Provide suggestions for type parameters missing bounds for associated types #70908

estebank · 2020-04-07T23:13:06Z

When implementing the binary operator traits it is easy to forget to restrict the Output associated type. rustc now accounts for different cases to lead users in the right direction to add the necessary restrictions. The structured suggestions in the following output are new:

error: equality constraints are not yet supported in `where` clauses
  --> $DIR/missing-bounds.rs:37:33
   |
LL | impl<B: Add> Add for E<B> where <B as Add>::Output = B {
   |                                 ^^^^^^^^^^^^^^^^^^^^^^ not supported
   |
   = note: see issue #20041 <https://github.com/rust-lang/rust/issues/20041> for more information
help: if `Output` is an associated type you're trying to set, use the associated type binding syntax
   |
LL | impl<B: Add> Add for E<B> where B: Add<Output = B> {
   |                                 ^^^^^^^^^^^^^^^^^

error[E0308]: mismatched types
  --> $DIR/missing-bounds.rs:11:11
   |
7  | impl<B> Add for A<B> where B: Add {
   |      - this type parameter
...
11 |         A(self.0 + rhs.0)
   |           ^^^^^^^^^^^^^^ expected type parameter `B`, found associated type
   |
   = note: expected type parameter `B`
             found associated type `<B as std::ops::Add>::Output`
help: consider further restricting this bound
   |
7  | impl<B> Add for A<B> where B: Add + std::ops::Add<Output = B> {
   |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0369]: cannot add `B` to `B`
  --> $DIR/missing-bounds.rs:31:21
   |
31 |         Self(self.0 + rhs.0)
   |              ------ ^ ----- B
   |              |
   |              B
   |
help: consider restricting type parameter `B`
   |
27 | impl<B: std::ops::Add<Output = B>> Add for D<B> {
   |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^

That output is given for the following cases:

struct A<B>(B);
impl<B> Add for A<B> where B: Add {
    type Output = Self;

    fn add(self, rhs: Self) -> Self {
        A(self.0 + rhs.0) //~ ERROR mismatched types
    }
}

struct D<B>(B);
impl<B> Add for D<B> {
    type Output = Self;

    fn add(self, rhs: Self) -> Self {
        Self(self.0 + rhs.0) //~ ERROR cannot add `B` to `B`
    }
}

struct E<B>(B);
impl<B: Add> Add for E<B> where <B as Add>::Output = B {
    type Output = Self;

    fn add(self, rhs: Self) -> Self {
        Self(self.0 + rhs.0)
    }
}

rust-highfive · 2020-04-07T23:13:09Z

r? @matthewjasper

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

src/librustc_hir/hir.rs

estebank · 2020-04-08T18:26:15Z

src/test/ui/generic-associated-types/construct_with_other_type.stderr

 LL | impl<T> Baz for T where T: Foo {
-   |         ^^^ expected type parameter `T`, found associated type
+   |      -  ^^^ expected type parameter `T`, found associated type
+   |      |
+   |      this type parameter
   |
   = note: expected associated type `<T as Foo>::Bar<'_, 'static>`
              found associated type `<<T as Baz>::Quux<'_> as Foo>::Bar<'_, 'static>`
-   = note: you might be missing a type parameter or trait bound
+help: consider further restricting this bound
+   |
+LL | impl<T> Baz for T where T: Foo + Baz<Quux = T> {
+   |                                ^^^^^^^^^^^^^^^


This suggestion will cause another error:

error[E0275]: overflow evaluating the requirement `<T as Baz>::Quux` --> src/test/ui/generic-associated-types/construct_with_other_type.rs:19:9 | 19 | impl<T> Baz for T where T: Foo + Baz<Quux = T> { | ^^^ | = note: required because of the requirements on the impl of `Baz` for `T` = note: required because of the requirements on the impl of `Baz` for `T`

I feel this is "fine" since the output was already confusing and the code itself is a bit contrived and requires #![feature(generic_associated_types)] to be enabled to even be triggered.

estebank · 2020-04-08T18:33:49Z

For tracking purposes, the original impetus to make this change came from seeing this thread https://users.rust-lang.org/t/you-might-be-missing-a-type-parameter-or-trait-bound/38333.

