Make more fns/macros const, update MSRV to 1.61.0

Make more functions and the `transmute!` macro const. This requires
updating our MSRV to 1.61.0.

While we're doing this, document our MSRV in the crate root and our
`README.md`.

.github/workflows/ci.yml

@@ -14,13 +14,13 @@ jobs:
       matrix:
         # See `INTERNAL.md` for an explanation of these pinned toolchain
         # versions.
-        channel: [ "1.56.1", "1.64.0", "nightly-2022-09-26" ]
+        channel: [ "1.61.0", "1.64.0", nightly-2022-09-26 ]
         target: [ "i686-unknown-linux-gnu", "x86_64-unknown-linux-gnu", "arm-unknown-linux-gnueabi", "aarch64-unknown-linux-gnu", "powerpc-unknown-linux-gnu", "powerpc64-unknown-linux-gnu", "wasm32-wasi" ]
         features: [ "" , "alloc,simd", "alloc,simd,simd-nightly" ]
         exclude:
           # Exclude any combination which uses a non-nightly toolchain but
           # enables nightly features.
-          - channel: "1.56.1"
+          - channel: "1.61.0"
             features: "alloc,simd,simd-nightly"
           - channel: "1.64.0"
             features: "alloc,simd,simd-nightly"

INTERNAL.md

@@ -9,12 +9,12 @@ locations.
 ## CI and toolchain versions
 
 In CI (`.github/workflows/ci.yml`), we pin to specific versions or dates of the
-stable and nightly toolchains. The reason is twofold: First, `zerocopy-derive`'s
-UI tests (see `zerocopy-derive/tests/trybuild.rs`) depend on the format of
-rustc's error messages, and that format can change between toolchain versions
-(we also maintain multiple copies of our UI tests - one for each toolchain
-version pinned in CI - for this reason). Second, not all nightlies have a
-working Miri, so we need to pin to one that does (see
+stable and nightly toolchains. The reason is twofold: First, our UI tests (see
+`tests/trybuild.rs` and `zerocopy-derive/tests/trybuild.rs`) depend on the
+format of rustc's error messages, and that format can change between toolchain
+versions (we also maintain multiple copies of our UI tests - one for each
+toolchain version pinned in CI - for this reason). Second, not all nightlies
+have a working Miri, so we need to pin to one that does (see
 https://rust-lang.github.io/rustup-components-history/).
 
 Updating the versions pinned in CI may cause the UI tests to break. In order to

README.md

@@ -41,6 +41,10 @@ packed SIMD vectors][simd-layout].
 which are only available on nightly. Since these types are unstable, support
 for any type may be removed at any point in the future.
 
+## Minimum Supported Rust Version (MSRV)
+
+zerocopy's MSRV is 1.61.0.
+
 [simd-layout]: https://rust-lang.github.io/unsafe-code-guidelines/layout/packed-simd-vectors.html
 
 ## Dislcaimer

src/lib.rs

@@ -44,6 +44,10 @@
 //! which are only available on nightly. Since these types are unstable, support
 //! for any type may be removed at any point in the future.
 //!
+//! # Minimum Supported Rust Version (MSRV)
+//!
+//! zerocopy's MSRV is 1.61.0.
+//!
 //! [simd-layout]: https://rust-lang.github.io/unsafe-code-guidelines/layout/packed-simd-vectors.html
 
 #![deny(
@@ -117,7 +121,7 @@ use core::{
     cmp::Ordering,
     fmt::{self, Debug, Display, Formatter},
     marker::PhantomData,
-    mem::{self, MaybeUninit},
+    mem::{self, ManuallyDrop, MaybeUninit},
     num::{
         NonZeroI128, NonZeroI16, NonZeroI32, NonZeroI64, NonZeroI8, NonZeroIsize, NonZeroU128,
         NonZeroU16, NonZeroU32, NonZeroU64, NonZeroU8, NonZeroUsize, Wrapping,
@@ -808,14 +812,29 @@ impl<T: Copy> Clone for Unalign<T> {
 
 impl<T> Unalign<T> {
     /// Constructs a new `Unalign`.
-    pub fn new(val: T) -> Unalign<T> {
+    pub const fn new(val: T) -> Unalign<T> {
         Unalign(val)
     }
 
