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tmiaskotmiasko
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Check if call return type is visibly uninhabited when building MIR
1 parent 4b133a7 commit 6f8a1ee

19 files changed

+167
-98
lines changed

Diff for: ‎compiler/rustc_mir_build/src/build/expr/into.rs

+9-4
Original file line numberDiff line numberDiff line change
@@ -255,10 +255,15 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
255255
func: fun,
256256
args,
257257
cleanup: None,
258-
// FIXME(varkor): replace this with an uninhabitedness-based check.
259-
// This requires getting access to the current module to call
260-
// `tcx.is_ty_uninhabited_from`, which is currently tricky to do.
261-
destination: if expr.ty.is_never() {
258+
// The presence or absence of a return edge affects control-flow sensitive
259+
// MIR checks and ultimately whether code is accepted or not. We can only
260+
// omit the return edge if a return type is visibly uninhabited to a module
261+
// that makes the call.
262+
destination: if this.tcx.is_ty_uninhabited_from(
263+
this.parent_module,
264+
expr.ty,
265+
this.param_env,
266+
) {
262267
None
263268
} else {
264269
Some((destination, success))

Diff for: ‎compiler/rustc_mir_build/src/build/mod.rs

+4-1
Original file line numberDiff line numberDiff line change
@@ -350,6 +350,7 @@ struct Builder<'a, 'tcx> {
350350

351351
def_id: DefId,
352352
hir_id: hir::HirId,
353+
parent_module: DefId,
353354
check_overflow: bool,
354355
fn_span: Span,
355356
arg_count: usize,
@@ -807,15 +808,17 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
807808
);
808809

809810
let lint_level = LintLevel::Explicit(hir_id);
811+
let param_env = tcx.param_env(def.did);
810812
let mut builder = Builder {
811813
thir,
812814
tcx,
813815
infcx,
814816
typeck_results: tcx.typeck_opt_const_arg(def),
815817
region_scope_tree: tcx.region_scope_tree(def.did),
816-
param_env: tcx.param_env(def.did),
818+
param_env,
817819
def_id: def.did.to_def_id(),
818820
hir_id,
821+
parent_module: tcx.parent_module(hir_id).to_def_id(),
819822
check_overflow,
820823
cfg: CFG { basic_blocks: IndexVec::new() },
821824
fn_span: span,

Diff for: ‎src/test/codegen/set-discriminant-invalid.rs

+1-1
Original file line numberDiff line numberDiff line change
@@ -28,7 +28,7 @@ impl IntoError<Error> for Api
2828
#[no_mangle]
2929
fn into_error(self, error: Self::Source) -> Error {
3030
Error::Api {
31-
source: (|v| v)(error),
31+
source: error,
3232
}
3333
}
3434
}

