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

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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`
This commit is contained in:
Matthias Krüger 2022-02-25 14:14:39 +01:00 • committed by GitHub
commit cf3bb09888
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7 changed files with 43 additions and 32 deletions
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compiler/rustc_const_eval/src/const_eval/machine.rs
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@ -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.