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author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-17 12:02:58 +0000 |
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committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-17 12:02:58 +0000 |
commit | 698f8c2f01ea549d77d7dc3338a12e04c11057b9 (patch) | |
tree | 173a775858bd501c378080a10dca74132f05bc50 /compiler/rustc_error_codes/src/error_codes/E0139.md | |
parent | Initial commit. (diff) | |
download | rustc-698f8c2f01ea549d77d7dc3338a12e04c11057b9.tar.xz rustc-698f8c2f01ea549d77d7dc3338a12e04c11057b9.zip |
Adding upstream version 1.64.0+dfsg1.upstream/1.64.0+dfsg1
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'compiler/rustc_error_codes/src/error_codes/E0139.md')
-rw-r--r-- | compiler/rustc_error_codes/src/error_codes/E0139.md | 84 |
1 files changed, 84 insertions, 0 deletions
diff --git a/compiler/rustc_error_codes/src/error_codes/E0139.md b/compiler/rustc_error_codes/src/error_codes/E0139.md new file mode 100644 index 000000000..a116cf293 --- /dev/null +++ b/compiler/rustc_error_codes/src/error_codes/E0139.md @@ -0,0 +1,84 @@ +#### Note: this error code is no longer emitted by the compiler. + +There are various restrictions on transmuting between types in Rust; for example +types being transmuted must have the same size. To apply all these restrictions, +the compiler must know the exact types that may be transmuted. When type +parameters are involved, this cannot always be done. + +So, for example, the following is not allowed: + +``` +use std::mem::transmute; + +struct Foo<T>(Vec<T>); + +fn foo<T>(x: Vec<T>) { + // we are transmuting between Vec<T> and Foo<F> here + let y: Foo<T> = unsafe { transmute(x) }; + // do something with y +} +``` + +In this specific case there's a good chance that the transmute is harmless (but +this is not guaranteed by Rust). However, when alignment and enum optimizations +come into the picture, it's quite likely that the sizes may or may not match +with different type parameter substitutions. It's not possible to check this for +_all_ possible types, so `transmute()` simply only accepts types without any +unsubstituted type parameters. + +If you need this, there's a good chance you're doing something wrong. Keep in +mind that Rust doesn't guarantee much about the layout of different structs +(even two structs with identical declarations may have different layouts). If +there is a solution that avoids the transmute entirely, try it instead. + +If it's possible, hand-monomorphize the code by writing the function for each +possible type substitution. It's possible to use traits to do this cleanly, +for example: + +``` +use std::mem::transmute; + +struct Foo<T>(Vec<T>); + +trait MyTransmutableType: Sized { + fn transmute(_: Vec<Self>) -> Foo<Self>; +} + +impl MyTransmutableType for u8 { + fn transmute(x: Vec<u8>) -> Foo<u8> { + unsafe { transmute(x) } + } +} + +impl MyTransmutableType for String { + fn transmute(x: Vec<String>) -> Foo<String> { + unsafe { transmute(x) } + } +} + +// ... more impls for the types you intend to transmute + +fn foo<T: MyTransmutableType>(x: Vec<T>) { + let y: Foo<T> = <T as MyTransmutableType>::transmute(x); + // do something with y +} +``` + +Each impl will be checked for a size match in the transmute as usual, and since +there are no unbound type parameters involved, this should compile unless there +is a size mismatch in one of the impls. + +It is also possible to manually transmute: + +``` +# use std::ptr; +# let v = Some("value"); +# type SomeType = &'static [u8]; +unsafe { + ptr::read(&v as *const _ as *const SomeType) // `v` transmuted to `SomeType` +} +# ; +``` + +Note that this does not move `v` (unlike `transmute`), and may need a +call to `mem::forget(v)` in case you want to avoid destructors being called. |