Rollup merge of #81176 - camsteffen:qpath-res, r=oli-obk

Improve safety of `LateContext::qpath_res`

This is my first rustc code change, inspired by hacking on clippy!

The first change is to clear cached `TypeckResults` from `LateContext` when visiting a nested item. I took a hint from [here](https://github.com/rust-lang/rust/blob/5e91c4ecc09312d8b63d250a432b0f3ef83f1df7/compiler/rustc_privacy/src/lib.rs#L1300).

Clippy has a `qpath_res` util function to avoid a possible ICE in `LateContext::qpath_res`. But the docs of `LateContext::qpath_res` promise no ICE. So this updates the `LateContext` method to keep its promises, and removes the util function.

Related: rust-lang/rust-clippy#4545

CC ````````````@eddyb```````````` since you've done related work
CC ````````````@flip1995```````````` FYI

1 files changed, 2 insertions, 2 deletions

diff --git a/clippy_lints/src/default.rs b/clippy_lints/src/default.rs
index f7224811e6e..6fa1378b8c7 100644
--- a/clippy_lints/src/default.rs
+++ b/clippy_lints/src/default.rs
@@ -1,5 +1,5 @@
 use crate::utils::{
-    any_parent_is_automatically_derived, contains_name, match_def_path, paths, qpath_res, snippet_with_macro_callsite,
+    any_parent_is_automatically_derived, contains_name, match_def_path, paths, snippet_with_macro_callsite,
 };
 use crate::utils::{span_lint_and_note, span_lint_and_sugg};
 use if_chain::if_chain;
@@ -231,7 +231,7 @@ fn is_expr_default<'tcx>(expr: &'tcx Expr<'tcx>, cx: &LateContext<'tcx>) -> bool
     if_chain! {
         if let ExprKind::Call(ref fn_expr, _) = &expr.kind;
         if let ExprKind::Path(qpath) = &fn_expr.kind;
-        if let Res::Def(_, def_id) = qpath_res(cx, qpath, fn_expr.hir_id);
+        if let Res::Def(_, def_id) = cx.qpath_res(qpath, fn_expr.hir_id);
         then {
             // right hand side of assignment is `Default::default`
             match_def_path(cx, def_id, &paths::DEFAULT_TRAIT_METHOD)