Introduce move expressions (`move($expr)`) by TaKO8Ki · Pull Request #155023 · rust-lang/rust

TaKO8Ki · 2026-04-09T02:53:10Z

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:

r? @nikomatsakis

replace TODO with FIXME

rustbot · 2026-04-16T12:26:16Z

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 · 2026-04-16T13:19:37Z

@@ -0,0 +1,12 @@
+error[E0658]: `move(expr)` syntax is experimental


I think it'd be nice to test more of the edge cases around move vs move(...). Something like this:

let x: bool = true; let y: bool = true; let a = move(x) || y; // this should be an error: move(x) outside of closure

let x: bool = true; let y: bool = true; let a = || move(x) || y; // this should successfully parse

let x: bool = true; let y: bool = true; let a = move[x] || y; // this should error

let x: bool = true; let y: bool = true; let a = move || move(x) || y; // this should successfully parse

Trying to think what else might come up. That's all I can think of right now.

View changes since the review

nikomatsakis · 2026-04-16T13:22:10Z

 use crate::errors::{InvalidLegacyConstGenericArg, UseConstGenericArg, YieldInClosure};
 use crate::{AllowReturnTypeNotation, FnDeclKind, ImplTraitPosition, TryBlockScope};

+struct MoveExprOccurrence<'a> {


comment please -- what is this struct for? what are the roles of its fields?

View changes since the review

nikomatsakis · 2026-04-16T13:22:21Z

+    expr: &'a Expr,
+}
+
+struct MoveExprCollector<'a> {


comment please: what does this struct do?

View changes since the review

my preference is to have comments that walk through an example, e.g.,

// Given an expression like this // // ``` // || move(foo.bar).clone() // ``` // // this will pass walk the AST for the closure and collect `move(foo.bar)` expressions into the vector.

nikomatsakis · 2026-04-16T13:23:57Z

+
+impl<'a> Visitor<'a> for MoveExprCollector<'a> {
+    fn visit_expr(&mut self, expr: &'a Expr) {
+        match &expr.kind {


...then here you can refer back to the example...

// If this is `move` expression (`move(foo.bar)` in the example above) push its id into the vector. // Else recurse.

or something lik ethat.

View changes since the review

nikomatsakis · 2026-04-16T13:24:41Z

+                    expr: inner,
+                });
+            }
+            ExprKind::Closure(..) | ExprKind::Gen(..) | ExprKind::ConstBlock(..) => {}


Worth calling this behavior out too: does not recurse into nested closures or const blocks. What about things like...

|| { fn bar() { move(x) } }

...I guess that'd be an error, but still, seems like we should stop recursing at items.

View changes since the review

nikomatsakis · 2026-04-16T13:32:44Z

+    pub explicit_captures: &'hir [ExplicitCapture],
+}
+
+#[derive(Debug, Clone, Copy, HashStable_Generic)]


comment, what does this struct represent in terms of Rust source code/

View changes since the review

nikomatsakis · 2026-04-16T13:32:56Z

+fn main() {
+    let s = String::from("hello");
+    let c = || {
+        let t = move(s);


add a test with more than one move

View changes since the review

nikomatsakis · 2026-04-16T13:34:39Z


        let _ = euv::ExprUseVisitor::new(&closure_fcx, &mut delegate).consume_body(body);

+        let explicit_captures = match self.tcx.hir_node(closure_hir_id).expect_expr().kind {


this paragraph merits a comment

View changes since the review

(...continuing previous store...) so we make it stronger here

nikomatsakis · 2026-04-16T13:36:25Z

@@ -210,6 +210,22 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {

        let _ = euv::ExprUseVisitor::new(&closure_fcx, &mut delegate).consume_body(body);


pre-existing, but this process ought to have a better comment showing the overall flow.

From what I see here, at the end of this phase, we'll have inferred uses based on the actual actions taken in the closure. But that is not strong enough for move($).clone(), which would introduce a variable $m that is only used by-ref, and we want it to capture $m by value.

View changes since the review

nikomatsakis · 2026-04-16T13:37:14Z

+        };
+        for capture in explicit_captures {
+            let place = closure_fcx.place_for_root_variable(closure_def_id, capture.var_hir_id);
+            delegate.capture_information.push((


I have to refresh my memory, I kinda remember there were methods on delegate for like "this place was moved" and so forth-- we are skipping those and directly writing this data structure, is there a helper method we could invoke instead?

View changes since the review

rust-bors · 2026-04-16T21:58:54Z

☔ The latest upstream changes (presumably #155380) made this pull request unmergeable. Please resolve the merge conflicts.

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Apr 9, 2026

TaKO8Ki added 4 commits April 9, 2026 20:55

add move_expr feature flag

17517fd

add move(expr) syntax

9f0f8c8

lower move(expr) in plain closures

9953aa0

force move(expr) captures to ByValue

0dcce73

TaKO8Ki force-pushed the move-expr-1 branch from 1a19a26 to 0dcce73 Compare April 9, 2026 13:05

support ast::ExprKind::Move in clippy

1203e52

rustbot added the T-clippy Relevant to the Clippy team. label Apr 9, 2026

support ast::ExprKind::Move in rustfmt

71d9fed

rustbot added the T-rustfmt Relevant to the rustfmt team, which will review and decide on the PR/issue. label Apr 9, 2026

rust-lang deleted a comment from rust-log-analyzer Apr 9, 2026

TaKO8Ki changed the title ~~Move expressions (move($expr))~~ Introduce move expressions (move($expr)) Apr 9, 2026

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TaKO8Ki mentioned this pull request Apr 9, 2026

Tracking Issue for move expressions (move(expr)) #155050

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TaKO8Ki added 2 commits April 16, 2026 21:25

fix tidy errors

cac3426

replace TODO with FIXME

add ui tests for move expr

5aba0a6

TaKO8Ki force-pushed the move-expr-1 branch from e25c9ae to 5aba0a6 Compare April 16, 2026 12:25

TaKO8Ki marked this pull request as ready for review April 16, 2026 12:26

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Apr 16, 2026

rustbot removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Apr 16, 2026

rustbot assigned nikomatsakis Apr 16, 2026

fmease requested changes Apr 16, 2026

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Comment thread compiler/rustc_ast_passes/src/feature_gate.rs Outdated

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Apr 16, 2026

use pre-expansion feature gate, gate_all! instead

c72426a

nikomatsakis requested changes Apr 16, 2026

View reviewed changes

		@@ -0,0 +1,12 @@
		error[E0658]: `move(expr)` syntax is experimental


		let _ = euv::ExprUseVisitor::new(&closure_fcx, &mut delegate).consume_body(body);

		let explicit_captures = match self.tcx.hir_node(closure_hir_id).expect_expr().kind {

		@@ -210,6 +210,22 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {

		let _ = euv::ExprUseVisitor::new(&closure_fcx, &mut delegate).consume_body(body);

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rust-bors bot commented Apr 16, 2026

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