Rollup merge of #118396 - compiler-errors:ast-lang-items, r=cjgillot

Collect lang items from AST, get rid of `GenericBound::LangItemTrait`

r? `@cjgillot`
cc #115178

Looking forward, the work to remove `QPath::LangItem` will also be significantly more difficult, but I plan on doing it as well. Specifically, we have to change:
1. A lot of `rustc_ast_lowering` for things like expr `..`
2. A lot of astconv, since we actually instantiate lang and non-lang paths quite differently.
3. A ton of diagnostics and clippy lints that are special-cased via `QPath::LangItem`

Meanwhile, it was pretty easy to remove `GenericBound::LangItemTrait`, so I just did that here.

compiler/rustc_hir/src/hir.rs

@@ -435,8 +435,6 @@ pub enum TraitBoundModifier {
 #[derive(Clone, Copy, Debug, HashStable_Generic)]
 pub enum GenericBound<'hir> {
     Trait(PolyTraitRef<'hir>, TraitBoundModifier),
-    // FIXME(davidtwco): Introduce `PolyTraitRef::LangItem`
-    LangItemTrait(LangItem, Span, HirId, &'hir GenericArgs<'hir>),
     Outlives(&'hir Lifetime),
 }
 
@@ -451,7 +449,6 @@ impl GenericBound<'_> {
     pub fn span(&self) -> Span {
         match self {
             GenericBound::Trait(t, ..) => t.span,
-            GenericBound::LangItemTrait(_, span, ..) => *span,
             GenericBound::Outlives(l) => l.ident.span,
         }
     }

compiler/rustc_hir/src/intravisit.rs

@@ -1075,10 +1075,6 @@ pub fn walk_param_bound<'v, V: Visitor<'v>>(visitor: &mut V, bound: &'v GenericB
         GenericBound::Trait(ref typ, _modifier) => {
             visitor.visit_poly_trait_ref(typ);
         }
-        GenericBound::LangItemTrait(_, _span, hir_id, args) => {
-            visitor.visit_id(hir_id);
-            visitor.visit_generic_args(args);
-        }
         GenericBound::Outlives(ref lifetime) => visitor.visit_lifetime(lifetime),
     }
 }

compiler/rustc_hir/src/target.rs

@@ -67,6 +67,42 @@ impl Display for Target {
 }
 
 impl Target {
+    pub fn is_associated_item(self) -> bool {
+        match self {
+            Target::AssocConst | Target::AssocTy | Target::Method(_) => true,
+            Target::ExternCrate
+            | Target::Use
+            | Target::Static
+            | Target::Const
+            | Target::Fn
+            | Target::Closure
+            | Target::Mod
+            | Target::ForeignMod
+            | Target::GlobalAsm
+            | Target::TyAlias
+            | Target::OpaqueTy
+            | Target::Enum
+            | Target::Variant
+            | Target::Struct
+            | Target::Field
+            | Target::Union
+            | Target::Trait
+            | Target::TraitAlias
+            | Target::Impl
+            | Target::Expression
+            | Target::Statement
+            | Target::Arm
+            | Target::ForeignFn
+            | Target::ForeignStatic
+            | Target::ForeignTy
+            | Target::GenericParam(_)
+            | Target::MacroDef
+            | Target::Param
+            | Target::PatField
+            | Target::ExprField => false,
+        }
+    }
+
     pub fn from_item(item: &Item<'_>) -> Target {
         match item.kind {
             ItemKind::ExternCrate(..) => Target::ExternCrate,