Auto merge of #93028 - compiler-errors:const_drop_bounds, r=fee1-dead

Check `const Drop` impls considering `~const` Bounds

 This PR adds logic to trait selection to account for `~const` bounds in custom `impl const Drop` for types, elaborates the `const Drop` check in `rustc_const_eval` to check those bounds, and steals some drop linting fixes from #92922, thanks `@DrMeepster.`

r? `@fee1-dead` `@oli-obk` <sup>(edit: guess I can't request review from two people, lol)</sup>
since each of you wrote and reviewed #88558, respectively.

Since the logic here is more complicated than what existed, it's possible that this is a perf regression. But it works correctly with tests, and that makes me happy.

Fixes #92881

4 files changed, 82 insertions, 67 deletions

diff --git a/src/test/ui/rfc-2632-const-trait-impl/const-drop-fail.precise.stderr b/src/test/ui/rfc-2632-const-trait-impl/const-drop-fail.precise.stderr
index 04c21101e75..721636e0743 100644
--- a/src/test/ui/rfc-2632-const-trait-impl/const-drop-fail.precise.stderr
+++ b/src/test/ui/rfc-2632-const-trait-impl/const-drop-fail.precise.stderr
@@ -1,13 +1,5 @@
-error: `~const` is not allowed here
-  --> $DIR/const-drop-fail.rs:27:35
-   |
-LL | struct ConstDropImplWithBounds<T: ~const A>(PhantomData<T>);
-   |                                   ^^^^^^^^
-   |
-   = note: only allowed on bounds on traits' associated types and functions, const fns, const impls and its associated functions
-
 error[E0277]: the trait bound `NonTrivialDrop: Drop` is not satisfied
-  --> $DIR/const-drop-fail.rs:45:5
+  --> $DIR/const-drop-fail.rs:44:5
    |
 LL |         const _: () = check($exp);
    |                       ----- required by a bound introduced by this call
@@ -16,50 +8,51 @@ LL |     NonTrivialDrop,
    |     ^^^^^^^^^^^^^^ the trait `Drop` is not implemented for `NonTrivialDrop`
    |
 note: required by a bound in `check`
-  --> $DIR/const-drop-fail.rs:36:19
+  --> $DIR/const-drop-fail.rs:35:19
    |
 LL | const fn check<T: ~const Drop>(_: T) {}
    |                   ^^^^^^^^^^^ required by this bound in `check`
 
-error[E0277]: the trait bound `ConstImplWithDropGlue: Drop` is not satisfied
-  --> $DIR/const-drop-fail.rs:47:5
+error[E0277]: the trait bound `NonTrivialDrop: Drop` is not satisfied in `ConstImplWithDropGlue`
+  --> $DIR/const-drop-fail.rs:46:5
    |
 LL |         const _: () = check($exp);
    |                       ----- required by a bound introduced by this call
 ...
 LL |     ConstImplWithDropGlue(NonTrivialDrop),
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Drop` is not implemented for `ConstImplWithDropGlue`
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ within `ConstImplWithDropGlue`, the trait `Drop` is not implemented for `NonTrivialDrop`
    |
+note: required because it appears within the type `ConstImplWithDropGlue`
+  --> $DIR/const-drop-fail.rs:17:8
+   |
+LL | struct ConstImplWithDropGlue(NonTrivialDrop);
+   |        ^^^^^^^^^^^^^^^^^^^^^
 note: required by a bound in `check`
-  --> $DIR/const-drop-fail.rs:36:19
+  --> $DIR/const-drop-fail.rs:35:19
    |
 LL | const fn check<T: ~const Drop>(_: T) {}
    |                   ^^^^^^^^^^^ required by this bound in `check`
 
