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Allow ~const bounds on non-const functions #102273

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merged 1 commit into from
Sep 26, 2022
Merged

Allow ~const bounds on non-const functions #102273

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7 changes: 3 additions & 4 deletions compiler/rustc_ast_passes/src/ast_validation.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1415,7 +1415,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
if !self.is_tilde_const_allowed {
self.err_handler()
.struct_span_err(bound.span(), "`~const` is not allowed here")
.note("only allowed on bounds on traits' associated types and functions, const fns, const impls and its associated functions")
.note("only allowed on bounds on functions, traits' associated types and functions, const impls and its associated functions")
.emit();
}
}
Expand Down Expand Up @@ -1523,9 +1523,8 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
});
}

let tilde_const_allowed =
matches!(fk.header(), Some(FnHeader { constness: Const::Yes(_), .. }))
|| matches!(fk.ctxt(), Some(FnCtxt::Assoc(_)));
let tilde_const_allowed = matches!(fk.header(), Some(FnHeader { .. }))
|| matches!(fk.ctxt(), Some(FnCtxt::Assoc(_)));

self.with_tilde_const(tilde_const_allowed, |this| visit::walk_fn(this, fk));
}
Expand Down
34 changes: 34 additions & 0 deletions src/test/ui/rfc-2632-const-trait-impl/tilde-const-and-const-params.rs
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@@ -0,0 +1,34 @@
// check-pass
#![feature(const_trait_impl)]
#![feature(generic_arg_infer)]
#![feature(generic_const_exprs)]
#![allow(incomplete_features)]

struct Foo<const N: usize>;

impl<const N: usize> Foo<N> {
fn add<A: ~const Add42>(self) -> Foo<{ A::add(N) }> {
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This can't be sound. ~const explicitly mean const but ehh. So it could be not const. We need something like A: const Trait which is "always const" to make this work. I will open a revert PR shortly.

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@lilasta lilasta Sep 28, 2022 •
edited
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As far as I recall, the following code was accepted before this PR (I changed only 'non-associated' non-const functions).

impl Type {
    fn function<T: ~const Trait>() {}
}

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Oh, so all cases like the following are currently unsound...?

somefunc<T: ~const Trait>() -> Foo<{ T::bar(value) }>

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I just came here because I broke this test in #101900 😆

so yea, this is unsound or depending on the impl, very confusing, as making a function const fn will make it stop compiling

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funky broken errors, but they point at the problem:

error[E0277]: the trait bound `A: Add42<_>` is not satisfied
  --> /home/ubuntu/rust2/src/test/ui/rfc-2632-const-trait-impl/tilde-const-and-const-params.rs:27:61
   |
LL | fn bar<A: ~const Add42, const N: usize>(_: Foo<N>) -> Foo<{ A::add(N) }> {
   |                                                             ^^^^^^ the trait `Add42<_>` is not implemented for `A`
   |
help: consider further restricting this bound
   |
LL | fn bar<A: ~const Add42 + Add42<_>, const N: usize>(_: Foo<N>) -> Foo<{ A::add(N) }> {
   |                        ++++++++++

error[E0277]: the trait bound `A: Add42<_>` is not satisfied
  --> /home/ubuntu/rust2/src/test/ui/rfc-2632-const-trait-impl/tilde-const-and-const-params.rs:11:43
   |
LL |    fn add<A: ~const Add42>(self) -> Foo<{ A::add(N) }> {
   |                                           ^^^^^^ the trait `Add42<_>` is not implemented for `A`
   |
help: consider further restricting this bound
   |
LL |    fn add<A: ~const Add42 + Add42<_>>(self) -> Foo<{ A::add(N) }> {
   |                           ++++++++++

error: aborting due to 2 previous errors

Foo
}
}

#[const_trait]
trait Add42 {
fn add(a: usize) -> usize;
}

impl const Add42 for () {
fn add(a: usize) -> usize {
a + 42
}
}

fn bar<A: ~const Add42, const N: usize>(_: Foo<N>) -> Foo<{ A::add(N) }> {
Foo
}

fn main() {
let foo = Foo::<0>;
let foo = bar::<(), _>(foo);
let _foo = bar::<(), _>(foo);
}
6 changes: 0 additions & 6 deletions src/test/ui/rfc-2632-const-trait-impl/tilde-const-invalid-places.rs
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Expand Up @@ -17,12 +17,6 @@ fn rpit_assoc_bound() -> impl IntoIterator<Item: ~const T> { Some(S) }
fn apit_assoc_bound(_: impl IntoIterator<Item: ~const T>) {}
//~^ ERROR `~const` is not allowed

fn generic<P: ~const T>() {}
//~^ ERROR `~const` is not allowed

fn where_clause<P>() where P: ~const T {}
//~^ ERROR `~const` is not allowed

struct TildeQuestion<T: ~const ?Sized>(std::marker::PhantomData<T>);
//~^ ERROR `~const` and `?` are mutually exclusive

Expand Down
28 changes: 6 additions & 22 deletions src/test/ui/rfc-2632-const-trait-impl/tilde-const-invalid-places.stderr
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Expand Up @@ -4,53 +4,37 @@ error: `~const` is not allowed here
LL | fn rpit() -> impl ~const T { S }
| ^^^^^^^^
|
= note: only allowed on bounds on traits' associated types and functions, const fns, const impls and its associated functions
= note: only allowed on bounds on functions, traits' associated types and functions, const impls and its associated functions

error: `~const` is not allowed here
--> $DIR/tilde-const-invalid-places.rs:11:17
|
LL | fn apit(_: impl ~const T) {}
| ^^^^^^^^
|
= note: only allowed on bounds on traits' associated types and functions, const fns, const impls and its associated functions
= note: only allowed on bounds on functions, traits' associated types and functions, const impls and its associated functions

error: `~const` is not allowed here
--> $DIR/tilde-const-invalid-places.rs:14:50
|
LL | fn rpit_assoc_bound() -> impl IntoIterator<Item: ~const T> { Some(S) }
| ^^^^^^^^
|
= note: only allowed on bounds on traits' associated types and functions, const fns, const impls and its associated functions
= note: only allowed on bounds on functions, traits' associated types and functions, const impls and its associated functions

error: `~const` is not allowed here
--> $DIR/tilde-const-invalid-places.rs:17:48
|
LL | fn apit_assoc_bound(_: impl IntoIterator<Item: ~const T>) {}
| ^^^^^^^^
|
= note: only allowed on bounds on traits' associated types and functions, const fns, const impls and its associated functions

error: `~const` is not allowed here
--> $DIR/tilde-const-invalid-places.rs:20:15
|
LL | fn generic<P: ~const T>() {}
| ^^^^^^^^
|
= note: only allowed on bounds on traits' associated types and functions, const fns, const impls and its associated functions

error: `~const` is not allowed here
--> $DIR/tilde-const-invalid-places.rs:23:31
|
LL | fn where_clause<P>() where P: ~const T {}
| ^^^^^^^^
|
= note: only allowed on bounds on traits' associated types and functions, const fns, const impls and its associated functions
= note: only allowed on bounds on functions, traits' associated types and functions, const impls and its associated functions

error: `~const` and `?` are mutually exclusive
--> $DIR/tilde-const-invalid-places.rs:26:25
--> $DIR/tilde-const-invalid-places.rs:20:25
|
LL | struct TildeQuestion<T: ~const ?Sized>(std::marker::PhantomData<T>);
| ^^^^^^^^^^^^^

error: aborting due to 7 previous errors
error: aborting due to 5 previous errors