Introduce move expressions (move($expr))

[TaKO8Ki]

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This is an experimental first version of move expressions.

This first version implements it just in plain closures. A support for coroutine closures will be added in follow up pull requests.

RFC: will be added later
Tracking issue: #155050
Project goal:

• Ergonomic ref-counting rust-project-goals#107
• https://rust-lang.github.io/rust-project-goals/2026/ergonomic-rc.html

r? @nikomatsakis

[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

The parser was modified, potentially altering the grammar of (stable) Rust
which would be a breaking change.

cc @fmease

Some changes occurred in src/tools/rustfmt

cc @rust-lang/rustfmt

[nikomatsakis]

under my proposed change, this would be Vec<Option<NodeMap<_>>>

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[nikomatsakis]

Looking good! Left a few nits.

View changes since this review

[nikomatsakis]

Would still like a comment :)

[nikomatsakis]

all of these methods with very similar names (lower_expr_closure_expr, lower_expr_closure, etc) could use with some comments showing how they relate to one another

[nikomatsakis]

Suggested change

	/// Stack of `move(...)` collection states. A plain closure body pushes
	/// Stack of `move(...)` collection states. A plain closure body pushes

[nikomatsakis]

Suggested change

	outer();
	// `outer` captures `x` by *reference* (and `inner` takes ownership of a clone)
	println!("{x");
	outer();

I expect that this will wind up with outer capturing x by ref, right? let's add something to demonstrate this by having println!("{x}");

[nikomatsakis]

I would also want a compile-fail test like

let mut x = String::from("hello");

let outer = || {
    let inner = || move(x.clone());
    let y = inner();
    assert_eq!(y, "hello");
    assert_eq!(x, "hello");
};

x.push("more test"); //~ ERROR

outer();

[nikomatsakis]

Suggested change

	let x = String::from("hello");
	let x = Arc::new(String::from("hello"));

[nikomatsakis]

we could try printing the ref count here and there to check?

[nikomatsakis]

I think this could be its own file

[nikomatsakis]

let's add coroutine tests (which will error for now), unless they already exist.