Rollup merge of #94343 - RalfJung:fn-ptr, r=oli-obk

Miri fn ptr check: don't use conservative null check

In https://github.com/rust-lang/rust/pull/94270 I used the wrong NULL check for function pointers: `memory.ptr_may_be_null` is conservative even on machines that support ptr-to-int casts, leading to false errors in Miri.

This fixes that problem, and also replaces that foot-fun of a method with `scalar_may_be_null` which is never unnecessarily conservative.

r? `@oli-obk`

1 files changed, 3 insertions, 2 deletions

diff --git a/compiler/rustc_const_eval/src/const_eval/machine.rs b/compiler/rustc_const_eval/src/const_eval/machine.rs
index b2019ce40c3..99888992bc8 100644
--- a/compiler/rustc_const_eval/src/const_eval/machine.rs
+++ b/compiler/rustc_const_eval/src/const_eval/machine.rs
@@ -217,8 +217,9 @@ impl<'mir, 'tcx: 'mir> CompileTimeEvalContext<'mir, 'tcx> {
             // Comparisons of abstract pointers with null pointers are known if the pointer
             // is in bounds, because if they are in bounds, the pointer can't be null.
             // Inequality with integers other than null can never be known for sure.
-            (Scalar::Int(int), Scalar::Ptr(ptr, _)) | (Scalar::Ptr(ptr, _), Scalar::Int(int)) => {
-                int.is_null() && !self.memory.ptr_may_be_null(ptr.into())
+            (Scalar::Int(int), ptr @ Scalar::Ptr(..))
+            | (ptr @ Scalar::Ptr(..), Scalar::Int(int)) => {
+                int.is_null() && !self.scalar_may_be_null(ptr)
             }
             // FIXME: return `true` for at least some comparisons where we can reliably
             // determine the result of runtime inequality tests at compile-time.