Normalize <X as Y>::T for rustdoc

- Only run for `QPath::Resolved` with `Some` self parameter (`<X as Y>::T`)
- Fall back to the previous behavior if the path can't be resolved
- Show what the behavior is if the type can't be normalized
- Run `resolve_vars_if_possible`

  It's not clear whether or not this is necessary. See
  https://github.com/rust-lang/rust/pull/77616 for more context.

- Add a test for cross-crate re-exports
- Use the same code for both `hir::Ty` and `Ty`

src/test/rustdoc/auxiliary/normalize-assoc-item.rs

@@ -0,0 +1,12 @@
+#![crate_name = "inner"]
+pub trait MyTrait {
+    type Y;
+}
+
+impl MyTrait for u32 {
+    type Y = i32;
+}
+
+pub fn foo() -> <u32 as MyTrait>::Y {
+    0
+}

src/test/rustdoc/normalize-assoc-item.rs

@@ -0,0 +1,63 @@
+// ignore-tidy-linelength
+// aux-build:normalize-assoc-item.rs
+// build-aux-docs
+
+pub trait Trait {
+    type X;
+}
+
+impl Trait for usize {
+    type X = isize;
+}
+
+// @has 'normalize_assoc_item/fn.f.html' '//pre[@class="rust fn"]' 'pub fn f() -> isize'
+pub fn f() -> <usize as Trait>::X {
+    0
+}
+
+pub struct S {
+    // @has 'normalize_assoc_item/struct.S.html' '//span[@id="structfield.box_me_up"]' 'box_me_up: Box<S>'
+    pub box_me_up: <S as Trait>::X,
+    // @has 'normalize_assoc_item/struct.S.html' '//span[@id="structfield.generic"]' 'generic: (usize, isize)'
+    pub generic: <Generic<usize> as Trait>::X,
+}
+
+impl Trait for S {
+    type X = Box<S>;
+}
+
+pub struct Generic<Inner>(Inner);
+
+impl<Inner: Trait> Trait for Generic<Inner> {
+    type X = (Inner, Inner::X);
+}
+
+// These can't be normalized because they depend on a generic parameter.
+// However the user can choose whether the text should be displayed as `Inner::X` or `<Inner as Trait>::X`.
+
+// @has 'normalize_assoc_item/struct.Unknown.html' '//pre[@class="rust struct"]' 'pub struct Unknown<Inner: Trait>(pub <Inner as Trait>::X);'
+pub struct Unknown<Inner: Trait>(pub <Inner as Trait>::X);
+
+// @has 'normalize_assoc_item/struct.Unknown2.html' '//pre[@class="rust struct"]' 'pub struct Unknown2<Inner: Trait>(pub Inner::X);'
+pub struct Unknown2<Inner: Trait>(pub Inner::X);
+
+trait Lifetimes<'a> {
+    type Y;
+}
+
+impl<'a> Lifetimes<'a> for usize {
+    type Y = &'a isize;
+}
+
+// @has 'normalize_assoc_item/fn.g.html' '//pre[@class="rust fn"]' "pub fn g() -> &isize"
+pub fn g() -> <usize as Lifetimes<'static>>::Y {
+    &0
+}
+
+// @has 'normalize_assoc_item/constant.A.html' '//pre[@class="rust const"]' "pub const A: &isize"
+pub const A: <usize as Lifetimes<'static>>::Y = &0;
+
+// test cross-crate re-exports
+extern crate inner;
+// @has 'normalize_assoc_item/fn.foo.html' '//pre[@class="rust fn"]' "pub fn foo() -> i32"
+pub use inner::foo;