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Suggest .as_ref() when appropriate for Option and Result 
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4 files changed

+165
-47
lines changed

4 files changed

+165
-47
lines changed

‎src/librustc/infer/error_reporting/mod.rs

+74
Original file line numberDiff line numberDiff line change
@@ -956,6 +956,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
956956
diag.span_label(span, message);
957957
}
958958
}
959+
self.suggest_as_ref_where_appropriate(span, &exp_found, diag);
959960
}
960961

961962
diag.note_expected_found(&"type", expected, found);
@@ -972,6 +973,79 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
972973
self.note_error_origin(diag, &cause);
973974
}
974975

976+
/// When encountering a case where `.as_ref()` on a `Result` or `Option` would be appropriate,
977+
/// suggest it.
978+
fn suggest_as_ref_where_appropriate(
979+
&self,
980+
span: Span,
981+
exp_found: &ty::error::ExpectedFound<Ty<'tcx>>,
982+
diag: &mut DiagnosticBuilder<'tcx>,
983+
) {
984+
match (&exp_found.expected.sty, &exp_found.found.sty) {
985+
(TyKind::Adt(exp_def, exp_substs), TyKind::Ref(_, found_ty, _)) => {
986+
if let TyKind::Adt(found_def, found_substs) = found_ty.sty {
987+
let path_str = format!("{:?}", exp_def);
988+
if exp_def == &found_def {
989+
let opt_msg = "you can convert from `&Option<T>` to `Option<&T>` using \
990+
`.as_ref()`";
991+
let result_msg = "you can convert from `&Result<T, E>` to \
992+
`Result<&T, &E>` using `.as_ref()`";
993+
let have_as_ref = &[
994+
("std::option::Option", opt_msg),
995+
("core::option::Option", opt_msg),
996+
("std::result::Result", result_msg),
997+
("core::result::Result", result_msg),
998+
];
999+
if let Some(msg) = have_as_ref.iter()
1000+
.filter_map(|(path, msg)| if &path_str == path {
1001+
Some(msg)
1002+
} else {
1003+
None
1004+
}).next()
1005+
{
1006+
let mut show_suggestion = true;
1007+
for (exp_ty, found_ty) in exp_substs.types().zip(found_substs.types()) {
1008+
if let TyKind::Ref(_, exp_ty, _) = exp_ty.sty {
1009+
match (&exp_ty.sty, &found_ty.sty) {
1010+
(TyKind::Adt(exp_did, _), TyKind::Adt(found_did, _))
1011+
if exp_did == found_did => {}
1012+
(TyKind::Bool, TyKind::Bool) |
1013+
(TyKind::Char, TyKind::Char) |
1014+
(TyKind::Str, TyKind::Str) |
1015+
(_, TyKind::Param(_)) |
1016+
(_, TyKind::Infer(_)) |
1017+
(TyKind::Param(_), _) |
1018+
(TyKind::Infer(_), _) => {}
1019+
(TyKind::Int(x), TyKind::Int(y)) if x == y => {}
1020+
(TyKind::Uint(x), TyKind::Uint(y)) if x == y => {}
1021+
(TyKind::Int(x), TyKind::Int(y)) if x == y => {}
1022+
(TyKind::Uint(x), TyKind::Uint(y)) if x == y => {}
1023+
(TyKind::Float(x), TyKind::Float(y)) if x == y => {}
1024+
_ => show_suggestion = false,
1025+
}
1026+
} else {
1027+
show_suggestion = false;
1028+
}
1029+
}
1030+
if let (Ok(snippet), true) = (
1031+
self.tcx.sess.source_map().span_to_snippet(span),
1032+
show_suggestion,
1033+
) {
1034+
diag.span_suggestion_with_applicability(
1035+
span,
1036+
msg,
1037+
format!("{}.as_ref()", snippet),
1038+
Applicability::MachineApplicable,
1039+
);
1040+
}
1041+
}
1042+
}
1043+
}
1044+
}
1045+
_ => {}
1046+
}
1047+
}
1048+
9751049
pub fn report_and_explain_type_error(
9761050
&self,
9771051
trace: TypeTrace<'tcx>,

‎src/test/ui/as-ref.stderr

-47
This file was deleted.

