On type mismatch involving associated type, suggest constraint #71108

estebank · 2020-04-13T20:37:32Z

When an associated type is found when a specific type was expected, if
possible provide a structured suggestion constraining the associated
type in a bound.

error[E0271]: type mismatch resolving `<T as Foo>::Y == i32`
  --> $DIR/associated-types-multiple-types-one-trait.rs:13:5
   |
LL |     want_y(t);
   |     ^^^^^^ expected `i32`, found associated type
...
LL | fn want_y<T:Foo<Y=i32>>(t: &T) { }
   |                 ----- required by this bound in `want_y`
   |
   = note:         expected type `i32`
           found associated type `<T as Foo>::Y`
help: consider constraining the associated type `<T as Foo>::Y` to `i32`
   |
LL | fn have_x_want_y<T:Foo<X=u32, Y = i32>>(t: &T)
   |                             ^^^^^^^^^

error[E0308]: mismatched types
  --> $DIR/trait-with-missing-associated-type-restriction.rs:12:9
   |
LL |     qux(x.func())
   |         ^^^^^^^^ expected `usize`, found associated type
   |
   = note:         expected type `usize`
           found associated type `<impl Trait as Trait>::A`
help: consider constraining the associated type `<impl Trait as Trait>::A` to `usize`
   |
LL | fn foo(x: impl Trait<A = usize>) {
   |                     ^^^^^^^^^^

Fix #71035. Related to #70908.

rust-highfive · 2020-04-13T20:37:36Z

r? @davidtwco

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

estebank · 2020-04-13T20:38:26Z

src/librustc_middle/ty/error.rs

+                for param in hir_generics.params {
+                    if self.hir().opt_local_def_id(param.hir_id).map(|id| id.to_def_id())
+                        == Some(def_id)


I would love it if we had a better way of doing this, but I guess this works 🤷.

if it occurs more often, we can introduce a helper for it.

Maybe LocalDefId just needs to implement PartialEq<DefId> and vice versa?

src/test/ui/suggestions/trait-with-missing-associated-type-restriction-fixable.stderr

estebank · 2020-04-13T20:41:20Z

src/test/ui/specialization/specialization-default-types.stderr

@@ -21,7 +21,7 @@ LL |     Example::generate(t)
   |
   = note:       expected struct `std::boxed::Box<T>`
           found associated type `<T as Example>::Output`
-   = note: consider constraining the associated type `<T as Example>::Output` to `std::boxed::Box<T>`
+   = help: consider constraining the associated type `<T as Example>::Output` to `std::boxed::Box<T>`


We should eventually handle the case of #![feature(specialization)], but I won't on this PR.

src/librustc_middle/ty/error.rs

src/test/ui/associated-types/issue-26681.stderr

estebank · 2020-04-15T06:12:47Z

src/test/ui/suggestions/trait-with-missing-associated-type-restriction.stderr

+error[E0308]: mismatched types
+  --> $DIR/trait-with-missing-associated-type-restriction.rs:25:12
+   |
+LL |     x.funk(3);
+   |            ^ expected associated type, found integer
+   |
+   = note: expected associated type `<T as Trait<i32>>::A`
+                         found type `{integer}`
+help: a method is available that returns `<T as Trait<i32>>::A`
+  --> $DIR/trait-with-missing-associated-type-restriction.rs:8:5
+   |
+LL |     fn func(&self) -> Self::A;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ consider calling `Trait::func`
+help: consider constraining the associated type `<T as Trait<i32>>::A` to `{integer}`
+   |
+LL | fn bar2<T: Trait<i32, A = {integer}>>(x: T) {
+   |                     ^^^^^^^^^^^^^^^


For this suggestion to use i32 instead of {integer} we need to run resolve_vars_if_possible, but that's not available in rustc_middle :-|

