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Add union justifications to conflicting borrows.
This commit adds justifications to error messages for conflicting borrows of union fields. Where previously an error message would say ``cannot borrow `u.b` as mutable..``, it now says ``cannot borrow `u` (via `u.b`) as mutable..``.
1 parent 79d8a0f commit 69bded2

7 files changed

+141
-127
lines changed

‎src/librustc_mir/borrow_check/error_reporting.rs

+48-10
Original file line numberDiff line numberDiff line change
@@ -329,10 +329,16 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
329329
"closure"
330330
};
331331

332-
let desc_place = self.describe_place(place).unwrap_or_else(|| "_".to_owned());
333-
let tcx = self.infcx.tcx;
334-
335-
let first_borrow_desc;
332+
let (desc_place, msg_place, msg_borrow) = if issued_borrow.borrowed_place == *place {
333+
let desc_place = self.describe_place(place).unwrap_or_else(|| "_".to_owned());
334+
(desc_place, "".to_string(), "".to_string())
335+
} else {
336+
let (desc_place, msg_place) = self.describe_place_for_conflicting_borrow(place);
337+
let (_, msg_borrow) = self.describe_place_for_conflicting_borrow(
338+
&issued_borrow.borrowed_place
339+
);
340+
(desc_place, msg_place, msg_borrow)
341+
};
336342

337343
let explanation = self.explain_why_borrow_contains_point(context, issued_borrow, None);
338344
let second_borrow_desc = if explanation.is_explained() {
@@ -342,6 +348,8 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
342348
};
343349

344350
// FIXME: supply non-"" `opt_via` when appropriate
351+
let tcx = self.infcx.tcx;
352+
let first_borrow_desc;
345353
let mut err = match (
346354
gen_borrow_kind,
347355
"immutable",
@@ -355,12 +363,12 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
355363
tcx.cannot_reborrow_already_borrowed(
356364
span,
357365
&desc_place,
358-
"",
366+
&msg_place,
359367
lft,
360368
issued_span,
361369
"it",
362370
rgt,
363-
"",
371+
&msg_borrow,
364372
None,
365373
Origin::Mir,
366374
)
@@ -370,12 +378,12 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
370378
tcx.cannot_reborrow_already_borrowed(
371379
span,
372380
&desc_place,
373-
"",
381+
&msg_place,
374382
lft,
375383
issued_span,
376384
"it",
377385
rgt,
378-
"",
386+
&msg_borrow,
379387
None,
380388
Origin::Mir,
381389
)
@@ -386,9 +394,9 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
386394
tcx.cannot_mutably_borrow_multiply(
387395
span,
388396
&desc_place,
389-
"",
397+
&msg_place,
390398
issued_span,
391-
"",
399+
&msg_borrow,
392400
None,
393401
Origin::Mir,
394402
)
@@ -518,6 +526,36 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
518526
err.buffer(&mut self.errors_buffer);
519527
}
520528

529+
/// Returns a description of a place and an associated message for the purposes of conflicting
530+
/// borrow diagnostics.
531+
///
532+
/// If the borrow is of the field `b` of a union `u`, then the return value will be
533+
/// `("u", " (via \`u.b\`)")`. Otherwise, for some variable `a`, the return value will be
534+
/// `("a", "")`.
535+
pub(super) fn describe_place_for_conflicting_borrow(
536+
&self,
537+
place: &Place<'tcx>,
538+
) -> (String, String) {
539+
place.base_local()
540+
.filter(|local| {
541+
// Filter out non-unions.
542+
self.mir.local_decls[*local].ty
543+
.ty_adt_def()
544+
.map(|adt| adt.is_union())
545+
.unwrap_or(false)
546+
})
547+
.and_then(|local| {
548+
let desc_base = self.describe_place(&Place::Local(local))
549+
.unwrap_or_else(|| "_".to_owned());
550+
let desc_original = self.describe_place(place)
551+
.unwrap_or_else(|| "_".to_owned());
552+
return Some((desc_base, format!(" (via `{}`)", desc_original)));
553+
})
554+
.unwrap_or_else(|| {
555+
(self.describe_place(place).unwrap_or_else(|| "_".to_owned()), "".to_string())
556+
})
557+
}
558+
521559
/// Reports StorageDeadOrDrop of `place` conflicts with `borrow`.
522560
///
523561
/// This means that some data referenced by `borrow` needs to live