‎src/test/ui/as-ref.rs renamed to ‎src/test/ui/suggestions/as-ref.rs

+10
Original file line numberDiff line numberDiff line change
@@ -12,4 +12,14 @@ fn main() {
1212
//~^ ERROR mismatched types [E0308]
1313
opt.and_then(|arg| Ok(takes_ref(arg)));
1414
//~^ ERROR mismatched types [E0308]
15+
let x: &Option<usize> = &Some(3);
16+
let y: Option<&usize> = x;
17+
//~^ ERROR mismatched types [E0308]
18+
let x: &Result<usize, usize> = &Ok(3);
19+
let y: Result<&usize, &usize> = x;
20+
//~^ ERROR mismatched types [E0308]
21+
// note: do not suggest because of `E: usize`
22+
let x: &Result<usize, usize> = &Ok(3);
23+
let y: Result<&usize, usize> = x;
24+
//~^ ERROR mismatched types [E0308]
1525
}

‎src/test/ui/suggestions/as-ref.stderr

+81
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,81 @@
1+
error[E0308]: mismatched types
2+
--> $DIR/as-ref.rs:6:27
3+
|
4+
LL | opt.map(|arg| takes_ref(arg));
5+
| - ^^^ expected &Foo, found struct `Foo`
6+
| |
7+
| help: consider using `as_ref` instead: `as_ref().`
8+
|
9+
= note: expected type `&Foo`
10+
found type `Foo`
11+
12+
error[E0308]: mismatched types
13+
--> $DIR/as-ref.rs:8:37
14+
|
15+
LL | opt.and_then(|arg| Some(takes_ref(arg)));
16+
| - ^^^ expected &Foo, found struct `Foo`
17+
| |
18+
| help: consider using `as_ref` instead: `as_ref().`
19+
|
20+
= note: expected type `&Foo`
21+
found type `Foo`
22+
23+
error[E0308]: mismatched types
24+
--> $DIR/as-ref.rs:11:27
25+
|
26+
LL | opt.map(|arg| takes_ref(arg));
27+
| - ^^^ expected &Foo, found struct `Foo`
28+
| |
29+
| help: consider using `as_ref` instead: `as_ref().`
30+
|
31+
= note: expected type `&Foo`
32+
found type `Foo`
33+
34+
error[E0308]: mismatched types
35+
--> $DIR/as-ref.rs:13:35
36+
|
37+
LL | opt.and_then(|arg| Ok(takes_ref(arg)));
38+
| - ^^^ expected &Foo, found struct `Foo`
39+
| |
40+
| help: consider using `as_ref` instead: `as_ref().`
41+
|
42+
= note: expected type `&Foo`
43+
found type `Foo`
44+
45+
error[E0308]: mismatched types
46+
--> $DIR/as-ref.rs:16:27
47+
|
48+
LL | let y: Option<&usize> = x;
49+
| ^
50+
| |
51+
| expected enum `std::option::Option`, found reference
52+
| help: you can convert from `&Option<T>` to `Option<&T>` using `.as_ref()`: `x.as_ref()`
53+
|
54+
= note: expected type `std::option::Option<&usize>`
55+
found type `&std::option::Option<usize>`
56+
57+
error[E0308]: mismatched types
58+
--> $DIR/as-ref.rs:19:35
59+
|
60+
LL | let y: Result<&usize, &usize> = x;
61+
| ^ expected enum `std::result::Result`, found reference
62+
|
63+
= note: expected type `std::result::Result<&usize, &usize>`
64+
found type `&std::result::Result<usize, usize>`
65+
help: you can convert from `&Result<T, E>` to `Result<&T, &E>` using `.as_ref()`
66+
|
67+
LL | let y: Result<&usize, &usize> = x.as_ref();
68+
| ^^^^^^^^^^
69+
70+
error[E0308]: mismatched types
71+
--> $DIR/as-ref.rs:23:34
72+
|
73+
LL | let y: Result<&usize, usize> = x;
74+
| ^ expected enum `std::result::Result`, found reference
75+
|
76+
= note: expected type `std::result::Result<&usize, usize>`
77+
found type `&std::result::Result<usize, usize>`
78+
79+
error: aborting due to 7 previous errors
80+
81+
For more information about this error, try `rustc --explain E0308`.

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