Auto merge of #129778 - RalfJung:interp-lossy-typed-copy, r=saethlin

interpret: make typed copies lossy wrt provenance and padding

A "typed copy" in Rust can be a lossy process: when copying at type `usize` (or any other non-pointer type), if the original memory had any provenance, that provenance is lost. When copying at pointer type, if the original memory had partial provenance (i.e., not the same provenance for all bytes), that provenance is lost. When copying any type with padding, the contents of padding are lost.

This PR equips our validity-checking pass with the ability to reset provenance and padding according to those rules. Can be reviewed commit-by-commit. The first three commits are just preparation without any functional change.

Fixes https://github.com/rust-lang/miri/issues/845
Fixes https://github.com/rust-lang/miri/issues/2182

3 files changed, 14 insertions, 0 deletions

diff --git a/compiler/rustc_middle/src/mir/interpret/allocation.rs b/compiler/rustc_middle/src/mir/interpret/allocation.rs
index 5fb8af576ae..cd56d0edc05 100644
--- a/compiler/rustc_middle/src/mir/interpret/allocation.rs
+++ b/compiler/rustc_middle/src/mir/interpret/allocation.rs
@@ -644,6 +644,12 @@ impl<Prov: Provenance, Extra, Bytes: AllocBytes> Allocation<Prov, Extra, Bytes>
         return Ok(());
     }
 
+    /// Remove all provenance in the given memory range.
+    pub fn clear_provenance(&mut self, cx: &impl HasDataLayout, range: AllocRange) -> AllocResult {
+        self.provenance.clear(range, cx)?;
+        return Ok(());
+    }
+
     /// Applies a previously prepared provenance copy.
     /// The affected range, as defined in the parameters to `provenance().prepare_copy` is expected
     /// to be clear of provenance.
diff --git a/compiler/rustc_middle/src/mir/interpret/value.rs b/compiler/rustc_middle/src/mir/interpret/value.rs
index 84c17b39a62..989f03d3d13 100644
--- a/compiler/rustc_middle/src/mir/interpret/value.rs
+++ b/compiler/rustc_middle/src/mir/interpret/value.rs
@@ -307,6 +307,13 @@ impl<'tcx, Prov: Provenance> Scalar<Prov> {
         }
     }
 
+    pub fn clear_provenance(&mut self) -> InterpResult<'tcx> {
+        if matches!(self, Scalar::Ptr(..)) {
+            *self = self.to_scalar_int()?.into();
+        }
+        Ok(())
+    }
+
     #[inline(always)]
     pub fn to_scalar_int(self) -> InterpResult<'tcx, ScalarInt> {
         self.try_to_scalar_int().map_err(|_| err_unsup!(ReadPointerAsInt(None)).into())
diff --git a/compiler/rustc_middle/src/ty/sty.rs b/compiler/rustc_middle/src/ty/sty.rs
index 1f4f2c62d70..730ba265b19 100644
--- a/compiler/rustc_middle/src/ty/sty.rs
+++ b/compiler/rustc_middle/src/ty/sty.rs
@@ -1136,6 +1136,7 @@ impl<'tcx> Ty<'tcx> {
     }
 
     /// Tests if this is any kind of primitive pointer type (reference, raw pointer, fn pointer).
+    /// `Box` is *not* considered a pointer here!
     #[inline]
     pub fn is_any_ptr(self) -> bool {
         self.is_ref() || self.is_unsafe_ptr() || self.is_fn_ptr()