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[NLL] Use span of the closure args in free region errors #53088

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merged 2 commits into from
Aug 8, 2018
Merged

[NLL] Use span of the closure args in free region errors #53088

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f72b8a4
Use span of the closure args in free region errors
matthewjasper Aug 5, 2018
b13e3f8
Name return type in free region messages
matthewjasper Aug 6, 2018
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62 changes: 53 additions & 9 deletions src/librustc_mir/borrow_check/nll/region_infer/error_reporting/region_name.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -10,6 +10,7 @@

use borrow_check::nll::region_infer::RegionInferenceContext;
use borrow_check::nll::ToRegionVid;
use borrow_check::nll::universal_regions::DefiningTy;
use rustc::hir;
use rustc::hir::def_id::DefId;
use rustc::infer::InferCtxt;
Expand Down Expand Up @@ -72,7 +73,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
})
.or_else(|| {
self.give_name_if_anonymous_region_appears_in_output(
infcx.tcx, mir, fr, counter, diag)
infcx.tcx, mir, mir_def_id, fr, counter, diag)
})
.unwrap_or_else(|| span_bug!(mir.span, "can't make a name for free region {:?}", fr))
}
Expand Down Expand Up @@ -107,13 +108,46 @@ impl<'tcx> RegionInferenceContext<'tcx> {
},

ty::BoundRegion::BrEnv => {
let closure_span = tcx.hir.span_if_local(mir_def_id).unwrap();
let region_name = self.synthesize_region_name(counter);
diag.span_label(
closure_span,
format!("lifetime `{}` represents the closure body", region_name),
);
Some(region_name)
let mir_node_id = tcx.hir.as_local_node_id(mir_def_id).expect("non-local mir");
let def_ty = self.universal_regions.defining_ty;

if let DefiningTy::Closure(def_id, substs) = def_ty {
let args_span = if let hir::ExprKind::Closure(_, _, _, span, _)
= tcx.hir.expect_expr(mir_node_id).node
{
span
} else {
bug!("Closure is not defined by a closure expr");
};
let region_name = self.synthesize_region_name(counter);
diag.span_label(
args_span,
format!("lifetime `{}` represents this closure's body", region_name),
);

let closure_kind_ty = substs.closure_kind_ty(def_id, tcx);
let note = match closure_kind_ty.to_opt_closure_kind() {
Some(ty::ClosureKind::Fn) => {
"closure implements `Fn`, so references to captured variables \
can't escape the closure"
}
Some(ty::ClosureKind::FnMut) => {
"closure implements `FnMut`, so references to captured variables \
can't escape the closure"
}
Some(ty::ClosureKind::FnOnce) => {
bug!("BrEnv in a `FnOnce` closure");
}
None => bug!("Closure kind not inferred in borrow check"),
};

diag.note(note);

Some(region_name)
} else {
// Can't have BrEnv in functions, constants or generators.
bug!("BrEnv outside of closure.");
}
}

ty::BoundRegion::BrAnon(_) | ty::BoundRegion::BrFresh(_) => None,
Expand Down Expand Up @@ -545,6 +579,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
&self,
tcx: TyCtxt<'_, '_, 'tcx>,
mir: &Mir<'tcx>,
mir_def_id: DefId,
fr: RegionVid,
counter: &mut usize,
diag: &mut DiagnosticBuilder<'_>,
Expand All @@ -558,9 +593,18 @@ impl<'tcx> RegionInferenceContext<'tcx> {
return None;
}

let mir_node_id = tcx.hir.as_local_node_id(mir_def_id).expect("non-local mir");
let args_span = if let hir::ExprKind::Closure(_, _, _, span, _)
= tcx.hir.expect_expr(mir_node_id).node
{
span
} else {
mir.span
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We can do better here, too. mir.span is pretty horrific in all cases. Maybe we leave that for another issue though. We would want to extract (from the HIR) the span of the return type, I suppose. Really, we should do the same "return type is &'1 u32" sort of thing we do for arguments, right?

I wonder if, for closures, we can highlight just the final |?

So it would look like:

error: unsatisfied lifetime constraints
  --> $DIR/E0621-does-not-trigger-for-closures.rs:25:26
   |
LL |     invoke(&x, |a, b| if a > b { a } else { b }); //~ ERROR E0495
   |                    --    ^^^^^ requires that `'1` must outlive `'2`
   |                    ||
   |                    |return type of closure is `&'2 i32`
   |                    has type `&'1 i32`

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This is the code to extract the type name:

rust/src/librustc_mir/borrow_check/nll/region_infer/error_reporting/region_name.rs

Lines 278 to 280 in 4b8089d

let type_name = with_highlight_region(needle_fr, *counter, || {
infcx.extract_type_name(&argument_ty)
});

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To get the final character you can use the end_point method

};

let region_name = self.synthesize_region_name(counter);
diag.span_label(
mir.span,
args_span,
format!("lifetime `{}` appears in return type", region_name),
);