‎src/test/ui/borrowck/borrowck-union-borrow.ast.nll.stderr renamed to ‎src/test/ui/borrowck/borrowck-union-borrow.nll.stderr

+34-45
Original file line numberDiff line numberDiff line change
@@ -1,132 +1,121 @@
11
error[E0502]: cannot borrow `u.a` as mutable because it is also borrowed as immutable
2-
--> $DIR/borrowck-union-borrow.rs:27:23
2+
--> $DIR/borrowck-union-borrow.rs:25:23
33
|
44
LL | let ra = &u.a;
55
| ---- immutable borrow occurs here
6-
LL | let rma = &mut u.a; //[ast]~ ERROR cannot borrow `u.a` as mutable because it is also borrowed as immutable
6+
LL | let rma = &mut u.a; //~ ERROR cannot borrow `u.a` as mutable because it is also borrowed as immutable
77
| ^^^^^^^^ mutable borrow occurs here
8-
LL | //[mir]~^ ERROR cannot borrow `u.a` as mutable because it is also borrowed as immutable
98
LL | drop(ra);
109
| -- immutable borrow later used here
1110

1211
error[E0506]: cannot assign to `u.a` because it is borrowed
13-
--> $DIR/borrowck-union-borrow.rs:33:13
12+
--> $DIR/borrowck-union-borrow.rs:30:13
1413
|
1514
LL | let ra = &u.a;
1615
| ---- borrow of `u.a` occurs here
17-
LL | u.a = 1; //[ast]~ ERROR cannot assign to `u.a` because it is borrowed
16+
LL | u.a = 1; //~ ERROR cannot assign to `u.a` because it is borrowed
1817
| ^^^^^^^ assignment to borrowed `u.a` occurs here
19-
LL | //[mir]~^ ERROR cannot assign to `u.a` because it is borrowed
2018
LL | drop(ra);
2119
| -- borrow later used here
2220

23-
error[E0502]: cannot borrow `u.b` as mutable because it is also borrowed as immutable
24-
--> $DIR/borrowck-union-borrow.rs:50:23
21+
error[E0502]: cannot borrow `u` (via `u.b`) as mutable because it is also borrowed as immutable (via `u.a`)
22+
--> $DIR/borrowck-union-borrow.rs:46:23
2523
|
2624
LL | let ra = &u.a;
27-
| ---- immutable borrow occurs here
28-
LL | let rmb = &mut u.b; //[ast]~ ERROR cannot borrow `u` (via `u.b`) as mutable because `u` is also borrowed as immutable (via `u.a`)
29-
| ^^^^^^^^ mutable borrow occurs here
30-
LL | //[mir]~^ ERROR cannot borrow `u.b` as mutable because it is also borrowed as immutable
25+
| ---- immutable borrow occurs here (via `u.a`)
26+
LL | let rmb = &mut u.b; //~ ERROR cannot borrow `u` (via `u.b`) as mutable because `u` is also borrowed as immutable (via `u.a`)
27+
| ^^^^^^^^ mutable borrow occurs here (via `u.b`)
3128
LL | drop(ra);
3229
| -- immutable borrow later used here
3330

3431
error[E0506]: cannot assign to `u.b` because it is borrowed
35-
--> $DIR/borrowck-union-borrow.rs:56:13
32+
--> $DIR/borrowck-union-borrow.rs:51:13
3633
|
3734
LL | let ra = &u.a;
3835
| ---- borrow of `u.b` occurs here
39-
LL | u.b = 1; //[ast]~ ERROR cannot assign to `u.b` because it is borrowed
36+
LL | u.b = 1; //~ ERROR cannot assign to `u.b` because it is borrowed
4037
| ^^^^^^^ assignment to borrowed `u.b` occurs here
41-
LL | //[mir]~^ ERROR cannot assign to `u.b` because it is borrowed
4238
LL | drop(ra);
4339
| -- borrow later used here
4440

