-
Notifications
You must be signed in to change notification settings - Fork 12.2k
Commit
This commit does not belong to any branch on this repository, and may belong to a fork outside of the repository.
Auto merge of #51131 - qnighy:unsized-locals, r=eddyb
Implement Unsized Rvalues This PR is the first step to implement RFC1909: unsized rvalues (#48055). ## Implemented - `Sized` is removed for arguments and local bindings. (under `#![feature(unsized_locals)]`) - Unsized locations are allowed in MIR - Unsized places and operands are correctly translated at codegen ## Not implemented in this PR - Additional `Sized` checks: - tuple struct constructor (accidentally compiles now) - closure arguments at closure generation (accidentally compiles now) - upvars (ICEs now) - Generating vtable for `fn method(self)` (ICEs now) - VLAs: `[e; n]` where `n` isn't const - Reduce unnecessary allocations ## Current status - [x] Fix `__rust_probestack` (rust-lang/compiler-builtins#244) - [x] Get the fix merged - [x] `#![feature(unsized_locals)]` - [x] Give it a tracking issue number - [x] Lift sized checks in typeck and MIR-borrowck - [ ] <del>Forbid `A(unsized-expr)`</del> will be another PR - [x] Minimum working codegen - [x] Add more examples and fill in unimplemented codegen paths - [ ] <del>Loosen object-safety rules (will be another PR)</del> - [ ] <del>Implement `Box<FnOnce>` (will be another PR)</del> - [ ] <del>Reduce temporaries (will be another PR)</del>
- Loading branch information
Showing
40 changed files
with
776 additions
and
64 deletions.
There are no files selected for viewing
180 changes: 180 additions & 0 deletions
180
src/doc/unstable-book/src/language-features/unsized-locals.md
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,180 @@ | ||
| # `unsized_locals` | ||
|
|
||
| The tracking issue for this feature is: [#48055] | ||
|
|
||
| [#48055]: https://github.com/rust-lang/rust/issues/48055 | ||
|
|
||
| ------------------------ | ||
|
|
||
| This implements [RFC1909]. When turned on, you can have unsized arguments and locals: | ||
|
|
||
| [RFC1909]: https://github.com/rust-lang/rfcs/blob/master/text/1909-coercions.md | ||
|
|
||
| ```rust | ||
| #![feature(unsized_locals)] | ||
|
|
||
| use std::any::Any; | ||
|
|
||
| fn main() { | ||
| let x: Box<dyn Any> = Box::new(42); | ||
| let x: dyn Any = *x; | ||
| // ^ unsized local variable | ||
| // ^^ unsized temporary | ||
| foo(x); | ||
| } | ||
|
|
||
| fn foo(_: dyn Any) {} | ||
| // ^^^^^^ unsized argument | ||
| ``` | ||
|
|
||
| The RFC still forbids the following unsized expressions: | ||
|
|
||
| ```rust,ignore | ||
| #![feature(unsized_locals)] | ||
| use std::any::Any; | ||
| struct MyStruct<T: ?Sized> { | ||
| content: T, | ||
| } | ||
| struct MyTupleStruct<T: ?Sized>(T); | ||
| fn answer() -> Box<dyn Any> { | ||
| Box::new(42) | ||
| } | ||
| fn main() { | ||
| // You CANNOT have unsized statics. | ||
| static X: dyn Any = *answer(); // ERROR | ||
| const Y: dyn Any = *answer(); // ERROR | ||
| // You CANNOT have struct initialized unsized. | ||
| MyStruct { content: *answer() }; // ERROR | ||
| MyTupleStruct(*answer()); // ERROR | ||
| (42, *answer()); // ERROR | ||
| // You CANNOT have unsized return types. | ||
| fn my_function() -> dyn Any { *answer() } // ERROR | ||
| // You CAN have unsized local variables... | ||
| let mut x: dyn Any = *answer(); // OK | ||
| // ...but you CANNOT reassign to them. | ||
| x = *answer(); // ERROR | ||
| // You CANNOT even initialize them separately. | ||
| let y: dyn Any; // OK | ||
| y = *answer(); // ERROR | ||
