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Better closure error message #42443

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Merged
merged 6 commits into from
Jun 8, 2017
Merged

Better closure error message #42443

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9d188e6
Better closure error message
tommyip Jun 5, 2017
9cbb5f9
Add ui tests for issue 26046
tommyip Jun 5, 2017
94c808c
Update closure errors to emit context for FnMut
tommyip Jun 6, 2017
2c282b8
Add additional ui tests for issue 26046
tommyip Jun 6, 2017
b1b6490
Group closure context ui tests
tommyip Jun 6, 2017
345b833
Cover all cases in closure errors
tommyip Jun 7, 2017
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11 changes: 6 additions & 5 deletions src/librustc/traits/error_reporting.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -648,6 +648,10 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
kind,
found_kind);

err.span_label(
obligation.cause.span,
format!("the requirement to implement `{}` derives from here", kind));

let infer_tables = match self.tables {
InferTables::Interned(tables) =>
Some(InferTablesRef::Interned(tables)),
Expand All @@ -656,6 +660,8 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
InferTables::Missing => None,
};

// Additional context information explaining why the closure only implements
// a particular trait.
if let Some(tables) = infer_tables {
match tables.closure_kinds.get(&node_id) {
Some(&(ty::ClosureKind::FnOnce, Some((span, name)))) => {
Expand All @@ -670,11 +676,6 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
},
_ => {}
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One minor point -- in this arm, we emit neither the new note nor the old one. Seems like the old one might still be helpful?

Actually, maybe we should always emit the old note -- but instead of a note, just make it a span_label? Seems like it's useful information either way. In other words, the old note tells you "why it has to be Fn", and the new note tells you why it cannot be...

If you agree, maybe move the body of the else below out and above this if, and convert span_note to span_label (so that it prints within one code snippet)

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@tommyip tommyip Jun 6, 2017 •
edited
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Good point, I didn't think about that!

}
} else {
err.span_note(
obligation.cause.span,
&format!("the requirement to implement `{}` \
derives from here", kind));
}

err.emit();
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3 changes: 3 additions & 0 deletions src/test/ui/closure_context/issue-26046-fn-mut.stderr
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Expand Up @@ -6,6 +6,9 @@ error[E0525]: expected a closure that implements the `Fn` trait, but this closur
15 | | num += 1;
16 | | };
| |_____^
17 |
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cc @estebank -- @tommyip mentioned that there is a trailing space after this 17 | -- this doesn't bother me per se, but is that just how the diagnostics output works in general?

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@nikomatsakis that space is to pad to compensate for the multiline markers. We can add a check for empty lines and avoid it.

18 | Box::new(closure)
| ----------------- the requirement to implement `Fn` derives from here
|
note: closure is `FnMut` because it mutates the variable `num` here
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slick

--> $DIR/issue-26046-fn-mut.rs:15:9
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3 changes: 3 additions & 0 deletions src/test/ui/closure_context/issue-26046-fn-once.stderr
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Expand Up @@ -6,6 +6,9 @@ error[E0525]: expected a closure that implements the `Fn` trait, but this closur
15 | | vec
16 | | };
| |_____^
17 |
18 | Box::new(closure)
| ----------------- the requirement to implement `Fn` derives from here
|
note: closure is `FnOnce` because it moves the variable `vec` out of its environment
--> $DIR/issue-26046-fn-once.rs:15:9
Expand Down