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Better closure error message

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Use tracked data introduced in #42196 to provide a better closure
error message by showing why a closure implements `FnOnce`.

```
error[E0525]: expected a closure that implements the `Fn` trait, but
this closure only implements `FnOnce`
 --> $DIR/issue_26046.rs:4:19
  |
4 |       let closure = move || {
  |  ___________________^
5 | |         vec
6 | |     };
  | |_____^
  |
note: closure is `FnOnce` because it moves the variable `vec` out of
its environment
 --> $DIR/issue_26046.rs:5:9
  |
5 |         vec
  |         ^^^

error: aborting due to previous error(s)
```

Fixes #26046
This commit is contained in:
Tommy Ip 2017-06-05 11:14:48 +01:00
parent c94a9ac8ae
commit 9d188e680e
1 changed files with 27 additions and 5 deletions
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32
src/librustc/traits/error_reporting.rs
View File

@ -29,7 +29,7 @@ use hir::{self, intravisit, Local, Pat, Body};
use hir::intravisit::{Visitor, NestedVisitorMap};
use hir::map::NodeExpr;
use hir::def_id::DefId;
use infer::{self, InferCtxt};
use infer::{self, InferCtxt, InferTables, InferTablesRef};
use infer::type_variable::TypeVariableOrigin;
use rustc::lint::builtin::EXTRA_REQUIREMENT_IN_IMPL;
use std::fmt;
@ -640,16 +640,38 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
ty::Predicate::ClosureKind(closure_def_id, kind) => {
let found_kind = self.closure_kind(closure_def_id).unwrap();
let closure_span = self.tcx.hir.span_if_local(closure_def_id).unwrap();
let node_id = self.tcx.hir.as_local_node_id(closure_def_id).unwrap();
let mut err = struct_span_err!(
self.tcx.sess, closure_span, E0525,
"expected a closure that implements the `{}` trait, \
but this closure only implements `{}`",
kind,
found_kind);
err.span_note(
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)),
InferTables::InProgress(tables) =>
Some(InferTablesRef::InProgress(tables.borrow())),
InferTables::Missing => None,
};
if let Some(tables) = infer_tables {
if let Some(&(ty::ClosureKind::FnOnce, Some((span, name)))) =
tables.closure_kinds.get(&node_id)
{
err.span_note(
span,
&format!("closure is `FnOnce` because it moves the \
variable `{}` out of its environment", name));
}
} else {
err.span_note(
obligation.cause.span,
&format!("the requirement to implement `{}` \
derives from here", kind));
}
err.emit();
return;
}