Diff for: ‎src/test/mir-opt/inline/inline_diverging.h.Inline.diff

+24-25
Original file line numberDiff line numberDiff line change
@@ -4,21 +4,22 @@
44
fn h() -> () {
55
let mut _0: (); // return place in scope 0 at $DIR/inline-diverging.rs:21:12: 21:12
66
let _1: (!, !); // in scope 0 at $DIR/inline-diverging.rs:22:5: 22:22
7-
+ let mut _2: fn() -> ! {sleep}; // in scope 0 at $DIR/inline-diverging.rs:22:5: 22:22
8-
+ let mut _9: (); // in scope 0 at $DIR/inline-diverging.rs:27:13: 27:16
9-
+ let mut _10: (); // in scope 0 at $DIR/inline-diverging.rs:28:13: 28:16
7+
+ let mut _2: (!, !); // in scope 0 at $DIR/inline-diverging.rs:22:5: 22:22
8+
+ let mut _3: fn() -> ! {sleep}; // in scope 0 at $DIR/inline-diverging.rs:22:5: 22:22
9+
+ let mut _10: (); // in scope 0 at $DIR/inline-diverging.rs:27:13: 27:16
10+
+ let mut _11: (); // in scope 0 at $DIR/inline-diverging.rs:28:13: 28:16
1011
+ scope 1 (inlined call_twice::<!, fn() -> ! {sleep}>) { // at $DIR/inline-diverging.rs:22:5: 22:22
11-
+ debug f => _2; // in scope 1 at $DIR/inline-diverging.rs:26:36: 26:37
12-
+ let _3: !; // in scope 1 at $DIR/inline-diverging.rs:27:9: 27:10
13-
+ let mut _4: &fn() -> ! {sleep}; // in scope 1 at $DIR/inline-diverging.rs:27:13: 27:14
14-
+ let mut _6: &fn() -> ! {sleep}; // in scope 1 at $DIR/inline-diverging.rs:28:13: 28:14
15-
+ let mut _7: !; // in scope 1 at $DIR/inline-diverging.rs:29:6: 29:7
16-
+ let mut _8: !; // in scope 1 at $DIR/inline-diverging.rs:29:9: 29:10
12+
+ debug f => _3; // in scope 1 at $DIR/inline-diverging.rs:26:36: 26:37
13+
+ let _4: !; // in scope 1 at $DIR/inline-diverging.rs:27:9: 27:10
14+
+ let mut _5: &fn() -> ! {sleep}; // in scope 1 at $DIR/inline-diverging.rs:27:13: 27:14
15+
+ let mut _7: &fn() -> ! {sleep}; // in scope 1 at $DIR/inline-diverging.rs:28:13: 28:14
16+
+ let mut _8: !; // in scope 1 at $DIR/inline-diverging.rs:29:6: 29:7
17+
+ let mut _9: !; // in scope 1 at $DIR/inline-diverging.rs:29:9: 29:10
1718
+ scope 2 {
18-
+ debug a => _3; // in scope 2 at $DIR/inline-diverging.rs:27:9: 27:10
19-
+ let _5: !; // in scope 2 at $DIR/inline-diverging.rs:28:9: 28:10
19+
+ debug a => _4; // in scope 2 at $DIR/inline-diverging.rs:27:9: 27:10
20+
+ let _6: !; // in scope 2 at $DIR/inline-diverging.rs:28:9: 28:10
2021
+ scope 3 {
21-
+ debug b => _5; // in scope 3 at $DIR/inline-diverging.rs:28:9: 28:10
22+
+ debug b => _6; // in scope 3 at $DIR/inline-diverging.rs:28:9: 28:10
2223
+ }
2324
+ scope 6 (inlined <fn() -> ! {sleep} as Fn<()>>::call - shim(fn() -> ! {sleep})) { // at $DIR/inline-diverging.rs:28:13: 28:16
2425
+ scope 7 (inlined sleep) { // at $SRC_DIR/core/src/ops/function.rs:LL:COL
@@ -33,27 +34,25 @@
3334