Expand Down
5 changes: 2 additions & 3 deletions src/test/ui/error-codes/E0621-does-not-trigger-for-closures.nll.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -8,9 +8,8 @@ error: unsatisfied lifetime constraints
--> $DIR/E0621-does-not-trigger-for-closures.rs:25:26
|
LL | invoke(&x, |a, b| if a > b { a } else { b }); //~ ERROR E0495
| ----------^^^^^-----------------
| | | |
| | | requires that `'1` must outlive `'2`
| ------ ^^^^^ requires that `'1` must outlive `'2`
| | |
| | has type `&'1 i32`
| lifetime `'2` appears in return type

Expand Down
20 changes: 9 additions & 11 deletions src/test/ui/issue-40510-1.nll.stderr
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Original file line number Diff line number Diff line change
@@ -1,17 +1,15 @@
error: unsatisfied lifetime constraints
--> $DIR/issue-40510-1.rs:18:9
|
LL | || {
| _____-
| |_____|
| ||
LL | || &mut x
| || ^^^^^^ return requires that `'1` must outlive `'2`
LL | || };
| || -
| ||_____|
| |______lifetime `'1` represents the closure body
| lifetime `'2` appears in return type
LL | || {
| --
| |
| lifetime `'1` represents this closure's body
| lifetime `'2` appears in return type
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I think it would be nice to say "in return type of this closure" -- this confused me for quite a bit

LL | &mut x
| ^^^^^^ return requires that `'1` must outlive `'2`
|
= note: closure implements `FnMut`, so references to captured variables can't escape the closure
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also so nice — I do wonder if we can "re-orient" the message around this note though. Something like this:

error: captured variable cannot escape a `FnMut` closure
  --> $DIR/issue-40510-1.rs:18:9
   |
LL |     || {
   |     -- closure implements `FnMut`, so references to captured variables can't escape the closure
LL |         &mut x
   |         ^^^^^^ returns a reference that is local to the closure

error: aborting due to previous error

maybe we should shelve this for a later improvement, though -- in particular, I would really like to do a bit more analysis and connect the message to the variable x. Something like this:

error: captured variable cannot escape a `FnMut` closure
  --> $DIR/issue-40510-1.rs:18:9
   |
LL |     || {
   |     -- closure implements `FnMut`, so references to captured variables can't escape the closure
LL |         &mut x
   |              ^ references to `x` cannot escape the closure body

error: aborting due to previous error


error: aborting due to previous error

25 changes: 11 additions & 14 deletions src/test/ui/issue-40510-3.nll.stderr
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Original file line number Diff line number Diff line change
@@ -1,20 +1,17 @@
error: unsatisfied lifetime constraints
--> $DIR/issue-40510-3.rs:18:9
|
LL | || {
| _____-
| |_____|
| ||
LL | || || {
| ||_________^
LL | ||| x.push(())
LL | ||| }
| |||_________^ requires that `'1` must outlive `'2`
LL | || };
| || -
| ||_____|
| |______lifetime `'1` represents the closure body
| lifetime `'2` appears in return type
LL | || {
| --
| |
| lifetime `'1` represents this closure's body
| lifetime `'2` appears in return type
LL | / || {
LL | | x.push(())
LL | | }
| |_________^ requires that `'1` must outlive `'2`
|
= note: closure implements `FnMut`, so references to captured variables can't escape the closure

error: aborting due to previous error

25 changes: 11 additions & 14 deletions src/test/ui/issue-49824.nll.stderr
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@@ -1,20 +1,17 @@
error: unsatisfied lifetime constraints
--> $DIR/issue-49824.rs:22:9
|
LL | || {
| _____-
| |_____|
| ||
LL | || || {
| ||_________^
LL | ||| let _y = &mut x;
LL | ||| }
| |||_________^ requires that `'1` must outlive `'2`
LL | || };
| || -
| ||_____|
| |______lifetime `'1` represents the closure body
| lifetime `'2` appears in return type
LL | || {
| --
| |
| lifetime `'1` represents this closure's body
| lifetime `'2` appears in return type
LL | / || {
LL | | let _y = &mut x;
LL | | }
| |_________^ requires that `'1` must outlive `'2`
|
= note: closure implements `FnMut`, so references to captured variables can't escape the closure

error: aborting due to previous error

9 changes: 5 additions & 4 deletions src/test/ui/nll/issue-48238.stderr
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Expand Up @@ -2,11 +2,12 @@ error: unsatisfied lifetime constraints
--> $DIR/issue-48238.rs:21:13
|
LL | move || use_val(&orig); //~ ERROR
| --------^^^^^^^^^^^^^^
| | |
| | argument requires that `'1` must outlive `'2`
| lifetime `'1` represents the closure body
| ------- ^^^^^^^^^^^^^^ argument requires that `'1` must outlive `'2`
| |
| lifetime `'1` represents this closure's body
| lifetime `'2` appears in return type
|
= note: closure implements `Fn`, so references to captured variables can't escape the closure

error: aborting due to previous error