Rollup merge of #137257 - compiler-errors:fake-borrow-of-packed-field, r=oli-obk

Ignore fake borrows for packed field check

We should not emit unaligned packed field reference errors for the fake borrows that we generate during match lowering.

These fake borrows are there to ensure in *borrow-checking* that we don't modify the value being matched (which is why this only occurs when there's a match guard, in this case `if true`), but they are removed after the MIR is processed by `CleanupPostBorrowck`, since they're really just there to cause borrowck errors if necessary.

I modified `PlaceContext::is_borrow` since that's used by the packed field check:
17c1c329a5/compiler/rustc_mir_transform/src/check_packed_ref.rs (L40)

It's only used in one other place, in the SROA optimization (by which fake borrows are removed, so it doesn't matter):
17c1c329a5/compiler/rustc_mir_dataflow/src/value_analysis.rs (L922)

Fixes https://github.com/rust-lang/rust/issues/137250

compiler/rustc_middle/src/mir/visit.rs

@@ -1364,13 +1364,13 @@ impl PlaceContext {
         matches!(self, PlaceContext::MutatingUse(MutatingUseContext::Drop))
     }
 
-    /// Returns `true` if this place context represents a borrow.
+    /// Returns `true` if this place context represents a borrow, excluding fake borrows
+    /// (which are an artifact of borrowck and not actually borrows in runtime MIR).
     pub fn is_borrow(self) -> bool {
         matches!(
             self,
-            PlaceContext::NonMutatingUse(
-                NonMutatingUseContext::SharedBorrow | NonMutatingUseContext::FakeBorrow
-            ) | PlaceContext::MutatingUse(MutatingUseContext::Borrow)
+            PlaceContext::NonMutatingUse(NonMutatingUseContext::SharedBorrow)
+                | PlaceContext::MutatingUse(MutatingUseContext::Borrow)
         )
     }
 

tests/ui/lint/unaligned_references_fake_borrow.rs

@@ -0,0 +1,27 @@
+//@ check-pass
+
+// Regression test for <https://github.com/rust-lang/rust/issues/137250>.
+
+// Ensure that we don't emit unaligned packed field reference errors for the fake
+// borrows that we generate during match lowering. These fake borrows are there to
+// ensure in *borrow-checking* that we don't modify the value being matched, but
+// they are removed after the MIR is processed by `CleanupPostBorrowck`.
+
+#[repr(packed)]
+pub struct Packed(i32);
+
+fn f(x: Packed) {
+    match &x {
+        Packed(4) => {},
+        _ if true => {},
+        _ => {}
+    }
+
+    match x {
+        Packed(4) => {},
+        _ if true => {},
+        _ => {}
+    }
+}
+
+fn main() {}