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indicate origin of where type parameter for uninferred types #67277

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nikomatsakis opened this issue Dec 13, 2019 · 2 comments · Fixed by #67285
Closed

indicate origin of where type parameter for uninferred types #67277

nikomatsakis opened this issue Dec 13, 2019 · 2 comments · Fixed by #67285
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A-diagnostics Area: Messages for errors, warnings, and lints E-mentor Call for participation: This issue has a mentor. Use #t-compiler/help on Zulip for discussion. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@nikomatsakis
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nikomatsakis commented Dec 13, 2019 •
edited by rustbot
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Over in #65951, @estebank and I were discussing that we could improve the error message in cases like:

rust/src/test/ui/async-await/unresolved_type_param.rs

Lines 8 to 14 in 3964a55

async fn foo() {
bar().await;
//~^ ERROR type inside `async fn` body must be known in this context
//~| NOTE cannot infer type for `T`
//~| NOTE the type is part of the `async fn` body because of this `await`
//~| NOTE in this expansion of desugaring of `await`
}

Presently, we say:

https://github.com/rust-lang/rust/blob/master/src/test/ui/async-await/unresolved_type_param.stderr#L1-L5

but this T we reference comes from the definition of bar:

rust/src/test/ui/async-await/unresolved_type_param.rs

Line 6 in 3964a55

async fn bar<T>() -> () {}

it'd be nice if we said "cannot infer value for type parameter T declared on the fn bar".

I believe that name comes from the TypeParameterDefinition information that we track as part of a type variable's origin:

rust/src/librustc/infer/error_reporting/need_type_info.rs

Lines 159 to 161 in 3964a55

if let TypeVariableOriginKind::TypeParameterDefinition(name) = var_origin.kind {
return (name.to_string(), Some(var_origin.span));
}

So we could do this by modifying that variant to track the DefId of the type parameter and then looking at the parent to derive the context.

This issue has been assigned to @ohadravid via this comment.
@nikomatsakis nikomatsakis mentioned this issue Dec 13, 2019
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@nikomatsakis nikomatsakis added A-diagnostics Area: Messages for errors, warnings, and lints T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. E-mentor Call for participation: This issue has a mentor. Use #t-compiler/help on Zulip for discussion. labels Dec 13, 2019
@nikomatsakis
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I'm tagging this as E-mentor because it has some rough mentoring instructions. Feel free to reach out to me for help, too.

@ohadravid
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@rustbot rustbot self-assigned this Dec 13, 2019
@ohadravid ohadravid mentioned this issue Dec 13, 2019
Merged
Centril added a commit to Centril/rust that referenced this issue Dec 15, 2019
@Centril
Rollup merge of rust-lang#67285 - ohadravid:indicate-origin-of-where-…
3a94151 
…type-parameter, r=estebank

Indicate origin of where type parameter for uninferred types

Based on rust-lang#65951 (which is not merge yet), fixes rust-lang#67277.

This PR improves a little the diagnostic for code like:

```
 async fn foo() {
     bar().await;
}

 async fn bar<T>() -> () {}
```

by showing:
```
error[E0698]: type inside `async fn` body must be known in this context
 --> unresolved_type_param.rs:9:5
  |
9 |     bar().await;
  |     ^^^ cannot infer type for type parameter `T` declared on the function `bar`
  |
...
```
(The
```
declared on the function `bar`
```
part is new)

A small side note: `Vec` and `slice` seem to resist this change, because querying `item_name()` panics, and `get_opt_name()` returns `None`.

r? @estebank
Centril added a commit to Centril/rust that referenced this issue Dec 15, 2019
@Centril
Rollup merge of rust-lang#67285 - ohadravid:indicate-origin-of-where-…
6cf51ad 
…type-parameter, r=estebank

Indicate origin of where type parameter for uninferred types

Based on rust-lang#65951 (which is not merge yet), fixes rust-lang#67277.

This PR improves a little the diagnostic for code like:

```
 async fn foo() {
     bar().await;
}

 async fn bar<T>() -> () {}
```

by showing:
```
error[E0698]: type inside `async fn` body must be known in this context
 --> unresolved_type_param.rs:9:5
  |
9 |     bar().await;
  |     ^^^ cannot infer type for type parameter `T` declared on the function `bar`
  |
...
```
(The
```
declared on the function `bar`
```
part is new)

A small side note: `Vec` and `slice` seem to resist this change, because querying `item_name()` panics, and `get_opt_name()` returns `None`.

r? @estebank
Centril added a commit to Centril/rust that referenced this issue Dec 20, 2019
@Centril
Rollup merge of rust-lang#67285 - ohadravid:indicate-origin-of-where-…
03169a9 
…type-parameter, r=estebank

Indicate origin of where type parameter for uninferred types

Based on rust-lang#65951 (which is not merge yet), fixes rust-lang#67277.

This PR improves a little the diagnostic for code like:

```
 async fn foo() {
     bar().await;
}

 async fn bar<T>() -> () {}
```

by showing:
```
error[E0698]: type inside `async fn` body must be known in this context
 --> unresolved_type_param.rs:9:5
  |
9 |     bar().await;
  |     ^^^ cannot infer type for type parameter `T` declared on the function `bar`
  |
...
```
(The
```
declared on the function `bar`
```
part is new)

A small side note: `Vec` and `slice` seem to resist this change, because querying `item_name()` panics, and `get_opt_name()` returns `None`.

r? @estebank
@bors bors closed this as completed in 403bb09 Dec 20, 2019
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Indicate origin of where type parameter for uninferred types ohadravid/rust
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