Add more detail to type inference error by estebank · Pull Request #61361 · rust-lang/rust

estebank · 2019-05-30T17:24:54Z

When encountering code where type inference fails, add more actionable
information:

fn main() {
let foo = Vec::new();
}

error[E0282]: type annotations needed in `std::vec::Vec<T>`
--> $DIR/vector-no-ann.rs:2:16
|
LL | let foo = Vec::new();
| --- ^^^^^^^^ cannot infer type for `T` in `std::vec::Vec<T>`
| |
| consider giving `foo` a type

Fix#25633.

rust-highfive · 2019-05-30T17:24:59Z

r? @varkor

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

estebank · 2019-05-30T17:28:16Z

cc @rust-lang/wg-diagnostics

We still need to modify type printing to optionally accept a TypeVariableTable in order to properly print std::vec::Vec<T>.

I think that's an important thing to focus on in the short/medium term, together with using the local ident for external paths (std::vec::Vec -> Vec).

Centril

I'm still not that sold on replacing _ but there seem to be bugs?

estebank · 2019-05-31T23:27:04Z

@varkor would you prefer the following output?

error[E0282]: type annotations needed for `std::vec::Vec<T>`
--> $DIR/vector-no-ann.rs:2:16
|
LL | let foo = Vec::new();
| --- ^^^^^^^^ cannot infer type for `T`
| |
| consider giving `foo` the type `std::vec::Vec<T>` with the type parameter `T` specified

We could also provide structured suggestions now that we have the type names, but I would like help to bikeshed the wording:

error[E0282]: type annotations needed for `T` in `std::vec::Vec<T>`
--> $DIR/vector-no-ann.rs:2:16
|
LL | let foo = Vec::new();
| --- ^^^^^^^^ cannot infer type
| |
| help: give `foo` a type with the type parameter `T` specified: `foo: std::vec::Vec<T>`

Also, I'm not sure we can verify with certainty that we're pointing at a let binding that doesn't have a type set, so I would prefer not to give a suggestion just yet.

For patterns I need to change the wording as it is subtly incorrect now.

When encountering code where type inference fails, add more actionable information: ``` fn main() { let foo = Vec::new(); } ``` ``` error[E0282]: type annotations needed for `std::vec::Vec<_>` --> $DIR/vector-no-ann.rs:2:16 | LL | let foo = Vec::new(); | --- ^^^^^^^^ cannot infer type for `T` | | | consider giving `foo` the type `std::vec::Vec<_>` with the type parameter `T` specified ``` We still need to modify type printing to optionally accept a `TypeVariableTable` in order to properly print `std::vec::Vec<T>`. CC rust-lang#25633.

varkor · 2019-06-01T19:55:10Z

 |
 LL | let mut closure0 = None;
- | ------------ consider giving `closure0` a type
+ | ------------ consider giving `closure0` the type `std::option::Option<T>` with the type parameter `_` specified


I think it's a problem when the type parameters don't match up. (It'd be better for both to be _, as long as it's unambiguous.)

I believe what is happening here is that _ and T are actually not unified, they are separate types as far as typeck is concerned. I'm not sure what the best way to go about it here would be :-/

Cheated a bit and changed it to be:
error[E0282]: type annotations needed for `std::option::Option<T>` --> $DIR/unboxed-closures-failed-recursive-fn-2.rs:16:32 | LL | let mut closure0 = None; | ------------ consider giving `closure0` the explicit type `std::option::Option<T>`, with the type parameters specified ... LL | return c(); | ^^^ cannot infer type | = note: type must be known at this point

Yeah, that's probably better.

varkor · 2019-06-01T20:00:37Z

I think a tweak to the message makes it slightly clearer (i.e. not to have users thinking: "but foo has a type already...").

error[E0282]: type annotations needed for `std::vec::Vec<T>`
--> $DIR/vector-no-ann.rs:2:16
|
LL | let foo = Vec::new();
| --- ^^^^^^^^ cannot infer type for `T`
| |
| consider giving `foo` the explicit type `std::vec::Vec<T>`, where the type parameter `T` is specified

Also, I'm not sure we can verify with certainty that we're pointing at a let binding that doesn't have a type set, so I would prefer not to give a suggestion just yet.

Structured suggestions would be nice, but only if we don't get it wrong, yeah.