4541
error[E0502]: cannot borrow `u.a` as immutable because it is also borrowed as mutable
46-
--> $DIR/borrowck-union-borrow.rs:63:22
42+
--> $DIR/borrowck-union-borrow.rs:57:22
4743
|
4844
LL | let rma = &mut u.a;
4945
| -------- mutable borrow occurs here
50-
LL | let ra = &u.a; //[ast]~ ERROR cannot borrow `u.a` as immutable because it is also borrowed as mutable
46+
LL | let ra = &u.a; //~ ERROR cannot borrow `u.a` as immutable because it is also borrowed as mutable
5147
| ^^^^ immutable borrow occurs here
52-
LL | //[mir]~^ ERROR cannot borrow `u.a` as immutable because it is also borrowed as mutable
5348
LL | drop(rma);
5449
| --- mutable borrow later used here
5550

5651
error[E0503]: cannot use `u.a` because it was mutably borrowed
57-
--> $DIR/borrowck-union-borrow.rs:69:21
52+
--> $DIR/borrowck-union-borrow.rs:62:21
5853
|
5954
LL | let ra = &mut u.a;
6055
| -------- borrow of `u.a` occurs here
61-
LL | let a = u.a; //[ast]~ ERROR cannot use `u.a` because it was mutably borrowed
56+
LL | let a = u.a; //~ ERROR cannot use `u.a` because it was mutably borrowed
6257
| ^^^ use of borrowed `u.a`
63-
LL | //[mir]~^ ERROR cannot use `u.a` because it was mutably borrowed
6458
LL | drop(ra);
6559
| -- borrow later used here
6660

6761
error[E0499]: cannot borrow `u.a` as mutable more than once at a time
68-
--> $DIR/borrowck-union-borrow.rs:75:24
62+
--> $DIR/borrowck-union-borrow.rs:67:24
6963
|
7064
LL | let rma = &mut u.a;
7165
| -------- first mutable borrow occurs here
72-
LL | let rma2 = &mut u.a; //[ast]~ ERROR cannot borrow `u.a` as mutable more than once at a time
66+
LL | let rma2 = &mut u.a; //~ ERROR cannot borrow `u.a` as mutable more than once at a time
7367
| ^^^^^^^^ second mutable borrow occurs here
74-
LL | //[mir]~^ ERROR cannot borrow `u.a` as mutable more than once at a time
7568
LL | drop(rma);
7669
| --- first borrow later used here
7770

7871
error[E0506]: cannot assign to `u.a` because it is borrowed
79-
--> $DIR/borrowck-union-borrow.rs:81:13
72+
--> $DIR/borrowck-union-borrow.rs:72:13
8073
|
8174
LL | let rma = &mut u.a;
8275
| -------- borrow of `u.a` occurs here
83-
LL | u.a = 1; //[ast]~ ERROR cannot assign to `u.a` because it is borrowed
76+
LL | u.a = 1; //~ ERROR cannot assign to `u.a` because it is borrowed
8477
| ^^^^^^^ assignment to borrowed `u.a` occurs here
85-
LL | //[mir]~^ ERROR cannot assign to `u.a` because it is borrowed
8678
LL | drop(rma);
8779
| --- borrow later used here
8880

89-
error[E0502]: cannot borrow `u.b` as immutable because it is also borrowed as mutable
90-
--> $DIR/borrowck-union-borrow.rs:88:22
81+
error[E0502]: cannot borrow `u` (via `u.b`) as immutable because it is also borrowed as mutable (via `u.a`)
82+
--> $DIR/borrowck-union-borrow.rs:78:22
9183
|
9284
LL | let rma = &mut u.a;
93-
| -------- mutable borrow occurs here
94-
LL | let rb = &u.b; //[ast]~ ERROR cannot borrow `u` (via `u.b`) as immutable because `u` is also borrowed as mutable (via `u.a`)
95-
| ^^^^ immutable borrow occurs here
96-
LL | //[mir]~^ ERROR cannot borrow `u.b` as immutable because it is also borrowed as mutable
85+
| -------- mutable borrow occurs here (via `u.a`)
86+
LL | let rb = &u.b; //~ ERROR cannot borrow `u` (via `u.b`) as immutable because `u` is also borrowed as mutable (via `u.a`)
87+
| ^^^^ immutable borrow occurs here (via `u.b`)
9788
LL | drop(rma);
9889
| --- mutable borrow later used here
9990

