Auto merge of rust-lang#10897 - y21:issue10887, r=Alexendoo

[`missing_fields_in_debug`]: don't ICE when self type is a generic param

Fixes rust-lang#10887

This PR fixes an ICE that happens when the implementor (self type) of a `Debug` impl is a generic parameter.
The lint calls `TyCtxt::type_of` with that self type, which ICEs when called with generic parameters, so this just adds a quick check before getting there to ignore them.

That can only happen inside of core itself (afaik) because the orphan rules forbid defining an impl such as `impl<T> Debug for T` outside of core, so I'm not sure how to add a test for this.
It seems like this impl in particular caused this: https://doc.rust-lang.org/stable/std/fmt/trait.Debug.html#impl-Debug-for-F

changelog: [`missing_fields_in_debug`]: don't ICE on blanket `Debug` impl in core

Author: bors

clippy_lints/src/missing_fields_in_debug.rs

@@ -207,6 +207,11 @@ 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, _))
             && 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)