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Auto merge of #112842 - lcnr:non-defining-use, r=oli-obk
check for non-defining uses of RPIT This PR requires defining uses of RPIT and the async functions return type to use unique generic parameters as type and const arguments, (mostly) fixing #111935. This changes the following snippet to an error (it compiled since 1.62): ```rust fn foo<T>() -> impl Sized { let _: () = foo::<u8>(); //~ ERROR non-defining use of `impl Sized` } ``` Since 1.62 we only checked that the generic arguments of opaque types are unique parameters for TAIT and ignored RPITs, so this PR changes the behavior here to be consistent. For defining uses which do not have unique params as arguments it is unclear how the hidden type should map to the generic params of the opaque. In the following snippet, should the hidden type of `foo<T>::opaque` be `T` or `u32`. ```rust fn foo<T>() -> impl Sized { let _: u32 = foo::<u32>(); foo::<T>() } ``` There are no crater regressions caused by this change. --- The same issue exists for lifetime arguments which is not fixed by this PR, currently resulting in an ICE in mir borrowck (I wasn't able to get an example which didn't ICE, it might be possible): ```rust fn foo<'a: 'a>() -> impl Sized { let _: &'static () = foo::<'static>(); //~^ ICE opaque type with non-universal region substs foo::<'a>() } ``` Fixing this for lifetimes as well is blocked on #113916. Due to this issue, functions returning an RPIT with lifetime parameters equal in the region constraint graph would always result in an error, resulting in breakage found via crater: #112842 (comment) ```rust trait Trait<'a, 'b> {} impl Trait<'_, '_> for () {} struct Type<'a>(&'a ()); impl<'a> Type<'a> { // `'b == 'a` fn do_stuff<'b: 'a>(&'b self) -> impl Trait<'a, 'b> { // This fails as long there is something in the body // which adds the outlives constraints to the constraint graph. // // This is the case for nested closures. (|| ())() } } ```
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‎compiler/rustc_borrowck/src/region_infer/opaque_types.rs

+14-27
Original file line numberDiff line numberDiff line change
@@ -371,40 +371,27 @@ fn check_opaque_type_parameter_valid(
371371
span: Span,
372372
) -> Result<(), ErrorGuaranteed> {
373373
let opaque_ty_hir = tcx.hir().expect_item(opaque_type_key.def_id);
374-
match opaque_ty_hir.expect_opaque_ty().origin {
375-
// No need to check return position impl trait (RPIT)
376-
// because for type and const parameters they are correct
377-
// by construction: we convert
378-
//
379-
// fn foo<P0..Pn>() -> impl Trait
380-
//
381-
// into
382-
//
383-
// type Foo<P0...Pn>
384-
// fn foo<P0..Pn>() -> Foo<P0...Pn>.
385-
//
386-
// For lifetime parameters we convert
387-
//
388-
// fn foo<'l0..'ln>() -> impl Trait<'l0..'lm>
389-
//
390-
// into
391-
//
392-
// type foo::<'p0..'pn>::Foo<'q0..'qm>
393-
// fn foo<l0..'ln>() -> foo::<'static..'static>::Foo<'l0..'lm>.
394-
//
395-
// which would error here on all of the `'static` args.
396-
OpaqueTyOrigin::FnReturn(..) | OpaqueTyOrigin::AsyncFn(..) => return Ok(()),
397-
// Check these
398-
OpaqueTyOrigin::TyAlias { .. } => {}
399-
}
374+
let is_ty_alias = match opaque_ty_hir.expect_opaque_ty().origin {
375+
OpaqueTyOrigin::TyAlias { .. } => true,
376+
OpaqueTyOrigin::AsyncFn(..) | OpaqueTyOrigin::FnReturn(..) => false,
377+
};
378+
400379
let opaque_generics = tcx.generics_of(opaque_type_key.def_id);
401380
let mut seen_params: FxIndexMap<_, Vec<_>> = FxIndexMap::default();
402381
for (i, arg) in opaque_type_key.args.iter().enumerate() {
382+
if let Err(guar) = arg.error_reported() {
383+
return Err(guar);
384+
}
385+
403386
let arg_is_param = match arg.unpack() {
404387
GenericArgKind::Type(ty) => matches!(ty.kind(), ty::Param(_)),
405-
GenericArgKind::Lifetime(lt) => {
388+
GenericArgKind::Lifetime(lt) if is_ty_alias => {
406389
matches!(*lt, ty::ReEarlyBound(_) | ty::ReFree(_))
407390
}
391+
// FIXME(#113916): we can't currently check for unique lifetime params,
392+
// see that issue for more. We will also have to ignore unused lifetime
393+
// params for RPIT, but that's comparatively trivial ✨
394+
GenericArgKind::Lifetime(_) => continue,
408395
GenericArgKind::Const(ct) => matches!(ct.kind(), ty::ConstKind::Param(_)),
409396
};
410397