 error[E0277]: the trait bound `NonTrivialDrop: A` is not satisfied
-  --> $DIR/const-drop-fail.rs:49:5
-   |
-LL |     ConstDropImplWithBounds::<NonTrivialDrop>(PhantomData),
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `A` is not implemented for `NonTrivialDrop`
-   |
-note: required by a bound in `ConstDropImplWithBounds`
-  --> $DIR/const-drop-fail.rs:27:35
-   |
-LL | struct ConstDropImplWithBounds<T: ~const A>(PhantomData<T>);
-   |                                   ^^^^^^^^ required by this bound in `ConstDropImplWithBounds`
-
-error[E0277]: the trait bound `NonTrivialDrop: A` is not satisfied
-  --> $DIR/const-drop-fail.rs:49:5
+  --> $DIR/const-drop-fail.rs:48:5
    |
+LL |         const _: () = check($exp);
+   |                       ----- required by a bound introduced by this call
+...
 LL |     ConstDropImplWithBounds::<NonTrivialDrop>(PhantomData),
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `A` is not implemented for `NonTrivialDrop`
    |
-note: required by a bound in `ConstDropImplWithBounds`
-  --> $DIR/const-drop-fail.rs:27:35
+note: required because of the requirements on the impl of `Drop` for `ConstDropImplWithBounds<NonTrivialDrop>`
+  --> $DIR/const-drop-fail.rs:29:25
    |
-LL | struct ConstDropImplWithBounds<T: ~const A>(PhantomData<T>);
-   |                                   ^^^^^^^^ required by this bound in `ConstDropImplWithBounds`
+LL | impl<T: ~const A> const Drop for ConstDropImplWithBounds<T> {
+   |                         ^^^^     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: required by a bound in `check`
+  --> $DIR/const-drop-fail.rs:35:19
+   |
+LL | const fn check<T: ~const Drop>(_: T) {}
+   |                   ^^^^^^^^^^^ required by this bound in `check`
 
-error: aborting due to 5 previous errors
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0277`.
diff --git a/src/test/ui/rfc-2632-const-trait-impl/const-drop-fail.rs b/src/test/ui/rfc-2632-const-trait-impl/const-drop-fail.rs
index 3d4de088f55..4622723c189 100644
--- a/src/test/ui/rfc-2632-const-trait-impl/const-drop-fail.rs
+++ b/src/test/ui/rfc-2632-const-trait-impl/const-drop-fail.rs
@@ -24,8 +24,7 @@ trait A { fn a() { println!("A"); } }
 
 impl A for NonTrivialDrop {}
 
-struct ConstDropImplWithBounds<T: ~const A>(PhantomData<T>);
-//~^ ERROR `~const` is not allowed
+struct ConstDropImplWithBounds<T: A>(PhantomData<T>);
 
 impl<T: ~const A> const Drop for ConstDropImplWithBounds<T> {
     fn drop(&mut self) {
@@ -48,7 +47,6 @@ check_all! {
     //~^ ERROR the trait bound
     ConstDropImplWithBounds::<NonTrivialDrop>(PhantomData),
     //~^ ERROR the trait bound
-    //~| ERROR the trait bound
 }
 
 fn main() {}
diff --git a/src/test/ui/rfc-2632-const-trait-impl/const-drop-fail.stock.stderr b/src/test/ui/rfc-2632-const-trait-impl/const-drop-fail.stock.stderr
index 04c21101e75..721636e0743 100644
--- a/src/test/ui/rfc-2632-const-trait-impl/const-drop-fail.stock.stderr
+++ b/src/test/ui/rfc-2632-const-trait-impl/const-drop-fail.stock.stderr
@@ -1,13 +1,5 @@
-error: `~const` is not allowed here
-  --> $DIR/const-drop-fail.rs:27:35
-   |
-LL | struct ConstDropImplWithBounds<T: ~const A>(PhantomData<T>);
-   |                                   ^^^^^^^^
-   |
-   = note: only allowed on bounds on traits' associated types and functions, const fns, const impls and its associated functions
-
 error[E0277]: the trait bound `NonTrivialDrop: Drop` is not satisfied
-  --> $DIR/const-drop-fail.rs:45:5
+  --> $DIR/const-drop-fail.rs:44:5
    |
 LL |         const _: () = check($exp);
    |                       ----- required by a bound introduced by this call
@@ -16,50 +8,51 @@ LL |     NonTrivialDrop,
    |     ^^^^^^^^^^^^^^ the trait `Drop` is not implemented for `NonTrivialDrop`
    |
 note: required by a bound in `check`
-  --> $DIR/const-drop-fail.rs:36:19
+  --> $DIR/const-drop-fail.rs:35:19
    |
 LL | const fn check<T: ~const Drop>(_: T) {}
    |                   ^^^^^^^^^^^ required by this bound in `check`
 
-error[E0277]: the trait bound `ConstImplWithDropGlue: Drop` is not satisfied
-  --> $DIR/const-drop-fail.rs:47:5
+error[E0277]: the trait bound `NonTrivialDrop: Drop` is not satisfied in `ConstImplWithDropGlue`
+  --> $DIR/const-drop-fail.rs:46:5
    |
 LL |         const _: () = check($exp);
    |                       ----- required by a bound introduced by this call
 ...
 LL |     ConstImplWithDropGlue(NonTrivialDrop),
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Drop` is not implemented for `ConstImplWithDropGlue`
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ within `ConstImplWithDropGlue`, the trait `Drop` is not implemented for `NonTrivialDrop`
    |
+note: required because it appears within the type `ConstImplWithDropGlue`
+  --> $DIR/const-drop-fail.rs:17:8
+   |
+LL | struct ConstImplWithDropGlue(NonTrivialDrop);
+   |        ^^^^^^^^^^^^^^^^^^^^^
 note: required by a bound in `check`
-  --> $DIR/const-drop-fail.rs:36:19
+  --> $DIR/const-drop-fail.rs:35:19
    |
 LL | const fn check<T: ~const Drop>(_: T) {}
    |                   ^^^^^^^^^^^ required by this bound in `check`
 
