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When moving out of a for loop head, suggest borrowing it #59195

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merged 2 commits into from
Mar 25, 2019
Merged

When moving out of a for loop head, suggest borrowing it #59195

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ae883dc
When moving out of a for loop head, suggest borrowing it
estebank Mar 14, 2019
66202c1
Add nll test
estebank Mar 24, 2019
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3 changes: 2 additions & 1 deletion src/librustc/hir/lowering.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -4334,13 +4334,14 @@ impl<'a> LoweringContext<'a> {
// }

// expand <head>
let head = self.lower_expr(head);
let mut head = self.lower_expr(head);
let head_sp = head.span;
let desugared_span = self.mark_span_with_reason(
CompilerDesugaringKind::ForLoop,
head_sp,
None,
);
head.span = desugared_span;

let iter = self.str_to_ident("iter");

Expand Down
14 changes: 14 additions & 0 deletions src/librustc_borrowck/borrowck/mod.rs
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Expand Up @@ -34,6 +34,7 @@ use std::fmt;
use std::rc::Rc;
use rustc_data_structures::sync::Lrc;
use std::hash::{Hash, Hasher};
use syntax::source_map::CompilerDesugaringKind;
use syntax_pos::{MultiSpan, Span};
use errors::{Applicability, DiagnosticBuilder, DiagnosticId};
use log::debug;
Expand Down Expand Up @@ -744,6 +745,19 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
},
moved_lp.ty));
}
if let (Some(CompilerDesugaringKind::ForLoop), Ok(snippet)) = (
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We might want to use a separate CompilerDesugaringKind variant to differentiate in errors, something along the lines of ForLoopHead, or something, or modify ForLoop to carry information on wether the source is completely synthetic or just moving around code that the user has written.

move_span.compiler_desugaring_kind(),
self.tcx.sess.source_map().span_to_snippet(move_span),
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Nit: not sure whether span_to_snippet is expensive or not, but if it is, them it may make sense to nest if lets to avoid pessimizing.

) {
if !snippet.starts_with("&") {
err.span_suggestion(
move_span,
"consider borrowing this to avoid moving it into the for loop",
format!("&{}", snippet),
Applicability::MaybeIncorrect,
);
}
}

// Note: we used to suggest adding a `ref binding` or calling
// `clone` but those suggestions have been removed because
Expand Down
24 changes: 24 additions & 0 deletions src/test/ui/suggestions/borrow-for-loop-head.nll.stderr
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Original file line number Diff line number Diff line change
@@ -0,0 +1,24 @@
error[E0505]: cannot move out of `a` because it is borrowed
--> $DIR/borrow-for-loop-head.rs:4:18
|
LL | for i in &a {
| --
| |
| borrow of `a` occurs here
| borrow later used here
LL | for j in a {
| ^ move out of `a` occurs here

error[E0382]: use of moved value: `a`
--> $DIR/borrow-for-loop-head.rs:4:18
|
LL | let a = vec![1, 2, 3];
| - move occurs because `a` has type `std::vec::Vec<i32>`, which does not implement the `Copy` trait
LL | for i in &a {
LL | for j in a {
| ^ value moved here, in previous iteration of loop

error: aborting due to 2 previous errors

Some errors occurred: E0382, E0505.
For more information about an error, try `rustc --explain E0382`.
10 changes: 10 additions & 0 deletions src/test/ui/suggestions/borrow-for-loop-head.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,10 @@
fn main() {
let a = vec![1, 2, 3];
for i in &a {
for j in a {
//~^ ERROR cannot move out of `a` because it is borrowed
//~| ERROR use of moved value: `a`
println!("{} * {} = {}", i, j, i * j);
}
}
}
24 changes: 24 additions & 0 deletions src/test/ui/suggestions/borrow-for-loop-head.stderr
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Original file line number Diff line number Diff line change
@@ -0,0 +1,24 @@
error[E0505]: cannot move out of `a` because it is borrowed
--> $DIR/borrow-for-loop-head.rs:4:18
|
LL | for i in &a {
| - borrow of `a` occurs here
LL | for j in a {
| ^ move out of `a` occurs here

error[E0382]: use of moved value: `a`
--> $DIR/borrow-for-loop-head.rs:4:18
|
LL | for j in a {
| ^ value moved here in previous iteration of loop
|
= note: move occurs because `a` has type `std::vec::Vec<i32>`, which does not implement the `Copy` trait
help: consider borrowing this to avoid moving it into the for loop
|
LL | for j in &a {
| ^^

error: aborting due to 2 previous errors

Some errors occurred: E0382, E0505.
For more information about an error, try `rustc --explain E0382`.