This PR should help

newcomers that want to apply a binary operation to two type arguments
people trying to preempt the missing bound error without knowing the whole trait definition
people who know that they want to specify the associated type but are unsure about the syntax and follow the code that unmet predicate errors show

These three cases are handled by different code (in different commits), but all should be handled in concert to catch people at any point of their "incorrect mental model" trip. The last one is particularly insidious because the associated type binding equality constraint syntax can be extrapolated from the rest of the syntax and displayed errors and is being worked on, but is not yet available. Pointing people in the right direction instead of just saying "sorry, computer says no" is important to the learnability of the language.

nikomatsakis · 2020-04-15T21:48:44Z

r? @nikomatsakis

nikomatsakis

Didn't get all the way through review this yet, just the first commit. Left some comments.

src/librustc_typeck/check/op.rs

src/librustc_middle/ty/diagnostics.rs

nikomatsakis

I left a few nits but they don't matter really

src/librustc_ast_passes/ast_validation.rs

nikomatsakis · 2020-05-02T10:32:52Z

src/librustc_middle/ty/diagnostics.rs

@@ -67,3 +72,180 @@ impl<'tcx> TyS<'tcx> {
        }
    }
 }
+
+/// Suggest restricting a type param with a new bound.


It'd be nice to add a brief example.

nikomatsakis · 2020-05-02T10:33:43Z

src/librustc_middle/ty/diagnostics.rs

+    if generics.where_clause.predicates.is_empty()
+        // Given `trait Base<T = String>: Super<T>` where `T: Copy`, suggest restricting in the
+        // `where` clause instead of `trait Base<T: Copy = String>: Super<T>`.
+        && !matches!(param.kind, hir::GenericParamKind::Type { default: Some(_), .. })


!matches!

what have we wrought

Clearly this should be ¡Mátches!.

nikomatsakis · 2020-05-02T10:34:37Z

src/librustc_middle/ty/diagnostics.rs

+
+    if generics.where_clause.predicates.is_empty()
+        // Given `trait Base<T = String>: Super<T>` where `T: Copy`, suggest restricting in the
+        // `where` clause instead of `trait Base<T: Copy = String>: Super<T>`.


Is this comment accurate? It seems like this covers the case where we don't suggest a where clause, at least some of the time

nikomatsakis · 2020-05-02T10:36:26Z

@bors r+

bors · 2020-05-02T10:36:28Z

📌 Commit c0a9d52 has been approved by nikomatsakis

Provide suggestions for type parameters missing bounds for associated types When implementing the binary operator traits it is easy to forget to restrict the `Output` associated type. `rustc` now accounts for different cases to lead users in the right direction to add the necessary restrictions. The structured suggestions in the following output are new: ``` error: equality constraints are not yet supported in `where` clauses --> $DIR/missing-bounds.rs:37:33 | LL | impl<B: Add> Add for E where ::Output = B { | ^^^^^^^^^^^^^^^^^^^^^^ not supported | = note: see issue rust-lang#20041 <rust-lang#20041> for more information help: if `Output` is an associated type you're trying to set, use the associated type binding syntax | LL | impl<B: Add> Add for E where B: Add<Output = B> { | ^^^^^^^^^^^^^^^^^ error[E0308]: mismatched types --> $DIR/missing-bounds.rs:11:11 | 7 | impl Add for A where B: Add { | - this type parameter ... 11 | A(self.0 + rhs.0) | ^^^^^^^^^^^^^^ expected type parameter `B`, found associated type | = note: expected type parameter `B` found associated type `::Output` help: consider further restricting this bound | 7 | impl Add for A where B: Add + std::ops::Add<Output = B> { | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ error[E0369]: cannot add `B` to `B` --> $DIR/missing-bounds.rs:31:21 | 31 | Self(self.0 + rhs.0) | ------ ^ ----- B | | | B | help: consider restricting type parameter `B` | 27 | impl<B: std::ops::Add<Output = B>> Add for D { | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ ``` That output is given for the following cases: ```rust struct A(B); impl Add for A where B: Add { type Output = Self; fn add(self, rhs: Self) -> Self { A(self.0 + rhs.0) //~ ERROR mismatched types } } struct D(B); impl Add for D { type Output = Self; fn add(self, rhs: Self) -> Self { Self(self.0 + rhs.0) //~ ERROR cannot add `B` to `B` } } struct E(B); impl<B: Add> Add for E where ::Output = B { type Output = Self; fn add(self, rhs: Self) -> Self { Self(self.0 + rhs.0) } } ```

Dylan-DPC-zz · 2020-05-03T08:42:42Z

failed in rollup

@bors r-

…onstraint, r=oli-obk On type mismatch involving associated type, suggest constraint 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 rust-lang#71035. Related to rust-lang#70908.

When encountering a binary operation involving a type parameter that has no bindings, suggest adding the appropriate bound.

When encountering a projection that isn't satisfied by a type parameter, suggest constraining the type parameter.

…ropriate syntax When encountering `where <A as Foo>::Bar = B`, it is possible that `Bar` is an associated type. If so, suggest `where A: Foo<Bar = B>`. CC rust-lang#20041.

estebank · 2020-05-04T18:12:24Z

@bors r=nikomatsakis

bors · 2020-05-04T18:12:26Z

📌 Commit b17b20c has been approved by nikomatsakis

bors · 2020-05-05T14:22:53Z

⌛ Testing commit b17b20c with merge 7ab8af3dd0318629a0a3d835468d9e3eb121a948...