 error[E0277]: the trait bound `NonTrivialDrop: A` is not satisfied
-  --> $DIR/const-drop-fail.rs:49:5
-   |
-LL |     ConstDropImplWithBounds::<NonTrivialDrop>(PhantomData),
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `A` is not implemented for `NonTrivialDrop`
-   |
-note: required by a bound in `ConstDropImplWithBounds`
-  --> $DIR/const-drop-fail.rs:27:35
-   |
-LL | struct ConstDropImplWithBounds<T: ~const A>(PhantomData<T>);
-   |                                   ^^^^^^^^ required by this bound in `ConstDropImplWithBounds`
-
-error[E0277]: the trait bound `NonTrivialDrop: A` is not satisfied
-  --> $DIR/const-drop-fail.rs:49:5
+  --> $DIR/const-drop-fail.rs:48:5
    |
+LL |         const _: () = check($exp);
+   |                       ----- required by a bound introduced by this call
+...
 LL |     ConstDropImplWithBounds::<NonTrivialDrop>(PhantomData),
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `A` is not implemented for `NonTrivialDrop`
    |
-note: required by a bound in `ConstDropImplWithBounds`
-  --> $DIR/const-drop-fail.rs:27:35
+note: required because of the requirements on the impl of `Drop` for `ConstDropImplWithBounds<NonTrivialDrop>`
+  --> $DIR/const-drop-fail.rs:29:25
    |
-LL | struct ConstDropImplWithBounds<T: ~const A>(PhantomData<T>);
-   |                                   ^^^^^^^^ required by this bound in `ConstDropImplWithBounds`
+LL | impl<T: ~const A> const Drop for ConstDropImplWithBounds<T> {
+   |                         ^^^^     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: required by a bound in `check`
+  --> $DIR/const-drop-fail.rs:35:19
+   |
+LL | const fn check<T: ~const Drop>(_: T) {}
+   |                   ^^^^^^^^^^^ required by this bound in `check`
 
-error: aborting due to 5 previous errors
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0277`.
diff --git a/src/test/ui/rfc-2632-const-trait-impl/const-drop.rs b/src/test/ui/rfc-2632-const-trait-impl/const-drop.rs
index 292017a1de2..13363c506d5 100644
--- a/src/test/ui/rfc-2632-const-trait-impl/const-drop.rs
+++ b/src/test/ui/rfc-2632-const-trait-impl/const-drop.rs
@@ -3,6 +3,7 @@
 #![feature(const_trait_impl)]
 #![feature(const_fn_trait_bound)]
 #![feature(const_mut_refs)]
+#![feature(never_type)]
 #![cfg_attr(precise, feature(const_precise_live_drops))]
 
 struct S<'a>(&'a mut u8);
@@ -45,6 +46,33 @@ mod t {
 
     pub struct HasConstDrop(pub ConstDrop);
     pub struct TrivialFields(pub u8, pub i8, pub usize, pub isize);
+
+    pub trait SomeTrait {
+        fn foo();
+    }
+    impl const SomeTrait for () {
+        fn foo() {}
+    }
+    // non-const impl
+    impl SomeTrait for i32 {
+        fn foo() {}
+    }
+
+    pub struct ConstDropWithBound<T: SomeTrait>(pub core::marker::PhantomData<T>);
+
+    impl<T: ~const SomeTrait> const Drop for ConstDropWithBound<T> {
+        fn drop(&mut self) {
+            T::foo();
+        }
+    }
+
+    pub struct ConstDropWithNonconstBound<T: SomeTrait>(pub core::marker::PhantomData<T>);
+
+    impl<T: SomeTrait> const Drop for ConstDropWithNonconstBound<T> {
+        fn drop(&mut self) {
+            // Note: we DON'T use the `T: SomeTrait` bound
+        }
+    }
 }
 
 use t::*;
@@ -61,6 +89,9 @@ implements_const_drop! {
     TrivialFields(1, 2, 3, 4),
     &1,
     &1 as *const i32,
+    ConstDropWithBound::<()>,
+    ConstDropWithNonconstBound::<i32>,
+    Result::<i32, !>::Ok(1),
 }
 
 fn main() {