-for-type-mismatch-in-closure-in-async.rs:6:16 | LL | call(|| -> Option<()> { | ---- ------^^^^^^^^^^ | | | | | expected `bool`, found `Option<()>` | required by a bound introduced by this call | = note: expected type `bool` found enum `Option<()>` note: required by a bound in `call` --> $DIR/dont-ice-for-type-mismatch-in-closure-in-async.rs:3:25 | LL | fn call(_: impl Fn() -> bool) {} | ^^^^ required by this bound in `call` ``` ``` error[E0271]: expected `{closure@f670.rs:28:13}` to be a closure that returns `Result<(), _>`, but it returns `!` --> f670.rs:28:20 | 28 | let c = |e| -> ! { | -------^ | | | expected `Result<(), _>`, found `!` ... 32 | f().or_else(c); | ------- required by a bound introduced by this call -Ztrack-diagnostics: created at compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs:1433:28 | = note: expected enum `Result<(), _>` found type `!` note: required by a bound in `Result::<T, E>::or_else` --> /home/gh-estebank/rust/library/core/src/result.rs:1406:39 | 1406 | pub fn or_else<F, O: FnOnce(E) -> Result<T, F>>(self, op: O) -> Result<T, F> { | ^^^^^^^^^^^^ required by this bound in `Result::<T, E>::or_else` ```

``` error[E0271]: expected `{closure@return-type-doesnt-match-bound.rs:18:13}` to be a closure that returns `Result<(), _>`, but it returns `!` --> tests/ui/closures/return-type-doesnt-match-bound.rs:18:20 | 18 | let c = |e| -> ! { //~ ERROR to be a closure that returns | -------^ | | | expected `Result<(), _>`, found `!` ... 22 | f().or_else(c); | ------- - | | | required by a bound introduced by this call | = note: expected enum `Result<(), _>` found type `!` note: required by a bound in `Result::<T, E>::or_else` --> /home/gh-estebank/rust/library/core/src/result.rs:1406:39 | 1406 | pub fn or_else<F, O: FnOnce(E) -> Result<T, F>>(self, op: O) -> Result<T, F> { | ^^^^^^^^^^^^ required by this bound in `Result::<T, E>::or_else` ```

bors · 2024-12-14T07:20:50Z

☔ The latest upstream changes (presumably #134292) made this pull request unmergeable. Please resolve the merge conflicts.

rustbot assigned Nadrieril Oct 25, 2024

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Oct 25, 2024

estebank commented Oct 26, 2024

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estebank force-pushed the closure-return branch from 5d6df06 to d2e4f44 Compare October 27, 2024 21:08

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estebank added 5 commits October 29, 2024 16:00

Add closure labels

5198b68

Remove unwrap()s

000d338

review comment: change span argument

63ae24a

estebank force-pushed the closure-return branch from d2e4f44 to 63ae24a Compare October 29, 2024 16:08

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When encountering unexpected closure return type, point at return type/expression #132156

When encountering unexpected closure return type, point at return type/expression #132156

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bors commented Dec 14, 2024

	let span = override_span.unwrap_or(cause.span);
	let span = override_span.unwrap_or(cause.span());

When encountering unexpected closure return type, point at return type/expression #132156

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When encountering unexpected closure return type, point at return type/expression #132156

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