Auto merge of rust-lang#11069 - y21:issue11063, r=Alexendoo

[`missing_fields_in_debug`]: make sure self type is an adt

Fixes rust-lang#11063, another ICE that can only happen in core.

This lint needs the `DefId` of the implementor to get its fields, but that ICEs if the implementor does not have a `DefId` (as is the case with primitive types, e.g. `impl Debug for bool`), which is where this ICE comes from.

This PR changes the check I added in rust-lang#10897 to be more... robust against `Debug` implementations we don't want to lint.
Instead of just checking if the self type is a type parameter and "special casing" one specific case we don't want to lint, we should probably rather just check that the self type is either a struct, an enum or a union and only then continue.
That prevents weird edge cases like this one that can only happen in core.

Again, I don't know if it's even possible to add a test case for this since one cannot implement `Debug` for primitive types outside of the crate that defined `Debug` (core).
I did make sure that this PR no longer ICEs on `impl<T> Debug for T` and `impl Debug for bool`.
Maybe writing such a test is possible with `#![no_core]` and then re-defining the `Debug` trait or something like that...?

changelog: [`missing_fields_in_debug`]: make sure self type is an adt (fixes an ICE in core)

r? `@Alexendoo` (reviewed the last PRs for this lint)

src/tools/clippy/clippy_lints/src/missing_fields_in_debug.rs

@@ -207,11 +207,10 @@ impl<'tcx> LateLintPass<'tcx> for MissingFieldsInDebug {
         if let ItemKind::Impl(Impl { of_trait: Some(trait_ref), self_ty, items, .. }) = item.kind
             && let Res::Def(DefKind::Trait, trait_def_id) = trait_ref.path.res
             && let TyKind::Path(QPath::Resolved(_, self_path)) = &self_ty.kind
-            // don't trigger if self is a generic parameter, e.g. `impl<T> Debug for T`
-            // this can only happen in core itself, where the trait is defined,
-            // but it caused ICEs in the past:
-            // https://github.com/rust-lang/rust-clippy/issues/10887
-            && !matches!(self_path.res, Res::Def(DefKind::TyParam, _))
+            // make sure that the self type is either a struct, an enum or a union
+            // this prevents ICEs such as when self is a type parameter or a primitive type
+            // (see #10887, #11063)
+            && let Res::Def(DefKind::Struct | DefKind::Enum | DefKind::Union, self_path_did) = self_path.res
             && cx.match_def_path(trait_def_id, &[sym::core, sym::fmt, sym::Debug])
             // don't trigger if this impl was derived
             && !cx.tcx.has_attr(item.owner_id, sym::automatically_derived)
@@ -222,7 +221,7 @@ impl<'tcx> LateLintPass<'tcx> for MissingFieldsInDebug {
             && let body = cx.tcx.hir().body(*body_id)
             && let ExprKind::Block(block, _) = body.value.kind
             // inspect `self`
-            && let self_ty = cx.tcx.type_of(self_path.res.def_id()).skip_binder().peel_refs()
+            && let self_ty = cx.tcx.type_of(self_path_did).skip_binder().peel_refs()
             && let Some(self_adt) = self_ty.ty_adt_def()
             && let Some(self_def_id) = self_adt.did().as_local()
             && let Some(Node::Item(self_item)) = cx.tcx.hir().find_by_def_id(self_def_id)