…name is `_`

varkor · 2019-06-03T00:04:16Z

Okay, I think this is good now! Thanks for being patient with all the back and forth about wording 😄

r=me if you're happy with the wording now too.

estebank · 2019-06-03T02:11:17Z

@bors r=varkor

Thanks for being patient with all the back and forth about wording

@varkor, we have high standards 😉

I'm just happy that the ty printing refactor made implementing this not only feasible, but rather straightforward, if hacky.

bors · 2019-06-03T02:11:19Z

📌 Commit e420f44 has been approved by varkor

bors · 2019-06-03T02:45:42Z

⌛ Testing commit e420f44 with merge d59dcb2...

Add more detail to type inference error When encountering code where type inference fails, add more actionable information: ``` fn main() { let foo = Vec::new(); } ``` ``` error[E0282]: type annotations needed in `std::vec::Vec<T>` --> $DIR/vector-no-ann.rs:2:16 | LL | let foo = Vec::new(); | --- ^^^^^^^^ cannot infer type for `T` in `std::vec::Vec<T>` | | | consider giving `foo` a type ``` Fix#25633.

bors · 2019-06-03T05:40:35Z

☀️ Test successful - checks-travis, status-appveyor
Approved by: varkor
Pushing d59dcb2 to master...

eddyb · 2019-06-10T08:38:46Z


 pub region_highlight_mode: RegionHighlightMode,
+
+ pub name_resolver: Option<Box<&'a dyn Fn(ty::sty::TyVid) -> Option<String>>>,


Please cc me on ty::print changes - but also, I have a few questions, such as "why is this a Box<&dyn Trait>?" - you can just remove the Box and everything would still work.

eddyb · 2019-06-10T08:39:57Z

- ty::Infer(infer_ty) => p!(write("{}", infer_ty)),
+ ty::Infer(infer_ty) => {
+ if let ty::TyVar(ty_vid) = infer_ty {
+ if let Some(name) = self.infer_ty_name(ty_vid) {


I'd probably override this in FmtPrinter's print_ty tbh, instead of adding infer_ty_name.

eddyb · 2019-06-10T08:40:54Z


 impl<F: fmt::Write> PrettyPrinter<'gcx, 'tcx> for FmtPrinter<'_, 'gcx, 'tcx, F> {
+ fn infer_ty_name(&self, id: ty::TyVid) -> Option<String> {
+ self.0.name_resolver.as_ref().and_then(|func| func(id))


There's a Deref impl, you don't need to write self.0.

rust-highfive assigned varkor May 30, 2019

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label May 30, 2019

estebank force-pushed the infer-type branch from 95cce7d to 90b5128Compare May 30, 2019 17:26

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varkor reviewed May 30, 2019
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Centril suggested changes May 30, 2019
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Comment threadsrc/librustc/infer/error_reporting/need_type_info.rs Outdated

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estebank force-pushed the infer-type branch from 82718cf to 9764380Compare May 30, 2019 22:55

estebank added 4 commits May 31, 2019 18:33

Alternative wording for inference failure
65c2a7b

On type inference errors use the type argument name when possible
74fb3bb
``` error[E0282]: type annotations needed in `std::result::Result<i32, E>` --> file7.rs:3:13 | 3 | let b = Ok(4); | - ^^ cannot infer type for `E` in `std::result::Result<i32, E>` | | | consider giving `b` a type` ```

Tweak wording
ebbe725

estebank force-pushed the infer-type branch from 9764380 to ebbe725Compare June 1, 2019 03:48

varkor reviewed Jun 1, 2019
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estebank added 2 commits June 1, 2019 14:47

review comment: tweak wording
9616b33

Account for cases where we can find the type arg name, but the local …
e420f44
…name is `_`

varkor added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jun 3, 2019

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jun 3, 2019

bors mentioned this pull request Jun 3, 2019
Update clippy #61467
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bors added the merged-by-bors This PR was explicitly merged by bors. label Jun 3, 2019

bors merged commit e420f44 into rust-lang:masterJun 3, 2019

eddyb reviewed Jun 10, 2019
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estebank mentioned this pull request Aug 12, 2019
When needing type annotation do not suggest using impl Trait or closure #63504
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		pub region_highlight_mode: RegionHighlightMode,

		pub name_resolver: Option<Box<&'a dyn Fn(ty::sty::TyVid) -> Option<String>>>,

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estebank commented May 30, 2019•
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