| // Not mentioned in the RFC, but by-move captured variables are also Sized. | ||
| let x: dyn Any = *answer(); | ||
| (move || { // ERROR | ||
| let y = x; | ||
| })(); | ||
| // You CAN create a closure with unsized arguments, | ||
| // but you CANNOT call it. | ||
| // This is an implementation detail and may be changed in the future. | ||
| let f = |x: dyn Any| {}; | ||
| f(*answer()); // ERROR | ||
| } | ||
| ``` | ||
|
|
||
| However, the current implementation allows `MyTupleStruct(..)` to be unsized. This will be fixed in the future. | ||
|
|
||
| ## By-value trait objects | ||
|
|
||
| With this feature, you can have by-value `self` arguments without `Self: Sized` bounds. | ||
|
|
||
| ```rust | ||
| #![feature(unsized_locals)] | ||
|
|
||
| trait Foo { | ||
| fn foo(self) {} | ||
| } | ||
|
|
||
| impl<T: ?Sized> Foo for T {} | ||
|
|
||
| fn main() { | ||
| let slice: Box<[i32]> = Box::new([1, 2, 3]); | ||
| <[i32] as Foo>::foo(*slice); | ||
| } | ||
| ``` | ||
|
|
||
| And `Foo` will also be object-safe. However, this object-safety is not yet implemented. | ||
|
|
||
| ```rust,ignore | ||
| #![feature(unsized_locals)] | ||
| trait Foo { | ||
| fn foo(self) {} | ||
| } | ||
| impl<T: ?Sized> Foo for T {} | ||
| fn main () { | ||
| let slice: Box<dyn Foo> = Box::new([1, 2, 3]); | ||
| // doesn't compile yet | ||
| <dyn Foo as Foo>::foo(*slice); | ||
| } | ||
| ``` | ||
|
|
||
| Unfortunately, this is not implemented yet. | ||
|
|
||
| One of the objectives of this feature is to allow `Box<dyn FnOnce>`, instead of `Box<dyn FnBox>` in the future. See [#28796] for details. | ||
|
|
||
| [#28796]: https://github.com/rust-lang/rust/issues/28796 | ||
|
|
||
| ## Variable length arrays | ||
|
|
||
| The RFC also describes an extension to the array literal syntax: `[e; dyn n]`. In the syntax, `n` isn't necessarily a constant expression. The array is dynamically allocated on the stack and has the type of `[T]`, instead of `[T; n]`. | ||
|
|
||
| ```rust,ignore | ||
| #![feature(unsized_locals)] | ||
| fn mergesort<T: Ord>(a: &mut [T]) { | ||
| let mut tmp = [T; dyn a.len()]; | ||
| // ... | ||
| } | ||
| fn main() { | ||
| let mut a = [3, 1, 5, 6]; | ||
| mergesort(&mut a); | ||
| assert_eq!(a, [1, 3, 5, 6]); | ||
| } | ||
| ``` | ||
|
|
||
| VLAs are not implemented yet. The syntax isn't final, either. We may need an alternative syntax for Rust 2015 because, in Rust 2015, expressions like `[e; dyn(1)]` would be ambiguous. One possible alternative proposed in the RFC is `[e; n]`: if `n` captures one or more local variables, then it is considered as `[e; dyn n]`. | ||
|
|
||
| ## Advisory on stack usage | ||
|
|
||
| It's advised not to casually use the `#![feature(unsized_locals)]` feature. Typical use-cases are: | ||
|
|
||
| - When you need a by-value trait objects. | ||
| - When you really need a fast allocation of small temporary arrays. | ||
|
|
||
| Another pitfall is repetitive allocation and temporaries. Currently the compiler simply extends the stack frame every time it encounters an unsized assignment. So for example, the code | ||
|
|
||
| ```rust | ||
| #![feature(unsized_locals)] | ||
|
|
||
| fn main() { | ||
| let x: Box<[i32]> = Box::new([1, 2, 3, 4, 5]); | ||
| let _x = {{{{{{{{{{*x}}}}}}}}}}; | ||
| } | ||
| ``` | ||
|
|
||
| and the code | ||
|
|
||
| ```rust | ||
| #![feature(unsized_locals)] | ||
|
|
||
| fn main() { | ||
| for _ in 0..10 { | ||
| let x: Box<[i32]> = Box::new([1, 2, 3, 4, 5]); | ||
| let _x = *x; | ||
| } | ||
| } | ||
| ``` | ||
|
|
||
| will unnecessarily extend the stack frame. |
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Oops, something went wrong.