Auto merge of #87123 - RalfJung:miri-provenance-overhaul, r=oli-obk

CTFE/Miri engine Pointer type overhaul

This fixes the long-standing problem that we are using `Scalar` as a type to represent pointers that might be integer values (since they point to a ZST). The main problem is that with int-to-ptr casts, there are multiple ways to represent the same pointer as a `Scalar` and it is unclear if "normalization" (i.e., the cast) already happened or not. This leads to ugly methods like `force_mplace_ptr` and `force_op_ptr`.
Another problem this solves is that in Miri, it would make a lot more sense to have the `Pointer::offset` field represent the full absolute address (instead of being relative to the `AllocId`). This means we can do ptr-to-int casts without access to any machine state, and it means that the overflow checks on pointer arithmetic are (finally!) accurate.

To solve this, the `Pointer` type is made entirely parametric over the provenance, so that we can use `Pointer<AllocId>` inside `Scalar` but use `Pointer<Option<AllocId>>` when accessing memory (where `None` represents the case that we could not figure out an `AllocId`; in that case the `offset` is an absolute address). Moreover, the `Provenance` trait determines if a pointer with a given provenance can be cast to an integer by simply dropping the provenance.

I hope this can be read commit-by-commit, but the first commit does the bulk of the work. It introduces some FIXMEs that are resolved later.
Fixes https://github.com/rust-lang/miri/issues/841
Miri PR: https://github.com/rust-lang/miri/pull/1851
r? `@oli-obk`

1 files changed, 4 insertions, 4 deletions

diff --git a/compiler/rustc_codegen_ssa/src/mir/operand.rs b/compiler/rustc_codegen_ssa/src/mir/operand.rs
index 3c42b2cc2ea..3e8386bc88f 100644
--- a/compiler/rustc_codegen_ssa/src/mir/operand.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/operand.rs
@@ -90,10 +90,10 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
                     Abi::ScalarPair(ref a, _) => a,
                     _ => bug!("from_const: invalid ScalarPair layout: {:#?}", layout),
                 };
-                let a = Scalar::from(Pointer::new(
-                    bx.tcx().create_memory_alloc(data),
-                    Size::from_bytes(start),
-                ));
+                let a = Scalar::from_pointer(
+                    Pointer::new(bx.tcx().create_memory_alloc(data), Size::from_bytes(start)),
+                    &bx.tcx(),
+                );
                 let a_llval = bx.scalar_to_backend(
                     a,
                     a_scalar,