‎tests/ui/impl-trait/issue-99073-2.rs

+1
Original file line numberDiff line numberDiff line change
@@ -8,6 +8,7 @@ fn test<T: Display>(t: T, recurse: bool) -> impl Display {
88
let f = || {
99
let i: u32 = test::<i32>(-1, false);
1010
//~^ ERROR concrete type differs from previous defining opaque type use
11+
//~| ERROR expected generic type parameter, found `i32`
1112
println!("{i}");
1213
};
1314
if recurse {

‎tests/ui/impl-trait/issue-99073-2.stderr

+14-4
Original file line numberDiff line numberDiff line change
@@ -1,14 +1,24 @@
1+
error[E0792]: expected generic type parameter, found `i32`
2+
--> $DIR/issue-99073-2.rs:9:22
3+
|
4+
LL | fn test<T: Display>(t: T, recurse: bool) -> impl Display {
5+
| - this generic parameter must be used with a generic type parameter
6+
LL | let f = || {
7+
LL | let i: u32 = test::<i32>(-1, false);
8+
| ^^^^^^^^^^^^^^^^^^^^^^
9+
110
error: concrete type differs from previous defining opaque type use
211
--> $DIR/issue-99073-2.rs:9:22
312
|
413
LL | let i: u32 = test::<i32>(-1, false);
514
| ^^^^^^^^^^^^^^^^^^^^^^ expected `T`, got `u32`
615
|
716
note: previous use here
8-
--> $DIR/issue-99073-2.rs:16:5
17+
--> $DIR/issue-99073-2.rs:7:45
918
|
10-
LL | t
11-
| ^
19+
LL | fn test<T: Display>(t: T, recurse: bool) -> impl Display {
20+
| ^^^^^^^^^^^^
1221

13-
error: aborting due to previous error
22+
error: aborting due to 2 previous errors
1423

24+
For more information about this error, try `rustc --explain E0792`.

‎tests/ui/impl-trait/issue-99073.rs

+1
Original file line numberDiff line numberDiff line change
@@ -5,4 +5,5 @@ fn main() {
55
fn fix<F: Fn(G), G: Fn()>(f: F) -> impl Fn() {
66
move || f(fix(&f))
77
//~^ ERROR concrete type differs from previous defining opaque type use
8+
//~| ERROR expected generic type parameter, found `&F`
89
}