     /// Consumes `self`, returning the inner `T`.
-    pub fn into_inner(self) -> T {
-        let Unalign(val) = self;
-        val
+    pub const fn into_inner(self) -> T {
+        // Use this instead of `mem::transmute` since the latter can't tell
+        // that `Unalign<T>` and `T` have the same size.
+        union Transmute<T> {
+            u: ManuallyDrop<Unalign<T>>,
+            t: ManuallyDrop<T>,
+        }
+
+        // SAFETY: Since `Unalign` is `#[repr(C, packed)]`, it has the same
+        // layout as `T`. `ManuallyDrop<U>` is guaranteed to have the same
+        // layout as `U`, and so `ManuallyDrop<Unalign<T>>` has the same layout
+        // as `ManuallyDrop<T>`. We do this instead of just destructuring in
+        // order to prevent `Unalign`'s `Drop::drop` from being run, since
+        // dropping is not supported in `const fn`s.
+        //
+        // TODO(https://github.com/rust-lang/rust/issues/73255): Destructure
+        // instead of using unsafe.
+        unsafe { ManuallyDrop::into_inner(Transmute { u: ManuallyDrop::new(self) }.t) }
     }
 
     /// Gets an unaligned raw pointer to the inner `T`.
@@ -832,7 +851,7 @@ impl<T> Unalign<T> {
     /// [`read_unaligned`].
     ///
     /// [`read_unaligned`]: core::ptr::read_unaligned
-    pub fn get_ptr(&self) -> *const T {
+    pub const fn get_ptr(&self) -> *const T {
         ptr::addr_of!(self.0)
     }
 
@@ -850,13 +869,15 @@ impl<T> Unalign<T> {
     /// [`read_unaligned`].
     ///
     /// [`read_unaligned`]: core::ptr::read_unaligned
+    // TODO(https://github.com/rust-lang/rust/issues/57349): Make this `const`.
     pub fn get_mut_ptr(&mut self) -> *mut T {
         ptr::addr_of_mut!(self.0)
     }
 }
 
 impl<T: Copy> Unalign<T> {
     /// Gets a copy of the inner `T`.
+    // TODO(https://github.com/rust-lang/rust/issues/57349): Make this `const`.
     pub fn get(&self) -> T {
         let Unalign(val) = *self;
         val
@@ -904,7 +925,7 @@ macro_rules! transmute {
             // This branch, though never taken, ensures that the type of `e` is
             // `AsBytes` and that the type of this macro invocation expression
             // is `FromBytes`.
-            fn transmute<T: $crate::AsBytes, U: $crate::FromBytes>(_t: T) -> U {
+            const fn transmute<T: $crate::AsBytes, U: $crate::FromBytes>(_t: T) -> U {
                 unreachable!()
             }
             transmute(e)
@@ -2462,6 +2483,12 @@ mod tests {
             }
         }
         let _: () = transmute!(PanicOnDrop(()));
+
+        // Test that `transmute!` is legal in a const context.
+        const ARRAY_OF_U8S: [u8; 8] = [0u8, 1, 2, 3, 4, 5, 6, 7];
+        const ARRAY_OF_ARRAYS: [[u8; 2]; 4] = [[0, 1], [2, 3], [4, 5], [6, 7]];
+        const X: [[u8; 2]; 4] = transmute!(ARRAY_OF_U8S);
+        assert_eq!(X, ARRAY_OF_ARRAYS);
     }
 
     #[test]

