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lcnrlcnr
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stepping into impls for norm is unproductive
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+201
-8
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6 files changed

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‎compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs

+13-8
Original file line numberDiff line numberDiff line change
@@ -286,18 +286,23 @@ where
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// fixing it may cause inference breakage or introduce ambiguity.
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GoalSource::Misc => PathKind::Unknown,
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GoalSource::NormalizeGoal(path_kind) => path_kind,
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GoalSource::ImplWhereBound => {
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GoalSource::ImplWhereBound => match self.current_goal_kind {
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// We currently only consider a cycle coinductive if it steps
291291
// into a where-clause of a coinductive trait.
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CurrentGoalKind::CoinductiveTrait => PathKind::Coinductive,
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// While normalizing via an impl does step into a where-clause of
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// an impl, accessing the associated item immediately steps out of
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// it again. This means cycles/recursive calls are not guarded
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// by impls used for normalization.
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//
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// See tests/ui/traits/next-solver/cycles/normalizes-to-is-not-productive.rs
299+
// for how this can go wrong.
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CurrentGoalKind::NormalizesTo => PathKind::Inductive,
293301
// We probably want to make all traits coinductive in the future,
294-
// so we treat cycles involving their where-clauses as ambiguous.
295-
if let CurrentGoalKind::CoinductiveTrait = self.current_goal_kind {
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PathKind::Coinductive
297-
} else {
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PathKind::Unknown
299-
}
300-
}
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// so we treat cycles involving where-clauses of not-yet coinductive
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// traits as ambiguous for now.
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CurrentGoalKind::Misc => PathKind::Unknown,
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},
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// Relating types is always unproductive. If we were to map proof trees to
302307
// corecursive functions as explained in #136824, relating types never
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// introduces a constructor which could cause the recursion to be guarded.

‎tests/ui/traits/next-solver/cycles/normalizes-to-is-not-productive-2.rs

+24
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@@ -0,0 +1,24 @@
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//@ revisions: current next
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//@ ignore-compare-mode-next-solver (explicit revisions)
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//@[next] compile-flags: -Znext-solver
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//@ check-pass
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// Regression test for trait-system-refactor-initiative#176.
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//
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// Normalizing `<Vec<T> as IntoIterator>::IntoIter` has two candidates
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// inside of the function:
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// - `impl<T> IntoIterator for Vec<T>` which trivially applies
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// - `impl<I: Iterator> IntoIterator for I`
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// - requires `Vec<T>: Iterator`
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// - where-clause requires `<Vec<T> as IntoIterator>::IntoIter eq Vec<T>`
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// - normalize `<Vec<T> as IntoIterator>::IntoIter` again, cycle
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//
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// We need to treat this cycle as an error to be able to use the actual impl.
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fn test<T>()
19+
where
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<Vec<T> as IntoIterator>::IntoIter: Iterator,
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{
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}
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fn main() {}

‎tests/ui/traits/next-solver/cycles/normalizes-to-is-not-productive.current.stderr

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@@ -0,0 +1,48 @@
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error[E0277]: the trait bound `Foo: Bound` is not satisfied
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--> $DIR/normalizes-to-is-not-productive.rs:41:1
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|
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LL | / fn generic<T>()
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LL | | where
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LL | | <Foo as Trait<T>>::Assoc: Bound,
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| |____________________________________^ the trait `Bound` is not implemented for `Foo`
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|
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= help: the trait `Bound` is implemented for `u32`
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note: required for `Foo` to implement `Trait<T>`
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--> $DIR/normalizes-to-is-not-productive.rs:24:19
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|
13+
LL | impl<T: Bound, U> Trait<U> for T {
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| ----- ^^^^^^^^ ^
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| |
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| unsatisfied trait bound introduced here
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error[E0277]: the trait bound `Foo: Bound` is not satisfied
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--> $DIR/normalizes-to-is-not-productive.rs:41:4
20+
|
21+
LL | fn generic<T>()
22+
| ^^^^^^^ the trait `Bound` is not implemented for `Foo`
23+
|
24+
= help: the trait `Bound` is implemented for `u32`
25+
note: required for `Foo` to implement `Trait<T>`
26+
--> $DIR/normalizes-to-is-not-productive.rs:24:19
27+
|
28+
LL | impl<T: Bound, U> Trait<U> for T {
29+
| ----- ^^^^^^^^ ^
30+
| |
31+
| unsatisfied trait bound introduced here
32+
33+
error[E0277]: the trait bound `Foo: Bound` is not satisfied
34+
--> $DIR/normalizes-to-is-not-productive.rs:48:19
35+
|
36+
LL | impls_bound::<Foo>();
37+
| ^^^ the trait `Bound` is not implemented for `Foo`
38+
|
39+
= help: the trait `Bound` is implemented for `u32`
40+
note: required by a bound in `impls_bound`
41+
--> $DIR/normalizes-to-is-not-productive.rs:28:19
42+
|
43+
LL | fn impls_bound<T: Bound>() {
44+
| ^^^^^ required by this bound in `impls_bound`
45+
46+
error: aborting due to 3 previous errors
47+
48+
For more information about this error, try `rustc --explain E0277`.