Dylan-DPC-zz · 2020-05-05T14:37:03Z

@bors retry yied

Provide suggestions for type parameters missing bounds for associated types When implementing the binary operator traits it is easy to forget to restrict the `Output` associated type. `rustc` now accounts for different cases to lead users in the right direction to add the necessary restrictions. The structured suggestions in the following output are new: ``` error: equality constraints are not yet supported in `where` clauses --> $DIR/missing-bounds.rs:37:33 | LL | impl<B: Add> Add for E where ::Output = B { | ^^^^^^^^^^^^^^^^^^^^^^ not supported | = note: see issue rust-lang#20041 <rust-lang#20041> for more information help: if `Output` is an associated type you're trying to set, use the associated type binding syntax | LL | impl<B: Add> Add for E where B: Add<Output = B> { | ^^^^^^^^^^^^^^^^^ error[E0308]: mismatched types --> $DIR/missing-bounds.rs:11:11 | 7 | impl Add for A where B: Add { | - this type parameter ... 11 | A(self.0 + rhs.0) | ^^^^^^^^^^^^^^ expected type parameter `B`, found associated type | = note: expected type parameter `B` found associated type `::Output` help: consider further restricting this bound | 7 | impl Add for A where B: Add + std::ops::Add<Output = B> { | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ error[E0369]: cannot add `B` to `B` --> $DIR/missing-bounds.rs:31:21 | 31 | Self(self.0 + rhs.0) | ------ ^ ----- B | | | B | help: consider restricting type parameter `B` | 27 | impl<B: std::ops::Add<Output = B>> Add for D { | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ ``` That output is given for the following cases: ```rust struct A(B); impl Add for A where B: Add { type Output = Self; fn add(self, rhs: Self) -> Self { A(self.0 + rhs.0) //~ ERROR mismatched types } } struct D(B); impl Add for D { type Output = Self; fn add(self, rhs: Self) -> Self { Self(self.0 + rhs.0) //~ ERROR cannot add `B` to `B` } } struct E(B); impl<B: Add> Add for E where ::Output = B { type Output = Self; fn add(self, rhs: Self) -> Self { Self(self.0 + rhs.0) } } ```

@ghost

Rollup of 5 pull requests Successful merges: - rust-lang#70908 (Provide suggestions for type parameters missing bounds for associated types) - rust-lang#71731 (Turn off rustc-dev-guide toolstate for now) - rust-lang#71888 (refactor suggest_traits_to_import) - rust-lang#71918 (Rename methods section) - rust-lang#71950 (Miri validation error handling cleanup) Failed merges: r? @ghost

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estebank mentioned this pull request Apr 8, 2020

Parse and accept type equality constraints in where clauses #20041

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estebank force-pushed the suggest-add branch 2 times, most recently from 4539157 to 654b851 Compare April 17, 2020 21:56

estebank mentioned this pull request Apr 19, 2020

On type mismatch involving associated type, suggest constraint #71108

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Rollup of 5 pull requests #71826

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estebank added 8 commits May 4, 2020 09:52

Handle binop on unbound type param

75f066d

When encountering a binary operation involving a type parameter that has no bindings, suggest adding the appropriate bound.

Suggest restricting type param when it doesn't satisfy projection

1473a66

When encountering a projection that isn't satisfied by a type parameter, suggest constraining the type parameter.

review comments: use or-pattern

3453db7

On incorrect equality constraint likely to be assoc type, suggest app…

d8d02f8

…ropriate syntax When encountering `where <A as Foo>::Bar = B`, it is possible that `Bar` is an associated type. If so, suggest `where A: Foo<Bar = B>`. CC rust-lang#20041.

review comments and rebase fix

c93c660

review comment: use body_id

5d64e91

fix rebase

b13f234

Add docstring to deny_equality_constraints

b17b20c

estebank force-pushed the suggest-add branch from c0a9d52 to b17b20c Compare May 4, 2020 18:12

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 May 4, 2020

Dylan-DPC-zz mentioned this pull request May 5, 2020

Rollup of 6 pull requests #71934

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Dylan-DPC-zz mentioned this pull request May 6, 2020

Rollup of 5 pull requests #71958

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bors merged commit ce14d6d into rust-lang:master May 7, 2020

estebank mentioned this pull request Jan 27, 2021

Suggest correct code for associated fn with type parameters #68982

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Provide suggestions for type parameters missing bounds for associated types #70908

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Provide suggestions for type parameters missing bounds for associated types #70908

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