tests/trybuild.rs

@@ -1,9 +1,29 @@
-// Copyright 2022 The Fuchsia Authors. All rights reserved.
+// Copyright 2019 The Fuchsia Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// UI tests depend on the exact error messages emitted by rustc, but those error
+// messages are not stable, and sometimes change between Rust versions. Thus, we
+// maintain one set of UI tests for each Rust version that we test in CI, and we
+// pin to specific versions in CI (a specific stable version, a specific date of
+// the nightly compiler, and a specific MSRV). Updating those pinned versions
+// may also require updating these tests.
+// - `tests/ui` - Contains the source of truth for our UI test source files
+//   (`.rs`), and contains `.err` and `.out` files for nightly and beta
+// - `tests/ui-stable` - Contains symlinks to the `.rs` files in `tests/ui`, and
+//   contains `.err` and `.out` files for stable
+// - `tests/ui-msrv` - Contains symlinks to the `.rs` files in `tests/ui`, and
+//   contains `.err` and `.out` files for MSRV
+
+#[rustversion::any(nightly, beta)]
+const SOURCE_FILES_GLOB: &str = "tests/ui/*.rs";
+#[rustversion::all(stable, not(stable(1.61.0)))]
+const SOURCE_FILES_GLOB: &str = "tests/ui-stable/*.rs";
+#[rustversion::stable(1.61.0)]
+const SOURCE_FILES_GLOB: &str = "tests/ui-msrv/*.rs";
+
 #[test]
 fn ui() {
     let t = trybuild::TestCases::new();
-    t.compile_fail("tests/ui/*.rs");
+    t.compile_fail(SOURCE_FILES_GLOB);
 }

tests/ui-msrv/transmute-illegal.rs

@@ -0,0 +1 @@
+../ui/transmute-illegal.rs

tests/ui-msrv/transmute-illegal.stderr

@@ -0,0 +1,24 @@
+error[E0601]: `main` function not found in crate `$CRATE`
+ --> tests/ui-msrv/transmute-illegal.rs:8:76
+  |
+8 | const POINTER_VALUE: usize = zerocopy::transmute!(&0usize as *const usize);
+  |                                                                            ^ consider adding a `main` function to `$DIR/tests/ui-msrv/transmute-illegal.rs`
+
+error[E0277]: the trait bound `*const usize: AsBytes` is not satisfied
+ --> tests/ui-msrv/transmute-illegal.rs:8:30
+  |
+8 | const POINTER_VALUE: usize = zerocopy::transmute!(&0usize as *const usize);
+  |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `AsBytes` is not implemented for `*const usize`
+  |
+  = help: the following implementations were found:
+            <usize as AsBytes>
+            <f32 as AsBytes>
+            <f64 as AsBytes>
+            <i128 as AsBytes>
+          and 10 others
+note: required by a bound in `POINTER_VALUE::transmute`
+ --> tests/ui-msrv/transmute-illegal.rs:8:30
+  |
+8 | const POINTER_VALUE: usize = zerocopy::transmute!(&0usize as *const usize);
+  |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `POINTER_VALUE::transmute`
+  = note: this error originates in the macro `zerocopy::transmute` (in Nightly builds, run with -Z macro-backtrace for more info)

tests/ui-stable/transmute-illegal.rs

@@ -0,0 +1 @@
+../ui/transmute-illegal.rs

tests/ui-stable/transmute-illegal.stderr

@@ -0,0 +1,28 @@
+error[E0601]: `main` function not found in crate `$CRATE`
+ --> tests/ui-stable/transmute-illegal.rs:8:76
+  |
+8 | const POINTER_VALUE: usize = zerocopy::transmute!(&0usize as *const usize);
+  |                                                                            ^ consider adding a `main` function to `$DIR/tests/ui-stable/transmute-illegal.rs`
+
+error[E0277]: the trait bound `*const usize: AsBytes` is not satisfied
+ --> tests/ui-stable/transmute-illegal.rs:8:30
+  |
+8 | const POINTER_VALUE: usize = zerocopy::transmute!(&0usize as *const usize);
+  |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `AsBytes` is not implemented for `*const usize`
+  |
+  = help: the following other types implement trait `AsBytes`:
+            f32
+            f64
+            i128
+            i16
+            i32
+            i64
+            i8
+            isize
+          and 6 others
+note: required by a bound in `POINTER_VALUE::transmute`
+ --> tests/ui-stable/transmute-illegal.rs:8:30
+  |
+8 | const POINTER_VALUE: usize = zerocopy::transmute!(&0usize as *const usize);
+  |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `POINTER_VALUE::transmute`
+  = note: this error originates in the macro `zerocopy::transmute` (in Nightly builds, run with -Z macro-backtrace for more info)

