Auto merge of #1854 - RalfJung:partial-ptr-overwrite, r=RalfJung

adjust for ERR_ON_PARTIAL_PTR_OVERWRITE

The Miri side of rust-lang/rust#87248

rust-version

@@ -1 +1 @@
-3bc9dd0dd293ab82945e35888ed6d7ab802761ef
+3227e35765bab6d02c581928e26ad1d34bacf394

src/machine.rs

@@ -123,13 +123,17 @@ impl fmt::Display for MiriMemoryKind {
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct Tag {
     pub alloc_id: AllocId,
-    // Stacked Borrows tag.
+    /// Stacked Borrows tag.
     pub sb: SbTag,
 }
 
 impl Provenance for Tag {
+    /// We use absolute addresses in the `offset` of a `Pointer<Tag>`.
     const OFFSET_IS_ADDR: bool = true;
 
+    /// We cannot err on partial overwrites, it happens too often in practice (due to unions).
+    const ERR_ON_PARTIAL_PTR_OVERWRITE: bool = false;
+
     fn fmt(ptr: &Pointer<Self>, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let (tag, addr) = ptr.into_parts(); // address is absolute
         write!(f, "0x{:x}", addr.bytes())?;

tests/compile-fail/overwriting_part_of_relocation_makes_the_rest_uninit.rs → tests/compile-fail/pointer_partial_overwrite.rs

@@ -1,6 +1,9 @@
 // Make sure we find these even with many checks disabled.
 // compile-flags: -Zmiri-disable-alignment-check -Zmiri-disable-stacked-borrows -Zmiri-disable-validation
 
+// Test what happens when we overwrite parts of a pointer.
+// Also see <https://github.com/rust-lang/rust/issues/87184>.
+
 fn main() {
     let mut p = &42;
     unsafe {

tests/compile-fail/pointer_byte_read.rs → tests/compile-fail/pointer_partial_read.rs

@@ -1,3 +1,5 @@
+// Test what happens when we read parts of a pointer.
+// Related to <https://github.com/rust-lang/rust/issues/69488>.
 fn main() {
     let x = 13;
     let y = &x;