add From<Box<T>> where T: ?Sized

.github/workflows/tests.yml

@@ -4,7 +4,6 @@ on: [push, pull_request]
 
 jobs:
   build:
-
     runs-on: ubuntu-latest
     strategy:
       matrix:
@@ -13,74 +12,83 @@ jobs:
           - 1.76
           - nightly
     steps:
-    - uses: actions/checkout@v4
-    - name: Build
-      run: cargo build --verbose
-    - name: Test
-      run: cargo test
-    - name: Test --no-default-features
-      run: cargo test --no-default-features --verbose
-    - name: Test --all-features
-      run: cargo test --all-features --verbose
+      - uses: actions/checkout@v4
+      - name: Install Rust ${{ matrix.rust }}
+        uses: actions-rs/toolchain@v1
+        with:
+          toolchain: ${{ matrix.rust }}
+          override: true
+      - name: Build
+        run: cargo build --verbose
+      - name: Test
+        run: cargo test
+      - name: Test --no-default-features
+        run: cargo test --no-default-features --verbose
+      - name: Test --all-features
+        if: matrix.rust == 'nightly'
+        run: cargo test --all-features --verbose
+      - name: Test --all-stable-features
+        if: matrix.rust != 'nightly'
+        run: cargo test --features "std,serde,stable_deref_trait,arc-swap,unsize" --verbose
 
   miri:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v4
-    - name: Install Rust Nightly
-      run: rustup toolchain install nightly --component miri --profile minimal
-    - name: Miri Test
-      run: cargo +nightly miri test
-    - name: Test --no-default-features
-      run: cargo +nightly miri test --no-default-features
-    - name: Test --all-features
-      run: cargo +nightly miri test --all-features
+      - uses: actions/checkout@v4
+      - name: Install Rust Nightly
+        run: rustup toolchain install nightly --component miri --profile minimal
+      - name: Miri Test
+        run: cargo +nightly miri test
+      - name: Test --no-default-features
+        run: cargo +nightly miri test --no-default-features
+      - name: Test --all-features
+        run: cargo +nightly miri test --all-features
 
   thread_sanitizer:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v4
-    - uses: actions/cache@v3
-      with:
-        path: |
-          ~/.cargo/bin/
-          ~/.cargo/registry/index/
-          ~/.cargo/registry/cache/
-          ~/.cargo/git/db/
-          target/
-        key: ${{ github.event.repository.name }}-${{ runner.os }}-cargo-thread_sanitizer-v2
-    - name: Install latest nightly
-      uses: actions-rs/toolchain@v1
-      with:
+      - uses: actions/checkout@v4
+      - uses: actions/cache@v3
+        with:
+          path: |
+            ~/.cargo/bin/
+            ~/.cargo/registry/index/
+            ~/.cargo/registry/cache/
+            ~/.cargo/git/db/
+            target/
+          key: ${{ github.event.repository.name }}-${{ runner.os }}-cargo-thread_sanitizer-v2
+      - name: Install latest nightly
+        uses: actions-rs/toolchain@v1
+        with:
           toolchain: nightly
           override: true
           components: rust-src
-    - name: Run thread sanitizer
-      run: |
-        export RUSTDOCFLAGS='-Zsanitizer=thread'
-        export RUSTFLAGS='-Zsanitizer=thread'
-        for _ in $(seq 1 10); do cargo +nightly test -Z build-std --target --target $(uname -m)-unknown-linux-gnu; done
+      - name: Run thread sanitizer
+        run: |
+          export RUSTDOCFLAGS='-Zsanitizer=thread'
+          export RUSTFLAGS='-Zsanitizer=thread'
+          for _ in $(seq 1 10); do cargo +nightly test -Z build-std --target --target $(uname -m)-unknown-linux-gnu; done
 
