Rollup merge of #98496 - BoxyUwU:instancers_bad_equality, r=lcnr

make `compare_const_impl` a query and use it in `instance.rs`

Fixes #88365

the bug in #88365 was caused by some `instance.rs` code using the `PartialEq` impl on `Ty` to check that the type of the associated const in an impl is the same as the type of the associated const in the trait definition. This was wrong for two reasons:
- the check typeck does is that the impl type is a subtype of the trait definition's type (see `mismatched_impl_ty_2.rs` which [was ICEing](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=f6d60ebe6745011f0d52ab2bc712025d) before this PR on stable)
- it assumes that if two types are equal then the `PartialEq` impl will reflect that which isnt true for higher ranked types or type level constants when `feature(generic_const_exprs)` is enabled (see `mismatched_impl_ty_3.rs` for higher ranked types which was [ICEing on stable](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=d7af131a655ed515b035624626c62c71))

r? `@lcnr`

5 files changed, 42 insertions, 2 deletions

diff --git a/src/test/ui/associated-consts/associated-const-impl-wrong-lifetime.stderr b/src/test/ui/associated-consts/associated-const-impl-wrong-lifetime.stderr
index de1d9589e99..742b81535df 100644
--- a/src/test/ui/associated-consts/associated-const-impl-wrong-lifetime.stderr
+++ b/src/test/ui/associated-consts/associated-const-impl-wrong-lifetime.stderr
@@ -2,7 +2,7 @@ error[E0308]: const not compatible with trait
   --> $DIR/associated-const-impl-wrong-lifetime.rs:7:5
    |
 LL |     const NAME: &'a str = "unit";
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime mismatch
+   |     ^^^^^^^^^^^^^^^^^^^ lifetime mismatch
    |
    = note: expected reference `&'static str`
               found reference `&'a str`
diff --git a/src/test/ui/associated-consts/mismatched_impl_ty_1.rs b/src/test/ui/associated-consts/mismatched_impl_ty_1.rs
new file mode 100644
index 00000000000..4dc6c2e47a9
--- /dev/null
+++ b/src/test/ui/associated-consts/mismatched_impl_ty_1.rs
@@ -0,0 +1,18 @@
+// run-pass
+#![feature(generic_const_exprs)]
+#![allow(incomplete_features)]
+
+trait MyTrait {
+    type ArrayType;
+    const SIZE: usize;
+    const ARRAY: Self::ArrayType;
+}
+impl MyTrait for () {
+    type ArrayType = [u8; Self::SIZE];
+    const SIZE: usize = 4;
+    const ARRAY: [u8; Self::SIZE] = [1, 2, 3, 4];
+}
+
+fn main() {
+    let _ = <() as MyTrait>::ARRAY;
+}
diff --git a/src/test/ui/associated-consts/mismatched_impl_ty_2.rs b/src/test/ui/associated-consts/mismatched_impl_ty_2.rs
new file mode 100644
index 00000000000..539becfdc7c
--- /dev/null
+++ b/src/test/ui/associated-consts/mismatched_impl_ty_2.rs
@@ -0,0 +1,11 @@
+// run-pass
+trait Trait {
+    const ASSOC: fn(&'static u32);
+}
+impl Trait for () {
+    const ASSOC: for<'a> fn(&'a u32) = |_| ();
+}
+
+fn main() {
+    let _ = <() as Trait>::ASSOC;
+}
diff --git a/src/test/ui/associated-consts/mismatched_impl_ty_3.rs b/src/test/ui/associated-consts/mismatched_impl_ty_3.rs
new file mode 100644
index 00000000000..17bcc8fe576
--- /dev/null
+++ b/src/test/ui/associated-consts/mismatched_impl_ty_3.rs
@@ -0,0 +1,11 @@
+// run-pass
+trait Trait {
+    const ASSOC: for<'a, 'b> fn(&'a u32, &'b u32);
+}
+impl Trait for () {
+    const ASSOC: for<'a> fn(&'a u32, &'a u32) = |_, _| ();
+}
+
+fn main() {
+    let _ = <() as Trait>::ASSOC;
+}
diff --git a/src/test/ui/nll/trait-associated-constant.stderr b/src/test/ui/nll/trait-associated-constant.stderr
index ae0ffd904e7..cf1c52ba7cc 100644
--- a/src/test/ui/nll/trait-associated-constant.stderr
+++ b/src/test/ui/nll/trait-associated-constant.stderr
@@ -2,7 +2,7 @@ error[E0308]: const not compatible with trait
   --> $DIR/trait-associated-constant.rs:21:5
    |
 LL |     const AC: Option<&'c str> = None;
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime mismatch
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime mismatch
    |
    = note: expected enum `Option<&'b str>`
               found enum `Option<&'c str>`