Auto merge of #10989 - ericmarkmartin:use-placeref-abstraction, r=Manishearth

Use placeref abstraction

rust-lang/rust#80647 suggests refactoring certain patterns with MIR places to use higher-level abstractions provided by the [`Place`](https://doc.rust-lang.org/stable/nightly-rustc/rustc_middle/mir/struct.Place.html)/[`PlaceRef`](https://doc.rust-lang.org/stable/nightly-rustc/rustc_middle/mir/struct.PlaceRef.html). While working on that issue, I found a couple candidates for such refactoring in clippy.

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changelog: none

tests/ui/missing_const_for_fn/cant_be_const.rs

@@ -157,3 +157,12 @@ impl Issue10617 {
         self.0
     }
 }
+
+union U {
+    f: u32,
+}
+
+// Do not lint because accessing union fields from const functions is unstable
+fn h(u: U) -> u32 {
+    unsafe { u.f }
+}