10091
error[E0503]: cannot use `u.b` because it was mutably borrowed
101-
--> $DIR/borrowck-union-borrow.rs:94:21
92+
--> $DIR/borrowck-union-borrow.rs:83:21
10293
|
10394
LL | let ra = &mut u.a;
10495
| -------- borrow of `u.a` occurs here
105-
LL | let b = u.b; //[ast]~ ERROR cannot use `u.b` because it was mutably borrowed
96+
LL | let b = u.b; //~ ERROR cannot use `u.b` because it was mutably borrowed
10697
| ^^^ use of borrowed `u.a`
107-
...
98+
LL |
10899
LL | drop(ra);
109100
| -- borrow later used here
110101

111-
error[E0499]: cannot borrow `u.b` as mutable more than once at a time
112-
--> $DIR/borrowck-union-borrow.rs:101:24
102+
error[E0499]: cannot borrow `u` (via `u.b`) as mutable more than once at a time
103+
--> $DIR/borrowck-union-borrow.rs:89:24
113104
|
114105
LL | let rma = &mut u.a;
115-
| -------- first mutable borrow occurs here
116-
LL | let rmb2 = &mut u.b; //[ast]~ ERROR cannot borrow `u` (via `u.b`) as mutable more than once at a time
117-
| ^^^^^^^^ second mutable borrow occurs here
118-
LL | //[mir]~^ ERROR cannot borrow `u.b` as mutable more than once at a time
106+
| -------- first mutable borrow occurs here (via `u.a`)
107+
LL | let rmb2 = &mut u.b; //~ ERROR cannot borrow `u` (via `u.b`) as mutable more than once at a time
108+
| ^^^^^^^^ second mutable borrow occurs here (via `u.b`)
119109
LL | drop(rma);
120110
| --- first borrow later used here
121111

122112
error[E0506]: cannot assign to `u.b` because it is borrowed
123-
--> $DIR/borrowck-union-borrow.rs:107:13
113+
--> $DIR/borrowck-union-borrow.rs:94:13
124114
|
125115
LL | let rma = &mut u.a;
126116
| -------- borrow of `u.b` occurs here
127-
LL | u.b = 1; //[ast]~ ERROR cannot assign to `u.b` because it is borrowed
117+
LL | u.b = 1; //~ ERROR cannot assign to `u.b` because it is borrowed
128118
| ^^^^^^^ assignment to borrowed `u.b` occurs here
129-
LL | //[mir]~^ ERROR cannot assign to `u.b` because it is borrowed
130119
LL | drop(rma);
131120
| --- borrow later used here
132121

‎src/test/ui/borrowck/borrowck-union-borrow.rs

+12-26
Original file line numberDiff line numberDiff line change
@@ -1,6 +1,4 @@
11
// ignore-tidy-linelength
2-
// revisions: ast mir
3-
//[mir]compile-flags: -Z borrowck=mir
42

