Rollup merge of #141811 - mejrs:bye_locals, r=compiler-errors

Unimplement unsized_locals

Implements https://github.com/rust-lang/compiler-team/issues/630

Tracking issue here: https://github.com/rust-lang/rust/issues/111942

Note that this just removes the feature, not the implementation, and does not touch `unsized_fn_params`. This is because it is required to support `Box<dyn FnOnce()>: FnOnce()`.

There may be more that should be removed (possibly in follow up prs)
- the `forget_unsized` function and `forget` intrinsic.
- the `unsized_locals` test directory; I've just fixed up the tests for now
- various codegen support for unsized values and allocas

cc ``@JakobDegen`` ``@oli-obk`` ``@Noratrieb`` ``@programmerjake`` ``@bjorn3``

``@rustbot`` label F-unsized_locals

Fixes rust-lang/rust#79409

2 files changed, 2 insertions, 177 deletions

diff --git a/src/doc/rustc-dev-guide/src/implementing_new_features.md b/src/doc/rustc-dev-guide/src/implementing_new_features.md
index d7561bbbad2..5d0e875cbc1 100644
--- a/src/doc/rustc-dev-guide/src/implementing_new_features.md
+++ b/src/doc/rustc-dev-guide/src/implementing_new_features.md
@@ -156,8 +156,8 @@ a new unstable feature:
    [`incomplete_features` lint]: https://doc.rust-lang.org/rustc/lints/listing/warn-by-default.html#incomplete-features
 
    ```rust ignore
-   /// Allows unsized rvalues at arguments and parameters.
-   (incomplete, unsized_locals, "CURRENT_RUSTC_VERSION", Some(48055), None),
+   /// Allows deref patterns.
+   (incomplete, deref_patterns, "CURRENT_RUSTC_VERSION", Some(87121), None),
    ```
 
    To avoid [semantic merge conflicts], please use `CURRENT_RUSTC_VERSION` instead of `1.70` or
diff --git a/src/doc/unstable-book/src/language-features/unsized-locals.md b/src/doc/unstable-book/src/language-features/unsized-locals.md
deleted file mode 100644
index d5b01a3d616..00000000000
--- a/src/doc/unstable-book/src/language-features/unsized-locals.md
+++ /dev/null
@@ -1,175 +0,0 @@
-# `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-unsized-rvalues.md
-
-```rust
-#![allow(incomplete_features)]
-#![feature(unsized_locals, unsized_fn_params)]
-
-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,compile_fail
-#![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
-}
-```
-
-## By-value trait objects
-
-With this feature, you can have by-value `self` arguments without `Self: Sized` bounds.
-
-```rust
-#![feature(unsized_fn_params)]
-
-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.
-
-```rust
-#![feature(unsized_fn_params)]
-
-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);
-}
-```
-
-One of the objectives of this feature is to allow `Box<dyn FnOnce>`.
-
-## 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 (not-yet-implemented)
-#![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.