Add `ConstArgKind::Path` and make `ConstArg` its own HIR node

This is a very large commit since a lot needs to be changed in order to
make the tests pass. The salient changes are:

- `ConstArgKind` gets a new `Path` variant, and all const params are now
  represented using it. Non-param paths still use `ConstArgKind::Anon`
  to prevent this change from getting too large, but they will soon use
  the `Path` variant too.

- `ConstArg` gets a distinct `hir_id` field and its own variant in
  `hir::Node`. This affected many parts of the compiler that expected
  the parent of an `AnonConst` to be the containing context (e.g., an
  array repeat expression). They have been changed to check the
  "grandparent" where necessary.

- Some `ast::AnonConst`s now have their `DefId`s created in
  rustc_ast_lowering rather than `DefCollector`. This is because in some
  cases they will end up becoming a `ConstArgKind::Path` instead, which
  has no `DefId`. We have to solve this in a hacky way where we guess
  whether the `AnonConst` could end up as a path const since we can't
  know for sure until after name resolution (`N` could refer to a free
  const or a nullary struct). If it has no chance as being a const
  param, then we create a `DefId` in `DefCollector` -- otherwise we
  decide during ast_lowering. This will have to be updated once all path
  consts use `ConstArgKind::Path`.

- We explicitly use `ConstArgHasType` for array lengths, rather than
  implicitly relying on anon const type feeding -- this is due to the
  addition of `ConstArgKind::Path`.

- Some tests have their outputs changed, but the changes are for the
  most part minor (including removing duplicate or almost-duplicate
  errors). One test now ICEs, but it is for an incomplete, unstable
  feature and is now tracked at #127009.

4 files changed, 4 insertions, 26 deletions

diff --git a/tests/ui/specialization/default-proj-ty-as-type-of-const-issue-125757.rs b/tests/ui/specialization/default-proj-ty-as-type-of-const-issue-125757.rs
index 59a015da84e..fb962ad24bf 100644
--- a/tests/ui/specialization/default-proj-ty-as-type-of-const-issue-125757.rs
+++ b/tests/ui/specialization/default-proj-ty-as-type-of-const-issue-125757.rs
@@ -14,6 +14,5 @@ struct Wrapper<const C: <i32 as Trait>::Type> {}
 
 impl<const C: usize> Wrapper<C> {}
 //~^ ERROR the constant `C` is not of type `<i32 as Trait>::Type`
-//~^^ ERROR mismatched types
 
 fn main() {}
diff --git a/tests/ui/specialization/default-proj-ty-as-type-of-const-issue-125757.stderr b/tests/ui/specialization/default-proj-ty-as-type-of-const-issue-125757.stderr
index 71d4277275f..7094ee8c67c 100644
--- a/tests/ui/specialization/default-proj-ty-as-type-of-const-issue-125757.stderr
+++ b/tests/ui/specialization/default-proj-ty-as-type-of-const-issue-125757.stderr
@@ -20,17 +20,5 @@ note: required by a const generic parameter in `Wrapper`
 LL | struct Wrapper<const C: <i32 as Trait>::Type> {}
    |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this const generic parameter in `Wrapper`
 
-error[E0308]: mismatched types
-  --> $DIR/default-proj-ty-as-type-of-const-issue-125757.rs:15:30
-   |
-LL | impl<const C: usize> Wrapper<C> {}
-   |                              ^ expected associated type, found `usize`
-   |
-   = note: expected associated type `<i32 as Trait>::Type`
-                         found type `usize`
-   = help: consider constraining the associated type `<i32 as Trait>::Type` to `usize`
-   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
-
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0308`.
diff --git a/tests/ui/specialization/min_specialization/bad-const-wf-doesnt-specialize.rs b/tests/ui/specialization/min_specialization/bad-const-wf-doesnt-specialize.rs
index f89a463bc58..a0ee7714417 100644
--- a/tests/ui/specialization/min_specialization/bad-const-wf-doesnt-specialize.rs
+++ b/tests/ui/specialization/min_specialization/bad-const-wf-doesnt-specialize.rs
@@ -6,7 +6,6 @@
 struct S<const L: usize>;
 
 impl<const N: i32> Copy for S<N> {}
-//~^ ERROR: mismatched types
 impl<const M: usize> Copy for S<M> {}
 //~^ ERROR: conflicting implementations of trait `Copy` for type `S<_>`
 
diff --git a/tests/ui/specialization/min_specialization/bad-const-wf-doesnt-specialize.stderr b/tests/ui/specialization/min_specialization/bad-const-wf-doesnt-specialize.stderr
index 1dac58e1f69..2953bc95917 100644
--- a/tests/ui/specialization/min_specialization/bad-const-wf-doesnt-specialize.stderr
+++ b/tests/ui/specialization/min_specialization/bad-const-wf-doesnt-specialize.stderr
@@ -1,19 +1,11 @@
 error[E0119]: conflicting implementations of trait `Copy` for type `S<_>`
-  --> $DIR/bad-const-wf-doesnt-specialize.rs:10:1
+  --> $DIR/bad-const-wf-doesnt-specialize.rs:9:1
    |
 LL | impl<const N: i32> Copy for S<N> {}
    | -------------------------------- first implementation here
-LL |
 LL | impl<const M: usize> Copy for S<M> {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `S<_>`
 
-error[E0308]: mismatched types
-  --> $DIR/bad-const-wf-doesnt-specialize.rs:8:31
-   |
-LL | impl<const N: i32> Copy for S<N> {}
-   |                               ^ expected `usize`, found `i32`
-
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
-Some errors have detailed explanations: E0119, E0308.
-For more information about an error, try `rustc --explain E0119`.
+For more information about this error, try `rustc --explain E0119`.