diff options
| -rw-r--r-- | src/libcore/intrinsics.rs | 81 |
1 files changed, 46 insertions, 35 deletions
diff --git a/src/libcore/intrinsics.rs b/src/libcore/intrinsics.rs index 1b52ea33d37..c7ea5e9da64 100644 --- a/src/libcore/intrinsics.rs +++ b/src/libcore/intrinsics.rs @@ -278,65 +278,74 @@ extern "rust-intrinsic" { /// Moves a value out of scope without running drop glue. pub fn forget<T>(_: T) -> (); - /// Bitcasts a value of one type to another. Both types must have the same - /// size. + /// Reinterprets the bits of a value of one type as another type. Both types + /// must have the same size. Neither the original, nor the result, may be an + /// invalid value, or else you'll have UB on your hands. /// /// `transmute::<T, U>(t)` is semantically equivalent to the following: /// /// ``` + /// // assuming that T and U are the same size /// fn transmute<T, U>(t: T) -> U { - /// let u: U = std::mem::uninitialized(); - /// std::ptr::copy_nonoverlapping(&t as *const T as *const u8, - /// &mut u as *mut U as *mut u8, - /// std::mem::size_of::<T>()); - /// std::mem::forget(t); - /// u + /// let u: U = std::mem::uninitialized(); + /// std::ptr::copy_nonoverlapping(&t as *const T as *const u8, + /// &mut u as *mut U as *mut u8, + /// std::mem::size_of::<T>()); + /// std::mem::forget(t); + /// u /// } /// ``` /// - /// `transmute` is incredibly unsafe. There are an incredible number of ways - /// to cause undefined behavior with this function. `transmute` should be + /// `transmute` is incredibly unsafe. There are a vast number of ways to + /// cause undefined behavior with this function. `transmute` should be /// the absolute last resort. /// /// The following is more complete documentation. Read it before using /// `transmute`: /// [nomicon](https://doc.rust-lang.org/nomicon/transmutes.html) /// - /// # Examples + /// # Alternatives + /// + /// There are very few good cases for `transmute`. Most can be achieved + /// through other means. Some more or less common uses, and a better way, + /// are as follows: /// /// ``` /// use std::mem; /// - /// let slice: &[u8] = unsafe { mem::transmute::<&str, &[u8]>("Rust") }; - /// assert_eq!(slice, [82, 117, 115, 116]); - /// // this is not a good way to do this. - /// // use .as_bytes() - /// let slice = "Rust".as_bytes(); - /// assert_eq!(slice, [82, 117, 115, 116]); - /// // Or, just use a byte string - /// assert_eq!(b"Rust", [82, 117, 116, 116]); - /// ``` + /// // turning a pointer into a usize + /// let ptr = &0; + /// let ptr_num_transmute = std::mem::transmute::<&i32, usize>(ptr); + /// // now with more `as` + /// let ptr_num_cast = ptr as *const i32 as usize; /// - /// There are very few good cases for `transmute`. Most can be achieved - /// through other means. Some commone uses, and the less unsafe way, are as - /// follows: /// - /// ``` /// // Turning a *mut T into an &mut T /// let ptr: *mut i32 = &mut 0; - /// let reF_transmuted = std::mem::transmute::<*mut i32, &mut i32>(ptr); + /// let ref_transmuted = std::mem::transmute::<*mut i32, &mut i32>(ptr); + /// // Use reborrows /// let ref_casted = &mut *ptr; - /// ``` /// - /// ``` + /// /// // Turning an &mut T into an &mut U /// let ptr = &mut 0; /// let val_transmuted = std::mem::transmute::<&mut i32, &mut u32>(ptr); - /// // There is a better way, using `as` and reborrowing: - /// let val_casts = &mut *(ptr as *mut T as *mut U); - /// ``` + /// // Reborrowing continues to play a role here, but now we add `as` casts + /// let val_casts = &mut *(ptr as *mut i32 as *mut u32); + /// + /// + /// // Turning an `&str` into an `&[u8]` + /// let slice = unsafe { mem::transmute::<&str, &[u8]>("Rust") }; + /// assert_eq!(slice, [82, 117, 115, 116]); + /// // this is not a good way to do this. + /// // use .as_bytes() + /// let slice = "Rust".as_bytes(); + /// assert_eq!(slice, [82, 117, 115, 116]); + /// // Or, just use a byte string, if you have control over the string + /// // literal + /// assert_eq!(b"Rust", [82, 117, 116, 116]); + /// /// - /// ``` /// // Copying an `&mut T` to reslice: /// fn split_at_mut_transmute<T>(slice: &mut [T], index: usize) /// -> (&mut [T], &mut [T]) { @@ -345,7 +354,7 @@ extern "rust-intrinsic" { /// let slice2 = std::mem::transmute::<&mut [T], &mut [T]>(slice); /// (slice[0..index], slice2[index..len]) /// } - /// // or: + /// // Again, use `as` and reborrowing /// fn split_at_mut_casts<T>(slice: &mut [T], index: usize) /// -> (&mut [T], &mut [T]) { /// let len = slice.len(); @@ -355,12 +364,15 @@ extern "rust-intrinsic" { /// } /// ``` /// - /// There are valid uses of transmute. + /// # Examples + /// + /// There are valid uses of transmute, though they are few and far between. /// /// ``` /// // getting the bitpattern of a floating point type /// let x = std::mem::transmute::<f32, u32>(0.0/0.0) /// + /// /// // turning a pointer into a function pointer /// // in file.c: `int foo(void) { ... }` /// let handle: *mut libc::c_void = libc::dlopen( @@ -372,6 +384,7 @@ extern "rust-intrinsic" { /// extern fn() -> libc::c_int>(foo); /// println!("{}", foo()); /// + /// /// // extending an invariant lifetime; this is advanced, very unsafe rust /// struct T<'a>(&'a i32); /// let value = 0; @@ -379,8 +392,6 @@ extern "rust-intrinsic" { /// let ptr = &mut t; /// let ptr_extended = std::mem::transmute::<&mut T, &mut T<'static>>(ptr); /// ``` - /// - /// But these are few and far between. #[stable(feature = "rust1", since = "1.0.0")] pub fn transmute<T, U>(e: T) -> U; |