‎tests/ui/traits/next-solver/cycles/normalizes-to-is-not-productive.next.stderr

+31
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@@ -0,0 +1,31 @@
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error[E0277]: the trait bound `Foo: Bound` is not satisfied
2+
--> $DIR/normalizes-to-is-not-productive.rs:43:31
3+
|
4+
LL | <Foo as Trait<T>>::Assoc: Bound,
5+
| ^^^^^ the trait `Bound` is not implemented for `Foo`
6+
|
7+
= help: the trait `Bound` is implemented for `u32`
8+
note: required for `Foo` to implement `Trait<T>`
9+
--> $DIR/normalizes-to-is-not-productive.rs:24:19
10+
|
11+
LL | impl<T: Bound, U> Trait<U> for T {
12+
| ----- ^^^^^^^^ ^
13+
| |
14+
| unsatisfied trait bound introduced here
15+
16+
error[E0277]: the trait bound `Foo: Bound` is not satisfied
17+
--> $DIR/normalizes-to-is-not-productive.rs:48:19
18+
|
19+
LL | impls_bound::<Foo>();
20+
| ^^^ the trait `Bound` is not implemented for `Foo`
21+
|
22+
= help: the trait `Bound` is implemented for `u32`
23+
note: required by a bound in `impls_bound`
24+
--> $DIR/normalizes-to-is-not-productive.rs:28:19
25+
|
26+
LL | fn impls_bound<T: Bound>() {
27+
| ^^^^^ required by this bound in `impls_bound`
28+
29+
error: aborting due to 2 previous errors
30+
31+
For more information about this error, try `rustc --explain E0277`.

‎tests/ui/traits/next-solver/cycles/normalizes-to-is-not-productive.rs

+54
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@@ -0,0 +1,54 @@
1+
//@ ignore-compare-mode-next-solver (explicit)
2+
//@ compile-flags: -Znext-solver
3+
4+
// Make sure that stepping into impl where-clauses of `NormalizesTo`
5+
// goals is unproductive. This must not compile, see the inline
6+
// comments.
7+
8+
trait Bound {
9+
fn method();
10+
}
11+
impl Bound for u32 {
12+
fn method() {}
13+
}
14+
trait Trait<T> {
15+
type Assoc: Bound;
16+
}
17+
18+
struct Foo;
19+
20+
impl Trait<u32> for Foo {
21+
type Assoc = u32;
22+
}
23+
impl<T: Bound, U> Trait<U> for T {
24+
type Assoc = T;
25+
}
26+
27+
fn impls_bound<T: Bound>() {
28+
T::method();
29+
}
30+
31+
// The where-clause requires `Foo: Trait<T>` to hold to be wf.
32+
// If stepping into where-clauses during normalization is considered
33+
// to be productive, this would be the case:
34+
//
35+
// - `Foo: Trait<T>`
36+
// - via blanket impls, requires `Foo: Bound`
37+
// - via where-bound, requires `Foo eq <Foo as Trait<T>>::Assoc`
38+
// - normalize `<Foo as Trait<T>>::Assoc`
39+
// - via blanket impl, requires where-clause `Foo: Bound` -> cycle
40+
fn generic<T>()
41+
where
42+
<Foo as Trait<T>>::Assoc: Bound,
43+
//~^ ERROR the trait bound `Foo: Bound` is not satisfied
44+
{
45+
// Requires proving `Foo: Bound` by normalizing
46+
// `<Foo as Trait<T>>::Assoc` to `Foo`.
47+
impls_bound::<Foo>();
48+
//~^ ERROR the trait bound `Foo: Bound` is not satisfied
49+
}
50+
fn main() {
51+
// Requires proving `<Foo as Trait<u32>>::Assoc: Bound`.
52+
// This is trivially true.
53+
generic::<u32>();
54+
}

‎tests/ui/traits/next-solver/cycles/normalizes-to-is-not-productive.stderr

+31
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@@ -0,0 +1,31 @@
1+
error[E0277]: the trait bound `Foo: Bound` is not satisfied
2+
--> $DIR/normalizes-to-is-not-productive.rs:42:31
3+
|
4+
LL | <Foo as Trait<T>>::Assoc: Bound,
5+
| ^^^^^ the trait `Bound` is not implemented for `Foo`
6+
|
7+
= help: the trait `Bound` is implemented for `u32`
8+
note: required for `Foo` to implement `Trait<T>`
9+
--> $DIR/normalizes-to-is-not-productive.rs:23:19
10+
|
11+
LL | impl<T: Bound, U> Trait<U> for T {
12+
| ----- ^^^^^^^^ ^
13+
| |
14+
| unsatisfied trait bound introduced here
15+
16+
error[E0277]: the trait bound `Foo: Bound` is not satisfied
17+
--> $DIR/normalizes-to-is-not-productive.rs:47:19
18+
|
19+
LL | impls_bound::<Foo>();
20+
| ^^^ the trait `Bound` is not implemented for `Foo`
21+
|
22+
= help: the trait `Bound` is implemented for `u32`
23+
note: required by a bound in `impls_bound`
24+
--> $DIR/normalizes-to-is-not-productive.rs:27:19
25+
|
26+
LL | fn impls_bound<T: Bound>() {
27+
| ^^^^^ required by this bound in `impls_bound`
28+
29+
error: aborting due to 2 previous errors
30+
31+
For more information about this error, try `rustc --explain E0277`.

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