3435
bb0: {
3536
StorageLive(_1); // scope 0 at $DIR/inline-diverging.rs:22:5: 22:22
36-
- _1 = call_twice::<!, fn() -> ! {sleep}>(sleep) -> bb1; // scope 0 at $DIR/inline-diverging.rs:22:5: 22:22
37+
- call_twice::<!, fn() -> ! {sleep}>(sleep); // scope 0 at $DIR/inline-diverging.rs:22:5: 22:22
3738
+ StorageLive(_2); // scope 0 at $DIR/inline-diverging.rs:22:5: 22:22
38-
+ _2 = sleep; // scope 0 at $DIR/inline-diverging.rs:22:5: 22:22
39+
+ StorageLive(_3); // scope 0 at $DIR/inline-diverging.rs:22:5: 22:22
40+
+ _3 = sleep; // scope 0 at $DIR/inline-diverging.rs:22:5: 22:22
3941
// mir::Constant
4042
- // + span: $DIR/inline-diverging.rs:22:5: 22:15
4143
- // + literal: Const { ty: fn(fn() -> ! {sleep}) -> (!, !) {call_twice::<!, fn() -> ! {sleep}>}, val: Value(Scalar(<ZST>)) }
4244
- // mir::Constant
4345
// + span: $DIR/inline-diverging.rs:22:16: 22:21
4446
// + literal: Const { ty: fn() -> ! {sleep}, val: Value(Scalar(<ZST>)) }
45-
+ StorageLive(_3); // scope 1 at $DIR/inline-diverging.rs:27:9: 27:10
46-
+ StorageLive(_4); // scope 1 at $DIR/inline-diverging.rs:27:13: 27:14
47-
+ _4 = &_2; // scope 1 at $DIR/inline-diverging.rs:27:13: 27:14
48-
+ StorageLive(_9); // scope 1 at $DIR/inline-diverging.rs:27:13: 27:16
49-
+ _9 = const (); // scope 1 at $DIR/inline-diverging.rs:27:13: 27:16
47+
+ StorageLive(_4); // scope 1 at $DIR/inline-diverging.rs:27:9: 27:10
48+
+ StorageLive(_5); // scope 1 at $DIR/inline-diverging.rs:27:13: 27:14
49+
+ _5 = &_3; // scope 1 at $DIR/inline-diverging.rs:27:13: 27:14
50+
+ StorageLive(_10); // scope 1 at $DIR/inline-diverging.rs:27:13: 27:16
51+
+ _10 = const (); // scope 1 at $DIR/inline-diverging.rs:27:13: 27:16
5052
+ goto -> bb1; // scope 5 at $DIR/inline-diverging.rs:39:5: 39:12
51-
}
52-
53-
bb1: {
54-
- StorageDead(_1); // scope 0 at $DIR/inline-diverging.rs:22:22: 22:23
55-
- _0 = const (); // scope 0 at $DIR/inline-diverging.rs:21:12: 23:2
56-
- return; // scope 0 at $DIR/inline-diverging.rs:23:2: 23:2
53+
+ }
54+
+
55+
+ bb1: {
5756
+ goto -> bb1; // scope 5 at $DIR/inline-diverging.rs:39:5: 39:12
5857
}
5958
}

Diff for: ‎src/test/mir-opt/issue_72181_1.main.mir_map.0.mir

+1-1
Original file line numberDiff line numberDiff line change
@@ -21,7 +21,7 @@ fn main() -> () {
2121
StorageLive(_2); // scope 0 at $DIR/issue-72181-1.rs:16:9: 16:10
2222
StorageLive(_3); // scope 2 at $DIR/issue-72181-1.rs:17:41: 17:43
2323
_3 = (); // scope 2 at $DIR/issue-72181-1.rs:17:41: 17:43
24-
_2 = transmute::<(), Void>(move _3) -> [return: bb1, unwind: bb4]; // scope 2 at $DIR/issue-72181-1.rs:17:9: 17:44
24+
transmute::<(), Void>(move _3) -> bb4; // scope 2 at $DIR/issue-72181-1.rs:17:9: 17:44
2525
// mir::Constant
2626
// + span: $DIR/issue-72181-1.rs:17:9: 17:40
2727
// + literal: Const { ty: unsafe extern "rust-intrinsic" fn(()) -> Void {transmute::<(), Void>}, val: Value(Scalar(<ZST>)) }

Diff for: ‎src/test/ui/consts/const-eval/ub-enum.32bit.stderr

+6-16
Original file line numberDiff line numberDiff line change
@@ -119,27 +119,17 @@ LL | const BAD_OPTION_CHAR: Option<(char, char)> = Some(('x', unsafe { mem::tran
119119
78 00 00 00 ff ff ff ff │ x.......
120120
}
121121

122-
error[E0080]: it is undefined behavior to use this value
123-
--> $DIR/ub-enum.rs:92:1
122+
error[E0080]: evaluation of constant value failed
123+
--> $DIR/ub-enum.rs:92:77
124124
|
125125
LL | const BAD_UNINHABITED_WITH_DATA1: Result<(i32, Never), (i32, !)> = unsafe { mem::transmute(0u64) };
126-
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed at .<enum-variant(Ok)>.0.1: encountered a value of uninhabited type Never
127-
|
128-
= note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
129-
= note: the raw bytes of the constant (size: 8, align: 4) {
130-
00 00 00 00 00 00 00 00 │ ........
131-
}
126+
| ^^^^^^^^^^^^^^^^^^^^ transmuting to uninhabited type
132127