estebank · 2020-04-19T00:28:57Z

r? @nikomatsakis

Dylan-DPC-zz · 2020-04-28T11:36:52Z

r? @oli-obk

When an associated type is found when a specific type was expected, if possible provide a structured suggestion constraining the associated type in a bound. ``` error[E0271]: type mismatch resolving `<T as Foo>::Y == i32` --> $DIR/associated-types-multiple-types-one-trait.rs:13:5 | LL | want_y(t); | ^^^^^^ expected `i32`, found associated type ... LL | fn want_y<T:Foo<Y=i32>>(t: &T) { } | ----- required by this bound in `want_y` | = note: expected type `i32` found associated type `<T as Foo>::Y` help: consider constraining the associated type `<T as Foo>::Y` to `i32` | LL | fn have_x_want_y<T:Foo<X=u32, Y = i32>>(t: &T) | ^^^^^^^^^ ``` ``` error[E0308]: mismatched types --> $DIR/trait-with-missing-associated-type-restriction.rs:12:9 | LL | qux(x.func()) | ^^^^^^^^ expected `usize`, found associated type | = note: expected type `usize` found associated type `<impl Trait as Trait>::A` help: consider constraining the associated type `<impl Trait as Trait>::A` to `usize` | LL | fn foo(x: impl Trait<A = usize>) { | ^^^^^^^^^^ ```

Associated types with a default type in a trait can't be relied upon to remain of that default type when in use, so literals of that type can't be used in the trait's items. Point at the associated type and state that information. Reduce verbosity for associated consts of the wrong type.

Fix rust-lang#71035.

src/librustc_middle/ty/error.rs

oli-obk · 2020-05-03T10:16:23Z

src/test/ui/issues/issue-69306.stderr

@@ -8,8 +8,6 @@ LL |     const C: S0<u8> = Self(0);
   |
   = note: expected type parameter `T`
                        found type `{integer}`
-   = help: type parameters must be constrained to match other types


maybe we should keep a help around that informs the user that a type parameter can never be equal to a concrete type, even if the caller would always choose that specific type?

I would prefer for us to detect these kind of cases proactively and give a more targeted error. In the case of

struct S0<T>(T); impl<T> S0<T> { const C: S0<u8> = Self(0); fn foo() { Self(0); } }

we should come up with a more extensive explanation of what Self resolves to, something closer to

error[E0308]: mismatched types --> $DIR/issue-69306.rs:5:28 | LL | impl<T> S0<T> { | - this type parameter LL | const C: S0<u8> = Self(0); | ---- ^ expected type parameter `T`, found integer | | | expected `T` because of this | = note: expected type parameter `T` found type `{integer}` note: `Self` resolves to `S0<T>`, which doesn't match `S0<{integer}>` becaue it is more general | LL | const C: S0<u8> = Self(0); | ^^^^ error[E0308]: mismatched types --> $DIR/issue-69306.rs:5:28 | LL | impl<T> S0<T> { | - this type parameter LL | const C: S0<u8> = Self(0); | ------ ^^^^^^^ expected `u8`, found type parameter `T` | | | expected `u8` because of this | = note: expected type parameter `T` found type `{integer}` note: `Self` resolves to `S0<T>`, which doesn't match `S0<u8>` becaue it is more general | LL | const C: S0<u8> = Self(0); | ^^^^ error[E0308]: mismatched types --> $DIR/issue-69306.rs:10:14 | LL | impl<T> S0<T> { | - this type parameter ... LL | Self(0); | ^ expected type parameter `T`, found integer | = note: expected type parameter `T` found type `{integer}`

oli-obk · 2020-05-04T06:15:03Z

@bors r+

bors · 2020-05-04T06:15:04Z

📌 Commit b368229 has been approved by oli-obk

bors · 2020-05-04T06:33:01Z

⌛ Testing commit b368229 with merge d6823ba...

bors · 2020-05-04T09:39:45Z

☀️ Test successful - checks-azure
Approved by: oli-obk
Pushing d6823ba to master...

rust-highfive assigned davidtwco Apr 13, 2020

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estebank commented Apr 13, 2020

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estebank mentioned this pull request Apr 14, 2020

Improve compiler error on associated types #71035

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estebank force-pushed the suggest-proj-type-mismatch-constraint branch from 48ee6bd to 2e46d52 Compare May 3, 2020 01:17

estebank added 8 commits May 2, 2020 18:23

When a projection is expected, suggest constraining or calling method

31b3566

review comment: use or patterns

ee96b8b

fix rebase

6648a08

Suggest constraint on impl Trait in return type

b0085c8

Fix rust-lang#71035.

Add docs

9bb5b54

trailing newlines

74b7ed7

estebank force-pushed the suggest-proj-type-mismatch-constraint branch from 2e46d52 to 74b7ed7 Compare May 3, 2020 01:24

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