   address_sanitizer:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v4
-    - uses: actions/cache@v3
-      with:
-        path: |
-          ~/.cargo/bin/
-          ~/.cargo/registry/index/
-          ~/.cargo/registry/cache/
-          ~/.cargo/git/db/
-          target/
-        key: ${{ github.event.repository.name }}-${{ runner.os }}-cargo-address_sanitizer-v2
-    - name: Install latest nightly
-      uses: actions-rs/toolchain@v1
-      with:
+      - uses: actions/checkout@v4
+      - uses: actions/cache@v3
+        with:
+          path: |
+            ~/.cargo/bin/
+            ~/.cargo/registry/index/
+            ~/.cargo/registry/cache/
+            ~/.cargo/git/db/
+            target/
+          key: ${{ github.event.repository.name }}-${{ runner.os }}-cargo-address_sanitizer-v2
+      - name: Install latest nightly
+        uses: actions-rs/toolchain@v1
+        with:
           toolchain: nightly
           override: true
-    - name: Run address sanitizer
-      run: |
-        export RUSTDOCFLAGS='-Zsanitizer=address'
-        export RUSTFLAGS='-Zsanitizer=address'
-        for _ in $(seq 1 10); do cargo +nightly test; done
+      - name: Run address sanitizer
+        run: |
+          export RUSTDOCFLAGS='-Zsanitizer=address'
+          export RUSTFLAGS='-Zsanitizer=address'
+          for _ in $(seq 1 10); do cargo +nightly test; done

Cargo.toml

@@ -1,7 +1,10 @@
 [package]
 name = "triomphe"
 version = "0.1.12"
-authors = ["Manish Goregaokar <[email protected]>", "The Servo Project Developers"]
+authors = [
+    "Manish Goregaokar <[email protected]>",
+    "The Servo Project Developers",
+]
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/Manishearth/triomphe"
 description = "A fork of std::sync::Arc with some extra functionality and without weak references (originally servo_arc)"
@@ -13,6 +16,7 @@ rust-version = "1.76"
 
 [features]
 std = []
+unstable = []
 default = ["serde", "stable_deref_trait", "std"]
 
 [package.metadata.docs.rs]

src/header.rs

@@ -203,9 +203,7 @@ impl From<String> for Arc<str> {
     }
 }
 
-// FIXME: once `pointer::with_metadata_of` is stable or
-//        implementable on stable without assuming ptr layout
-//        this will be able to accept `T: ?Sized`.
+#[cfg(not(feature = "unstable"))]
 impl<T> From<Box<T>> for Arc<T> {
     fn from(b: Box<T>) -> Self {
         let layout = Layout::for_value::<T>(&b);
@@ -240,6 +238,43 @@ impl<T> From<Box<T>> for Arc<T> {
     }
 }
 
+#[cfg(feature = "unstable")]
+impl<T: ?Sized> From<Box<T>> for Arc<T> {
+    fn from(b: Box<T>) -> Self {
+        let src = Box::into_raw(b);
+        unsafe {
+            // Safety: `src` is a valid pointer
+            let layout = Layout::for_value_raw(src);
+
+            // Safety: the closure only changes the type of the pointer
+            let inner = Self::allocate_for_layout(layout, |mem| {
+                mem.with_metadata_of(src as *const ArcInner<T>) as *mut ArcInner<T>
+            });
+
+            // Safety: inner is a valid pointer, so this can't go out of bounds
+            let dst = addr_of_mut!((*inner.as_ptr()).data);
+
+            // Safety:
+            // - `src` is valid for reads (got from `Box`)
+            // - `dst` is valid for writes (just allocated)
+            // - `src` and `dst` don't overlap (separate allocations)
+            ptr::copy_nonoverlapping(src as *const u8, dst as *mut u8, layout.size());
+
+            // Deallocate box without dropping `T`
+            //
+            // Safety:
+            // - `src` has been got from `Box::into_raw`
+            // - `ManuallyDrop<T>` is guaranteed to have the same layout as `T`
+            drop(Box::<ManuallyDrop<T>>::from_raw(src as _));
+
+            Arc {
+                p: inner,
+                phantom: PhantomData,
+            }
+        }
+    }
+}
+
 impl<T> From<Vec<T>> for Arc<[T]> {
     fn from(v: Vec<T>) -> Self {
         Arc::from_header_and_vec((), v).into()
@@ -383,6 +418,14 @@ mod tests {
         );
     }
 
+    #[cfg(feature = "unstable")]
+    #[test]
+    fn from_dyn_box() {
+        let b = String::from("xxx").into_boxed_str();
+        let b = Arc::<str>::from(b);
+        assert_eq!(&*b, "xxx");
+    }
+
     /// It’s possible to make a generic `Arc` wrapper that supports both:
     ///
     /// * `T: !Sized`

src/lib.rs

@@ -20,6 +20,7 @@
 
 #![allow(missing_docs)]
 #![cfg_attr(not(feature = "std"), no_std)]
+#![cfg_attr(feature = "unstable", feature(layout_for_ptr, set_ptr_value))]
 
 extern crate alloc;
 #[cfg(feature = "std")]