delegation: move statements out of the first arg by aerooneqq · Pull Request #157248 · rust-lang/rust

aerooneqq · 2026-06-01T12:47:34Z

This PR moves statements that precede final block expression out of the first argument, so we can apply adjustments to final block expression in more cases.

This changes behavior a bit, for example if we specify an empty block:

fnb<C>(e:C){}reuse b {}// Desugaring:fnb<C>(arg0:_) -> _{b::<C>(arg0)}

So we generated the first arg and block is no-op.

In general scenario:

traitTrait:Sized{//~ WARN trait `Trait` is never usedfnby_value(self,x:i32) -> i32{ x }fnby_ref(&self,x:i32) -> i32{ x }fnby_mut_ref(&mutself,x:i32) -> i32{ x }}structF;//~ WARN struct `F` is never constructedimplTraitforF{}structS(F);//~ WARN struct `S` is never constructedimplTraitforS{
reuse <FasTrait>::*{
println!();// Final expression is adjusted even if it is in a block.self.0}}// Desugaring:#[attr = Inline(Hint)]fnby_value(self:_,arg1:_)
->
_{// Final expression is adjusted even if it is in a block.{::std::io::_print(format_arguments::from_str("\n"));};
<FasTrait>::by_value(self.0, arg1)}#[attr = Inline(Hint)]fnby_ref(self:_,arg1:_)
->
_{{::std::io::_print(format_arguments::from_str("\n"));};
<FasTrait>::by_ref(self.0, arg1)}#[attr = Inline(Hint)]fnby_mut_ref(self:_,arg1:_)
->
_{{::std::io::_print(format_arguments::from_str("\n"));};
<FasTrait>::by_mut_ref(self.0, arg1)}

If we change target expression to

reuse <FasTrait>::*{println!();let x = 1;}// Desugaring:#[attr = Inline(Hint)]fnby_value(self:_,arg1:_)
->
_{{::std::io::_print(format_arguments::from_str("\n"));};let x = 1;
<FasTrait>::by_value(self, arg1)}

Once again we generated default arguments for function and block is no-op now (meaning it does not affect callee arguments).

Part of #118212.
r? @petrochenkov

aerooneqq · 2026-06-03T06:18:12Z

@rustbot ready

petrochenkov · 2026-06-03T10:04:37Z

r=me after squashing commits.
@rustbot author
@bors delegate+

rust-bors · 2026-06-03T10:04:40Z

✌️ @aerooneqq, you can now approve this pull request!

If @petrochenkov told you to "r=me" after making some further change, then please make that change and post @bors r=petrochenkov.

View changes since this delegation.

rustbot · 2026-06-03T10:04:41Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

aerooneqq · 2026-06-03T11:09:55Z

@bors r=petrochenkov

rust-bors · 2026-06-03T11:09:59Z

📌 Commit 28cfce7 has been approved by petrochenkov

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 5. This pull request will be tested once the tree is reopened.

@petrochenkov

…t-of-first-arg, r=petrochenkov delegation: move statements out of the first arg This PR moves statements that precede final block expression out of the first argument, so we can apply adjustments to final block expression in more cases. This changes behavior a bit, for example if we specify an empty block: ```rust fn b<C>(e: C) {} reuse b {} // Desugaring: fn b<C>(arg0: _) -> _ { b::<C>(arg0) } ``` So we generated the first arg and block is no-op. In general scenario: ```rust trait Trait: Sized { //~ WARN trait `Trait` is never used fn by_value(self, x: i32) -> i32 { x } fn by_ref(&self, x: i32) -> i32 { x } fn by_mut_ref(&mut self, x: i32) -> i32 { x } } struct F; //~ WARN struct `F` is never constructed impl Trait for F {} struct S(F); //~ WARN struct `S` is never constructed impl Trait for S { reuse <F as Trait>::* { println!(); // Final expression is adjusted even if it is in a block. self.0 } } // Desugaring: #[attr = Inline(Hint)] fn by_value(self: _, arg1: _) -> _ { // Final expression is adjusted even if it is in a block. { ::std::io::_print(format_arguments::from_str("\n")); }; <F as Trait>::by_value(self.0, arg1) } #[attr = Inline(Hint)] fn by_ref(self: _, arg1: _) -> _ { { ::std::io::_print(format_arguments::from_str("\n")); }; <F as Trait>::by_ref(self.0, arg1) } #[attr = Inline(Hint)] fn by_mut_ref(self: _, arg1: _) -> _ { { ::std::io::_print(format_arguments::from_str("\n")); }; <F as Trait>::by_mut_ref(self.0, arg1) } ``` If we change target expression to ```rust reuse <F as Trait>::* { println!(); let x = 1; } // Desugaring: #[attr = Inline(Hint)] fn by_value(self: _, arg1: _) -> _ { { ::std::io::_print(format_arguments::from_str("\n")); }; let x = 1; <F as Trait>::by_value(self, arg1) } ``` Once again we generated default arguments for function and block is no-op now (meaning it does not affect callee arguments). Part of rust-lang#118212. r? @petrochenkov

…uwer Rollup of 15 pull requests Successful merges: - #155763 (Promotes 5 Thumb-mode bare-metal Arm targets to Tier 2) - #156953 (delegation: emit error when there is an infer lifetime in user-specified args) - #157248 (delegation: move statements out of the first arg) - #157263 (rustc_codegen_ssa: Refactor `ArchiveEntry` to include entry kind) - #157311 (Use weak linkage for EII defaults) - #156089 (Fix unused_parens for pinned reference patterns) - #156928 (Remove -Zemscripten-wasm-eh) - #157236 (Reorganize `tests/ui/issues` [3/N]) - #157287 (Const generics: remove AliasTerm::kind(), and small fixes) - #157294 (Split coroutine layout computation to its own file) - #157328 (windows: Elide division-by-zero checks in Instant::now()) - #157331 (Rewrite target checking for `#[link]`) - #157336 (Enable `clippy::mem_replace_with_default`) - #157362 (Fix trivial wf module argument/doc comment name mismatches) - #157364 (Rewrite target checking of `rustc_dummy`) Failed merges: - #157332 (Rewrite target checking for `#[sanitize]`)