tests/ui/transmute-illegal.rs

@@ -0,0 +1,8 @@
+// Copyright 2022 The Fuchsia Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+extern crate zerocopy;
+
+// It is unsound to inspect the usize value of a pointer during const eval.
+const POINTER_VALUE: usize = zerocopy::transmute!(&0usize as *const usize);

tests/ui/transmute-illegal.stderr

@@ -0,0 +1,31 @@
+error[E0601]: `main` function not found in crate `$CRATE`
+ --> tests/ui/transmute-illegal.rs:8:76
+  |
+8 | const POINTER_VALUE: usize = zerocopy::transmute!(&0usize as *const usize);
+  |                                                                            ^ consider adding a `main` function to `$DIR/tests/ui/transmute-illegal.rs`
+
+error[E0277]: the trait bound `*const usize: AsBytes` is not satisfied
+ --> tests/ui/transmute-illegal.rs:8:30
+  |
+8 | const POINTER_VALUE: usize = zerocopy::transmute!(&0usize as *const usize);
+  |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |                              |
+  |                              the trait `AsBytes` is not implemented for `*const usize`
+  |                              required by a bound introduced by this call
+  |
+  = help: the following other types implement trait `AsBytes`:
+            f32
+            f64
+            i128
+            i16
+            i32
+            i64
+            i8
+            isize
+          and 6 others
+note: required by a bound in `POINTER_VALUE::transmute`
+ --> tests/ui/transmute-illegal.rs:8:30
+  |
+8 | const POINTER_VALUE: usize = zerocopy::transmute!(&0usize as *const usize);
+  |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `POINTER_VALUE::transmute`
+  = note: this error originates in the macro `zerocopy::transmute` (in Nightly builds, run with -Z macro-backtrace for more info)

zerocopy-derive/tests/trybuild.rs

@@ -17,9 +17,9 @@
 
 #[rustversion::any(nightly, beta)]
 const SOURCE_FILES_GLOB: &str = "tests/ui/*.rs";
-#[rustversion::all(stable, not(stable(1.56.1)))]
+#[rustversion::all(stable, not(stable(1.61.0)))]
 const SOURCE_FILES_GLOB: &str = "tests/ui-stable/*.rs";
-#[rustversion::stable(1.56.1)]
+#[rustversion::stable(1.61.0)]
 const SOURCE_FILES_GLOB: &str = "tests/ui-msrv/*.rs";
 
 #[test]

zerocopy-derive/tests/ui-msrv/late_compile_pass.stderr

@@ -32,6 +32,9 @@ error[E0277]: the trait bound `u16: Unaligned` is not satisfied
 38 | #[derive(Unaligned)]
    |          ^^^^^^^^^ the trait `Unaligned` is not implemented for `u16`
    |
+   = help: the following implementations were found:
+             <i8 as Unaligned>
+             <u8 as Unaligned>
 note: required by a bound in `<Unaligned1 as Unaligned>::only_derive_is_allowed_to_implement_this_trait::ImplementsUnaligned`
   --> tests/ui-msrv/late_compile_pass.rs:38:10
    |
@@ -45,6 +48,9 @@ error[E0277]: the trait bound `u16: Unaligned` is not satisfied
 46 | #[derive(Unaligned)]
    |          ^^^^^^^^^ the trait `Unaligned` is not implemented for `u16`
    |
+   = help: the following implementations were found:
+             <i8 as Unaligned>
+             <u8 as Unaligned>
 note: required by a bound in `<Unaligned2 as Unaligned>::only_derive_is_allowed_to_implement_this_trait::ImplementsUnaligned`
   --> tests/ui-msrv/late_compile_pass.rs:46:10
    |
@@ -58,6 +64,9 @@ error[E0277]: the trait bound `u16: Unaligned` is not satisfied
 53 | #[derive(Unaligned)]
    |          ^^^^^^^^^ the trait `Unaligned` is not implemented for `u16`
    |
+   = help: the following implementations were found:
+             <i8 as Unaligned>
+             <u8 as Unaligned>
 note: required by a bound in `<Unaligned3 as Unaligned>::only_derive_is_allowed_to_implement_this_trait::ImplementsUnaligned`
   --> tests/ui-msrv/late_compile_pass.rs:53:10
    |