133-
error[E0080]: it is undefined behavior to use this value
134-
--> $DIR/ub-enum.rs:94:1
128+
error[E0080]: evaluation of constant value failed
129+
--> $DIR/ub-enum.rs:94:77
135130
|
136131
LL | const BAD_UNINHABITED_WITH_DATA2: Result<(i32, !), (i32, Never)> = unsafe { mem::transmute(0u64) };
137-
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed at .<enum-variant(Ok)>.0.1: encountered a value of the never type `!`
138-
|
139-
= note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
140-
= note: the raw bytes of the constant (size: 8, align: 4) {
141-
00 00 00 00 00 00 00 00 │ ........
142-
}
132+
| ^^^^^^^^^^^^^^^^^^^^ transmuting to uninhabited type
143133

144134
error: aborting due to 13 previous errors
145135

Diff for: ‎src/test/ui/consts/const-eval/ub-enum.64bit.stderr

+6-16
Original file line numberDiff line numberDiff line change
@@ -119,27 +119,17 @@ LL | const BAD_OPTION_CHAR: Option<(char, char)> = Some(('x', unsafe { mem::tran
119119
78 00 00 00 ff ff ff ff │ x.......
120120
}
121121

122-
error[E0080]: it is undefined behavior to use this value
123-
--> $DIR/ub-enum.rs:92:1
122+
error[E0080]: evaluation of constant value failed
123+
--> $DIR/ub-enum.rs:92:77
124124
|
125125
LL | const BAD_UNINHABITED_WITH_DATA1: Result<(i32, Never), (i32, !)> = unsafe { mem::transmute(0u64) };
126-
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed at .<enum-variant(Ok)>.0.1: encountered a value of uninhabited type Never
127-
|
128-
= note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
129-
= note: the raw bytes of the constant (size: 8, align: 4) {
130-
00 00 00 00 00 00 00 00 │ ........
131-
}
126+
| ^^^^^^^^^^^^^^^^^^^^ transmuting to uninhabited type
132127

133-
error[E0080]: it is undefined behavior to use this value
134-
--> $DIR/ub-enum.rs:94:1
128+
error[E0080]: evaluation of constant value failed
129+
--> $DIR/ub-enum.rs:94:77
135130
|
136131
LL | const BAD_UNINHABITED_WITH_DATA2: Result<(i32, !), (i32, Never)> = unsafe { mem::transmute(0u64) };
137-
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed at .<enum-variant(Ok)>.0.1: encountered a value of the never type `!`
138-
|
139-
= note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
140-
= note: the raw bytes of the constant (size: 8, align: 4) {
141-
00 00 00 00 00 00 00 00 │ ........
142-
}
132+
| ^^^^^^^^^^^^^^^^^^^^ transmuting to uninhabited type
143133

144134
error: aborting due to 13 previous errors
145135

Diff for: ‎src/test/ui/consts/const-eval/ub-enum.rs

+2-2
Original file line numberDiff line numberDiff line change
@@ -90,9 +90,9 @@ const BAD_OPTION_CHAR: Option<(char, char)> = Some(('x', unsafe { mem::transmute
9090
// All variants are uninhabited but also have data.
9191
// Use `0` as constant to make behavior endianess-independent.
9292
const BAD_UNINHABITED_WITH_DATA1: Result<(i32, Never), (i32, !)> = unsafe { mem::transmute(0u64) };
93-
//~^ ERROR is undefined behavior
93+
//~^ ERROR evaluation of constant value failed
9494
const BAD_UNINHABITED_WITH_DATA2: Result<(i32, !), (i32, Never)> = unsafe { mem::transmute(0u64) };
95-
//~^ ERROR is undefined behavior
95+
//~^ ERROR evaluation of constant value failed
9696