@petrochenkov

Rollup merge of #157248 - aerooneqq:delegation-statements-out-of-first-arg, r=petrochenkov delegation: move statements out of the first arg This PR moves statements that precede final block expression out of the first argument, so we can apply adjustments to final block expression in more cases. This changes behavior a bit, for example if we specify an empty block: ```rust fn b<C>(e: C) {} reuse b {} // Desugaring: fn b<C>(arg0: _) -> _ { b::<C>(arg0) } ``` So we generated the first arg and block is no-op. In general scenario: ```rust trait Trait: Sized { //~ WARN trait `Trait` is never used fn by_value(self, x: i32) -> i32 { x } fn by_ref(&self, x: i32) -> i32 { x } fn by_mut_ref(&mut self, x: i32) -> i32 { x } } struct F; //~ WARN struct `F` is never constructed impl Trait for F {} struct S(F); //~ WARN struct `S` is never constructed impl Trait for S { reuse <F as Trait>::* { println!(); // Final expression is adjusted even if it is in a block. self.0 } } // Desugaring: #[attr = Inline(Hint)] fn by_value(self: _, arg1: _) -> _ { // Final expression is adjusted even if it is in a block. { ::std::io::_print(format_arguments::from_str("\n")); }; <F as Trait>::by_value(self.0, arg1) } #[attr = Inline(Hint)] fn by_ref(self: _, arg1: _) -> _ { { ::std::io::_print(format_arguments::from_str("\n")); }; <F as Trait>::by_ref(self.0, arg1) } #[attr = Inline(Hint)] fn by_mut_ref(self: _, arg1: _) -> _ { { ::std::io::_print(format_arguments::from_str("\n")); }; <F as Trait>::by_mut_ref(self.0, arg1) } ``` If we change target expression to ```rust reuse <F as Trait>::* { println!(); let x = 1; } // Desugaring: #[attr = Inline(Hint)] fn by_value(self: _, arg1: _) -> _ { { ::std::io::_print(format_arguments::from_str("\n")); }; let x = 1; <F as Trait>::by_value(self, arg1) } ``` Once again we generated default arguments for function and block is no-op now (meaning it does not affect callee arguments). Part of #118212. r? @petrochenkov

…uwer Rollup of 15 pull requests Successful merges: - rust-lang/rust#155763 (Promotes 5 Thumb-mode bare-metal Arm targets to Tier 2) - rust-lang/rust#156953 (delegation: emit error when there is an infer lifetime in user-specified args) - rust-lang/rust#157248 (delegation: move statements out of the first arg) - rust-lang/rust#157263 (rustc_codegen_ssa: Refactor `ArchiveEntry` to include entry kind) - rust-lang/rust#157311 (Use weak linkage for EII defaults) - rust-lang/rust#156089 (Fix unused_parens for pinned reference patterns) - rust-lang/rust#156928 (Remove -Zemscripten-wasm-eh) - rust-lang/rust#157236 (Reorganize `tests/ui/issues` [3/N]) - rust-lang/rust#157287 (Const generics: remove AliasTerm::kind(), and small fixes) - rust-lang/rust#157294 (Split coroutine layout computation to its own file) - rust-lang/rust#157328 (windows: Elide division-by-zero checks in Instant::now()) - rust-lang/rust#157331 (Rewrite target checking for `#[link]`) - rust-lang/rust#157336 (Enable `clippy::mem_replace_with_default`) - rust-lang/rust#157362 (Fix trivial wf module argument/doc comment name mismatches) - rust-lang/rust#157364 (Rewrite target checking of `rustc_dummy`) Failed merges: - rust-lang/rust#157332 (Rewrite target checking for `#[sanitize]`)

rustbot assigned petrochenkov Jun 1, 2026

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 Jun 1, 2026

aerooneqq changed the title ~~delegation: move out statements out of the first arg~~delegation: move statements out of the first argJun 1, 2026

aerooneqq force-pushed the delegation-statements-out-of-first-arg branch from aff0ceb to ba754c4Compare June 2, 2026 05:57

aerooneqq marked this pull request as ready for review June 2, 2026 05:59

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jun 2, 2026

petrochenkov added the F-fn_delegation `#![feature(fn_delegation)]` label Jun 2, 2026

petrochenkov reviewed Jun 2, 2026
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petrochenkov 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 Jun 2, 2026

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jun 3, 2026

rustbot removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jun 3, 2026

rustbot added the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Jun 3, 2026

Move out statements out of delegation's callee first arg
28cfce7

aerooneqq force-pushed the delegation-statements-out-of-first-arg branch from 8f6baf5 to 28cfce7Compare June 3, 2026 11:09

rust-borsBot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jun 3, 2026

JonathanBrouwer mentioned this pull request Jun 3, 2026
Rollup of 15 pull requests #157375
Closed

petrochenkov mentioned this pull request Jun 3, 2026
delegation: do not always generate first argument #156798
Merged

JonathanBrouwer mentioned this pull request Jun 3, 2026
Rollup of 15 pull requests #157376
Merged

rust-borsBot merged commit d7c1249 into rust-lang:mainJun 3, 2026
12 checks passed

rustbot added this to the 1.98.0 milestone Jun 3, 2026

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delegation: move statements out of the first arg - #157248

delegation: move statements out of the first arg#157248
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