NLL lacks various special case handling of closures #54976

davidtwco · 2018-10-10T23:30:10Z

Part of #52663.

Firstly, this PR extends existing handling of closures to also support generators.

Second, this PR adds the note found in the AST when a closure is invoked twice and captures a variable by-value:

note: closure cannot be invoked more than once because it moves the variable `dict` out of its environment
  --> $DIR/issue-42065.rs:16:29
   |
LL |         for (key, value) in dict {
   |                             ^^^^

r? @nikomatsakis
cc @pnkfelix

davidtwco · 2018-10-10T23:31:16Z

src/test/ui/issues/issue-12127.nll.stderr

+  --> $DIR/issue-12127.rs:18:39
+   |
+LL |     let f = to_fn_once(move|| do_it(&*x));
+   |                                       ^


This test doesn't have this note in the AST borrow checker. A brief glance made me think it was right but I'm not too sure.

The closure only implements FnOnce because of to_fn_once, otherwise it would be a Fn closure (it doesn't drop/move x when called).

nikomatsakis

Looks good but the labeling of freevars is too eager.

nikomatsakis · 2018-10-16T15:40:04Z

src/librustc_mir/borrow_check/error_reporting.rs

+                                 freevar={:?} upvar_ty={:?}",
+                                freevar, upvar_ty,
+                            );
+                            if !upvar_ty.is_region_ptr() {


This is too broad -- the code is only looking at the type of the value that was captured (and crudely at that), but it needs to take into consideration how the value is used (e.g., the &*x in the issue-12127.rs does not require moving x).

There are a few ways to handle this.

One way would be to go through the point where the closure is created and look at whether it "moves" its arguments. That seems a bit complex though.

Another way would be to consult the closure_kind_origins map in the typeck-tables. This map indicates why a given closure has the kind it does (and in particular it will have an entry if this is due to a user of some freevar).

See the code in librustc_borrowck for an example:

rust/src/librustc_borrowck/borrowck/mod.rs

Lines 704 to 713 in 5ea8eb5

if let Some((span, name)) = self.tables.closure_kind_origins().get(hir_id) {

err.span_note(*span, &format!(

"closure cannot be invoked more than once because \

it moves the variable `{}` out of its environment",

name

));

false

} else {

true

}

(I think that in the case of issue-12127, there will be no entry in the map at all.)

Pushed a fix for this.

nikomatsakis · 2018-10-17T19:20:12Z

@bors r+

kennytm · 2018-10-18T14:07:50Z

@bors p=26

rollup fairness

This commit extends existing special-casing of closures to highlight the use of variables within generators that are causing the generator to borrow them.

This commit adds a note that was present in the AST borrow checker when closures are invoked more than once and have captured variables by-value.

davidtwco · 2018-10-18T16:24:57Z

@bors r=nikomatsakis

bors · 2018-10-18T16:24:58Z

📌 Commit d088edc has been approved by nikomatsakis

bors · 2018-10-18T18:15:00Z

⌛ Testing commit d088edc with merge e7f5d48...

@nikomatsakis

…=nikomatsakis NLL lacks various special case handling of closures Part of #52663. Firstly, this PR extends existing handling of closures to also support generators. Second, this PR adds the note found in the AST when a closure is invoked twice and captures a variable by-value: ```text note: closure cannot be invoked more than once because it moves the variable `dict` out of its environment --> $DIR/issue-42065.rs:16:29 | LL | for (key, value) in dict { | ^^^^ ``` r? @nikomatsakis cc @pnkfelix

bors · 2018-10-18T21:13:30Z

☀️ Test successful - status-appveyor, status-travis
Approved by: nikomatsakis
Pushing e7f5d48 to master...

rust-highfive assigned nikomatsakis Oct 10, 2018

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davidtwco commented Oct 10, 2018

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pnkfelix mentioned this pull request Oct 10, 2018

NLL: need a (new) round of review of deltas between .stderr and .nll.stderr output #52663

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memoryruins added the A-NLL Area: Non Lexical Lifetimes (NLL) label Oct 12, 2018

nikomatsakis requested changes Oct 16, 2018

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davidtwco added 3 commits October 18, 2018 17:48

Extend closure special-casing for generators.

aa70115

This commit extends existing special-casing of closures to highlight the use of variables within generators that are causing the generator to borrow them.

Add by-value captured variable note on second use.

375645a

This commit adds a note that was present in the AST borrow checker when closures are invoked more than once and have captured variables by-value.

Improve check to consider how value is used.

d088edc

davidtwco force-pushed the issue-52663-special-case-closures branch from 270f802 to d088edc Compare October 18, 2018 16:24

bors 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-review Status: Awaiting review from the assignee but also interested parties. labels Oct 18, 2018

bors merged commit d088edc into rust-lang:master Oct 18, 2018

bors mentioned this pull request Oct 18, 2018

Prefer unwrap_or_else to unwrap_or in case of function calls/allocations #55014

Merged

davidtwco deleted the issue-52663-special-case-closures branch October 18, 2018 21:47

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NLL lacks various special case handling of closures #54976

NLL lacks various special case handling of closures #54976

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bors commented Oct 18, 2018

	if let Some((span, name)) = self.tables.closure_kind_origins().get(hir_id) {
	err.span_note(*span, &format!(
	"closure cannot be invoked more than once because \
	it moves the variable `{}` out of its environment",
	name
	));
	false
	} else {
	true
	}

NLL lacks various special case handling of closures #54976

NLL lacks various special case handling of closures #54976

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