‎tests/ui/impl-trait/issue-99073.stderr

+15-6
Original file line numberDiff line numberDiff line change
@@ -1,14 +1,23 @@
1-
error: concrete type differs from previous defining opaque type use
1+
error[E0792]: expected generic type parameter, found `&F`
22
--> $DIR/issue-99073.rs:6:11
33
|
4+
LL | fn fix<F: Fn(G), G: Fn()>(f: F) -> impl Fn() {
5+
| - this generic parameter must be used with a generic type parameter
6+
LL | move || f(fix(&f))
7+
| ^^^^^^^^^^
8+
9+
error: concrete type differs from previous defining opaque type use
10+
--> $DIR/issue-99073.rs:6:13
11+
|
412
LL | move || f(fix(&f))
5-
| ^^^^^^^^^^ expected `[closure@$DIR/issue-99073.rs:6:3: 6:10]`, got `G`
13+
| ^^^^^^^ expected `[closure@$DIR/issue-99073.rs:6:3: 6:10]`, got `G`
614
|
715
note: previous use here
8-
--> $DIR/issue-99073.rs:6:3
16+
--> $DIR/issue-99073.rs:5:36
917
|
10-
LL | move || f(fix(&f))
11-
| ^^^^^^^^^^^^^^^^^^
18+
LL | fn fix<F: Fn(G), G: Fn()>(f: F) -> impl Fn() {
19+
| ^^^^^^^^^
1220

13-
error: aborting due to previous error
21+
error: aborting due to 2 previous errors
1422

23+
For more information about this error, try `rustc --explain E0792`.

‎tests/ui/impl-trait/rpit/equal-lifetime-params-ok.rs

+19
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,19 @@
1+
// check-pass
2+
3+
// related to #113916, check that using RPITs in functions with lifetime params
4+
// which are constrained to be equal compiles.
5+
6+
trait Trait<'a, 'b> {}
7+
impl Trait<'_, '_> for () {}
8+
fn pass<'a: 'b, 'b: 'a>() -> impl Trait<'a, 'b> {
9+
(|| {})()
10+
}
11+
12+
struct Foo<'a>(&'a ());
13+
impl<'a> Foo<'a> {
14+
fn bar<'b: 'a>(&'b self) -> impl Trait<'a, 'b> {
15+
let _: &'a &'b &'a ();
16+
}
17+
}
18+
19+
fn main() {}

‎tests/ui/impl-trait/rpit/non-defining-use.rs

+14
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,14 @@
1+
// Regression test for #111935 that non-defining uses of RPIT result in errors
2+
#![allow(unconditional_recursion)]
3+
fn foo<T>() -> impl Sized {
4+
let _: () = foo::<u8>(); //~ ERROR expected generic type parameter, found `u8`
5+
}
6+
7+
fn bar<T>(val: T) -> impl Sized {
8+
let _: u8 = bar(0u8);
9+
//~^ ERROR concrete type differs from previous defining opaque type use
10+
//~| ERROR expected generic type parameter, found `u8`
11+
val
12+
}
13+
14+
fn main() {}

‎tests/ui/impl-trait/rpit/non-defining-use.stderr

+31
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,31 @@
1+
error[E0792]: expected generic type parameter, found `u8`
2+
--> $DIR/non-defining-use.rs:4:12
3+
|
4+
LL | fn foo<T>() -> impl Sized {
5+
| - this generic parameter must be used with a generic type parameter
6+
LL | let _: () = foo::<u8>();
7+
| ^^
8+
9+
error[E0792]: expected generic type parameter, found `u8`
10+
--> $DIR/non-defining-use.rs:8:12
11+
|
12+
LL | fn bar<T>(val: T) -> impl Sized {
13+
| - this generic parameter must be used with a generic type parameter
14+
LL | let _: u8 = bar(0u8);
15+
| ^^
16+
17+
error: concrete type differs from previous defining opaque type use
18+
--> $DIR/non-defining-use.rs:8:17
19+
|
20+
LL | let _: u8 = bar(0u8);
21+
| ^^^^^^^^ expected `T`, got `u8`
22+
|
23+
note: previous use here
24+
--> $DIR/non-defining-use.rs:7:22
25+
|
26+
LL | fn bar<T>(val: T) -> impl Sized {
27+
| ^^^^^^^^^^
28+
29+
error: aborting due to 3 previous errors
30+
31+
For more information about this error, try `rustc --explain E0792`.

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