add DerefMut for Utf8PathBuf

Also add a miri check for unsafe code.

Resolves camino-rs#57.

.github/workflows/ci.yml

@@ -38,8 +38,19 @@ jobs:
       matrix:
         os: [ ubuntu-latest, macos-latest, windows-latest ]
         # 1.34 is the MSRV, and the other versions add new features through build.rs.
+        # The nightly versions are used to ensure that build.rs's version detection is compatible
+        # with them:
         # nightly-2022-06-17 is the last toolchain before `try_reserve_2` was stabilized.
-        rust-version: [ 1.34, 1.44, 1.56, nightly-2022-06-17, 1.63, stable ]
+        # nightly-2022-12-14 is the last toolchain before `path_buf_deref_mut` was stabilized.
+        rust-version:
+          - 1.34
+          - 1.44
+          - 1.56
+          - nightly-2022-06-17
+          - 1.63
+          - nightly-2022-12-14
+          - 1.68
+          - stable
         exclude:
           # These versions started failing with "archive member 'lib.rmeta' with length 26456 is not
           # mach-o or llvm bitcode file".
@@ -58,6 +69,13 @@ jobs:
       - uses: dtolnay/rust-toolchain@v1
         with:
           toolchain: ${{ matrix.rust-version }}
+      - name: Disable sparse registries on nightly-2022-12-14
+        # Sparse registries are experimental on this nightly, but are enabled by
+        # dtolnay/rust-toolchain.
+        if: matrix.rust-version == 'nightly-2022-12-14'
+        shell: bash
+        run: |
+          echo CARGO_REGISTRIES_CRATES_IO_PROTOCOL=git >> $GITHUB_ENV
       - name: Build the library
         run: cargo build
       - name: Test
@@ -70,3 +88,19 @@ jobs:
         # Some optional features are not compatible with earlier versions
         if: ${{ matrix.rust-version == 'stable' }}
         run: cargo test --workspace --all-features
+
+  miri:
+    name: Check unsafe code against miri
+    runs-on: ubuntu-latest
+    env:
+      RUSTFLAGS: -D warnings
+    steps:
+      - uses: actions/checkout@v3
+      - uses: dtolnay/rust-toolchain@v1
+        with:
+          toolchain: nightly
+          components: rust-src, miri
+      - name: Test the library
+        run: cargo miri test
+        env:
+          MIRIFLAGS: -Zmiri-disable-isolation

CHANGELOG.md

@@ -3,6 +3,12 @@
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## Unreleased
+
+### Added
+
+- Implement `DerefMut` for `Utf8PathBuf` on Rust 1.68 and above.
+
 ## [1.1.3] - 2023-02-21
 
 ### Added

build.rs

@@ -24,13 +24,22 @@ fn main() {
     if compiler.minor >= 56 {
         println!("cargo:rustc-cfg=shrink_to");
     }
-    // Stable and beta 1.63 have a stable try_reserve_2.
+    // NOTE: the below checks use == rather than `matches!`. This is because `matches!` isn't stable
+    // on Rust 1.34.
+    // try_reserve_2 was added in a 1.63 nightly.
     if (compiler.minor >= 63
         && (compiler.channel == ReleaseChannel::Stable || compiler.channel == ReleaseChannel::Beta))
         || compiler.minor >= 64
     {
         println!("cargo:rustc-cfg=try_reserve_2");
     }
+    // path_buf_deref_mut was added in a 1.68 nightly.
+    if (compiler.minor >= 68
+        && (compiler.channel == ReleaseChannel::Stable || compiler.channel == ReleaseChannel::Beta))
+        || compiler.minor >= 69
+    {
+        println!("cargo:rustc-cfg=path_buf_deref_mut");
+    }
 }
 
 struct Compiler {

src/lib.rs

@@ -484,6 +484,13 @@ impl Deref for Utf8PathBuf {
     }
 }
 
+#[cfg(path_buf_deref_mut)]
+impl std::ops::DerefMut for Utf8PathBuf {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        unsafe { Utf8Path::assume_utf8_mut(&mut self.0) }
+    }
+}
+
 impl fmt::Debug for Utf8PathBuf {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         fmt::Debug::fmt(&**self, f)
@@ -1457,6 +1464,11 @@ impl Utf8Path {
         // *const Path to a *const Utf8Path is valid.
         &*(path as *const Path as *const Utf8Path)
     }
+
+    #[cfg(path_buf_deref_mut)]
+    unsafe fn assume_utf8_mut(path: &mut Path) -> &mut Utf8Path {
+        &mut *(path as *mut Path as *mut Utf8Path)
+    }
 }
 
 impl Clone for Box<Utf8Path> {

src/tests.rs

@@ -44,3 +44,11 @@ where
 {
     let _ = orig.into();
 }
+
+#[cfg(path_buf_deref_mut)]
+#[test]
+fn test_deref_mut() {
+    // This test is mostly for miri.
+    let mut path_buf = Utf8PathBuf::from("foobar");
+    let _: &mut Utf8Path = &mut path_buf;
+}