53
#[derive(Clone, Copy)]
64
union U {
@@ -24,14 +22,12 @@ fn main() {
2422
}
2523
{
2624
let ra = &u.a;
27-
let rma = &mut u.a; //[ast]~ ERROR cannot borrow `u.a` as mutable because it is also borrowed as immutable
28-
//[mir]~^ ERROR cannot borrow `u.a` as mutable because it is also borrowed as immutable
25+
let rma = &mut u.a; //~ ERROR cannot borrow `u.a` as mutable because it is also borrowed as immutable
2926
drop(ra);
3027
}
3128
{
3229
let ra = &u.a;
33-
u.a = 1; //[ast]~ ERROR cannot assign to `u.a` because it is borrowed
34-
//[mir]~^ ERROR cannot assign to `u.a` because it is borrowed
30+
u.a = 1; //~ ERROR cannot assign to `u.a` because it is borrowed
3531
drop(ra);
3632
}
3733
// Imm borrow, other field
@@ -47,65 +43,55 @@ fn main() {
4743
}
4844
{
4945
let ra = &u.a;
50-
let rmb = &mut u.b; //[ast]~ ERROR cannot borrow `u` (via `u.b`) as mutable because `u` is also borrowed as immutable (via `u.a`)
51-
//[mir]~^ ERROR cannot borrow `u.b` as mutable because it is also borrowed as immutable
46+
let rmb = &mut u.b; //~ ERROR cannot borrow `u` (via `u.b`) as mutable because `u` is also borrowed as immutable (via `u.a`)
5247
drop(ra);
5348
}
5449
{
5550
let ra = &u.a;
56-
u.b = 1; //[ast]~ ERROR cannot assign to `u.b` because it is borrowed
57-
//[mir]~^ ERROR cannot assign to `u.b` because it is borrowed
51+
u.b = 1; //~ ERROR cannot assign to `u.b` because it is borrowed
5852
drop(ra);
5953
}
6054
// Mut borrow, same field
6155
{
6256
let rma = &mut u.a;
63-
let ra = &u.a; //[ast]~ ERROR cannot borrow `u.a` as immutable because it is also borrowed as mutable
64-
//[mir]~^ ERROR cannot borrow `u.a` as immutable because it is also borrowed as mutable
57+
let ra = &u.a; //~ ERROR cannot borrow `u.a` as immutable because it is also borrowed as mutable
6558
drop(rma);
6659
}
6760
{
6861
let ra = &mut u.a;
69-
let a = u.a; //[ast]~ ERROR cannot use `u.a` because it was mutably borrowed
70-
//[mir]~^ ERROR cannot use `u.a` because it was mutably borrowed
62+
let a = u.a; //~ ERROR cannot use `u.a` because it was mutably borrowed
7163
drop(ra);
7264
}
7365
{
7466
let rma = &mut u.a;
75-
let rma2 = &mut u.a; //[ast]~ ERROR cannot borrow `u.a` as mutable more than once at a time
76-
//[mir]~^ ERROR cannot borrow `u.a` as mutable more than once at a time
67+
let rma2 = &mut u.a; //~ ERROR cannot borrow `u.a` as mutable more than once at a time
7768
drop(rma);
7869
}
7970
{
8071
let rma = &mut u.a;
81-
u.a = 1; //[ast]~ ERROR cannot assign to `u.a` because it is borrowed
82-
//[mir]~^ ERROR cannot assign to `u.a` because it is borrowed
72+
u.a = 1; //~ ERROR cannot assign to `u.a` because it is borrowed
8373
drop(rma);
8474
}
8575
// Mut borrow, other field
8676
{
8777
let rma = &mut u.a;
88-
let rb = &u.b; //[ast]~ ERROR cannot borrow `u` (via `u.b`) as immutable because `u` is also borrowed as mutable (via `u.a`)
89-
//[mir]~^ ERROR cannot borrow `u.b` as immutable because it is also borrowed as mutable
78+
let rb = &u.b; //~ ERROR cannot borrow `u` (via `u.b`) as immutable because `u` is also borrowed as mutable (via `u.a`)
9079
drop(rma);
9180
}
9281
{
9382
let ra = &mut u.a;
94-
let b = u.b; //[ast]~ ERROR cannot use `u.b` because it was mutably borrowed
95-
//[mir]~^ ERROR cannot use `u.b` because it was mutably borrowed
83+
let b = u.b; //~ ERROR cannot use `u.b` because it was mutably borrowed
9684

9785
drop(ra);
9886
}
9987
{
10088
let rma = &mut u.a;
101-
let rmb2 = &mut u.b; //[ast]~ ERROR cannot borrow `u` (via `u.b`) as mutable more than once at a time
102-
//[mir]~^ ERROR cannot borrow `u.b` as mutable more than once at a time
89+
let rmb2 = &mut u.b; //~ ERROR cannot borrow `u` (via `u.b`) as mutable more than once at a time
10390
drop(rma);
10491
}
10592
{
10693
let rma = &mut u.a;
107-
u.b = 1; //[ast]~ ERROR cannot assign to `u.b` because it is borrowed
108-
//[mir]~^ ERROR cannot assign to `u.b` because it is borrowed
94+
u.b = 1; //~ ERROR cannot assign to `u.b` because it is borrowed
10995
drop(rma);
11096
}
11197
}

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