Rollup merge of #70048 - TyPR124:mutable_osstr, r=dtolnay

Allow obtaining &mut OsStr

```rust
impl DerefMut for OsString {...}              // type Target = OsStr
impl IndexMut<RangeFull> for OsString {...}   // type Output = OsStr
```

---

This change is pulled out of #69937 per @dtolnay

This implements `DerefMut for OsString` to allow obtaining a `&mut OsStr`. This also implements `IndexMut for OsString`, which is used by `DerefMut`. This pattern is the same as is used by `Deref`.

This is necessary to for methods like `make_ascii_lowercase` which need to mutate the underlying value.

1 files changed, 12 insertions, 0 deletions

diff --git a/src/libstd/sys/windows/os_str.rs b/src/libstd/sys/windows/os_str.rs
index ef260f9c5d2..ff6885cb274 100644
--- a/src/libstd/sys/windows/os_str.rs
+++ b/src/libstd/sys/windows/os_str.rs
@@ -77,9 +77,21 @@ impl Buf {
     }
 
     pub fn as_slice(&self) -> &Slice {
+        // Safety: Slice is just a wrapper for Wtf8,
+        // and self.inner.as_slice() returns &Wtf8.
+        // Therefore, transmuting &Wtf8 to &Slice is safe.
         unsafe { mem::transmute(self.inner.as_slice()) }
     }
 
+    pub fn as_mut_slice(&mut self) -> &mut Slice {
+        // Safety: Slice is just a wrapper for Wtf8,
+        // and self.inner.as_mut_slice() returns &mut Wtf8.
+        // Therefore, transmuting &mut Wtf8 to &mut Slice is safe.
+        // Additionally, care should be taken to ensure the slice
+        // is always valid Wtf8.
+        unsafe { mem::transmute(self.inner.as_mut_slice()) }
+    }
+
     pub fn into_string(self) -> Result<String, Buf> {
         self.inner.into_string().map_err(|buf| Buf { inner: buf })
     }