EUV: fix place of deref pattern's interior's scrutinee

The place previously used here was that of the temporary holding the
reference returned by `Deref::deref` or `DerefMut::deref_mut`. However,
since the inner pattern of `deref!(inner)` expects the deref-target type
itself, this would ICE when that type was inspected (e.g. by the EUV
case for slice patterns). This adds a deref projection to fix that.

Since current in-tree consumers of EUV (upvar inference and clippy)
don't care about Rvalues, the place could be simplified to
`self.cat_rvalue(pat.hir_id, self.pat_ty_adjusted(subpat)?)` to save
some cycles. I personally find EUV to be a bit fragile, so I've opted
for pedantic correctness. Maybe a `HACK` comment would suffice though?

Author: dianne

compiler/rustc_hir_typeck/src/expr_use_visitor.rs

@@ -1840,7 +1840,8 @@ impl<'tcx, Cx: TypeInformationCtxt<'tcx>, D: Delegate<'tcx>> ExprUseVisitor<'tcx
                 let ty = self.pat_ty_adjusted(subpat)?;
                 let ty = Ty::new_ref(self.cx.tcx(), re_erased, ty, mutability);
                 // A deref pattern generates a temporary.
-                let place = self.cat_rvalue(pat.hir_id, ty);
+                let base = self.cat_rvalue(pat.hir_id, ty);
+                let place = self.cat_deref(pat.hir_id, base)?;
                 self.cat_pattern(place, subpat, op)?;
             }
 

tests/crashes/125059.rs

@@ -0,0 +1,15 @@
+//@ check-pass
+//! Regression test for ICE in `rustc_hir_typeck::expr_use_visitor` on nesting a slice pattern
+//! inside a deref pattern inside a closure: rust-lang/rust#125059
+
+#![feature(deref_patterns)]
+#![allow(incomplete_features, unused)]
+
+fn simple_vec(vec: Vec<u32>) -> u32 {
+   (|| match Vec::<u32>::new() {
+        deref!([]) => 100,
+        _ => 2000,
+    })()
+}
+
+fn main() {}