Add more detail to type inference error #61361

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).

src/test/ui/issues/issue-23046.stderr

src/librustc/infer/error_reporting/need_type_info.rs

src/librustc/ty/print/pretty.rs

Centril

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

src/librustc/ty/print/pretty.rs

src/test/ui/issues/issue-20261.stderr

src/test/ui/issues/issue-17551.stderr

src/test/ui/issues/issue-7813.stderr

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

src/test/ui/unboxed-closures/unboxed-closures-failed-recursive-fn-2.stderr

  --> $DIR/unboxed-closures-failed-recursive-fn-2.rs:16:32
   |
 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

src/librustc/ty/print/pretty.rs

@@ -931,6 +945,8 @@ pub struct FmtPrinterData<'a, 'gcx, 'tcx, F> {
    binder_depth: usize,

    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

src/librustc/ty/print/pretty.rs

-            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

src/librustc/ty/print/pretty.rs

@@ -1206,6 +1223,10 @@ impl<F: fmt::Write> Printer<'gcx, 'tcx> for FmtPrinter<'_, 'gcx, 'tcx, F> {
 }

 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

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estebank force-pushed the infer-type branch from 82718cf to 9764380 Compare 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 ebbe725 Compare June 1, 2019 03:48

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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 `_`

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bors mentioned this pull request Jun 3, 2019

Update clippy #61467

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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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Add more detail to type inference error #61361

Add more detail to type inference error #61361

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Add more detail to type inference error #61361

Add more detail to type inference error #61361

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