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-nursery/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>

1 files changed, 3 insertions, 0 deletions

diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index 3b9e1b3c265..e8245a553eb 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -503,6 +503,9 @@ declare_features! (
 
     // Allows `Self` in type definitions
     (active, self_in_typedefs, "1.30.0", Some(49303), None),
+
+    // unsized rvalues at arguments and parameters
+    (active, unsized_locals, "1.30.0", Some(48055), None),
 );
 
 declare_features! (