Auto merge of #77546 - lcnr:impl-trait-closure, r=eddyb

fix def collector for impl trait

fixes #77329

We now consistently make `impl Trait` a hir owner, requiring some special casing for synthetic generic params.

r? `@eddyb`

compiler/rustc_hir/src/hir.rs

@@ -486,6 +486,8 @@ impl Generics<'hir> {
 #[derive(HashStable_Generic)]
 pub enum SyntheticTyParamKind {
     ImplTrait,
+    // Created by the `#[rustc_synthetic]` attribute.
+    FromAttr,
 }
 
 /// A where-clause in a definition.