9797
fn main() {
9898
}

Diff for: ‎src/test/ui/consts/const-eval/validate_uninhabited_zsts.32bit.stderr

+3-6
Original file line numberDiff line numberDiff line change
@@ -10,14 +10,11 @@ LL | unsafe { std::mem::transmute(()) }
1010
LL | const FOO: [Empty; 3] = [foo(); 3];
1111
| ----- inside `FOO` at $DIR/validate_uninhabited_zsts.rs:13:26
1212

13-
error[E0080]: it is undefined behavior to use this value
14-
--> $DIR/validate_uninhabited_zsts.rs:16:1
13+
error[E0080]: evaluation of constant value failed
14+
--> $DIR/validate_uninhabited_zsts.rs:16:35
1515
|
1616
LL | const BAR: [Empty; 3] = [unsafe { std::mem::transmute(()) }; 3];
17-
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed at [0]: encountered a value of uninhabited type Empty
18-
|
19-
= note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
20-
= note: the raw bytes of the constant (size: 0, align: 1) {}
17+
| ^^^^^^^^^^^^^^^^^^^^^^^ transmuting to uninhabited type
2118

2219
warning: the type `!` does not permit zero-initialization
2320
--> $DIR/validate_uninhabited_zsts.rs:4:14

Diff for: ‎src/test/ui/consts/const-eval/validate_uninhabited_zsts.64bit.stderr

+3-6
Original file line numberDiff line numberDiff line change
@@ -10,14 +10,11 @@ LL | unsafe { std::mem::transmute(()) }
1010
LL | const FOO: [Empty; 3] = [foo(); 3];
1111
| ----- inside `FOO` at $DIR/validate_uninhabited_zsts.rs:13:26
1212

13-
error[E0080]: it is undefined behavior to use this value
14-
--> $DIR/validate_uninhabited_zsts.rs:16:1
13+
error[E0080]: evaluation of constant value failed
14+
--> $DIR/validate_uninhabited_zsts.rs:16:35
1515
|
1616
LL | const BAR: [Empty; 3] = [unsafe { std::mem::transmute(()) }; 3];
17-
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed at [0]: encountered a value of uninhabited type Empty
18-
|
19-
= note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
20-
= note: the raw bytes of the constant (size: 0, align: 1) {}
17+
| ^^^^^^^^^^^^^^^^^^^^^^^ transmuting to uninhabited type
2118

2219
warning: the type `!` does not permit zero-initialization
2320
--> $DIR/validate_uninhabited_zsts.rs:4:14

Diff for: ‎src/test/ui/consts/const-eval/validate_uninhabited_zsts.rs

+1-1
Original file line numberDiff line numberDiff line change
@@ -14,7 +14,7 @@ const FOO: [Empty; 3] = [foo(); 3];
1414

1515
#[warn(const_err)]
1616
const BAR: [Empty; 3] = [unsafe { std::mem::transmute(()) }; 3];
17-
//~^ ERROR it is undefined behavior to use this value
17+
//~^ ERROR evaluation of constant value failed
1818
//~| WARN the type `Empty` does not permit zero-initialization
1919

2020
fn main() {

Diff for: ‎src/test/ui/generator/issue-93161.rs

+2-1
Original file line numberDiff line numberDiff line change
@@ -1,5 +1,6 @@
11
// edition:2021
22
// run-pass
3+
// compile-flags: -Zdrop-tracking
34

45
#![feature(never_type)]
56

@@ -32,7 +33,7 @@ fn never() -> Never {
3233
}
3334

3435
async fn includes_never(crash: bool, x: u32) -> u32 {
35-
let mut result = async { x * x }.await;
36+
let result = async { x * x }.await;
3637
if !crash {
3738
return result;
3839
}

Diff for: ‎src/test/ui/statics/uninhabited-static.rs

+2-2
Original file line numberDiff line numberDiff line change
@@ -11,11 +11,11 @@ extern {
1111

1212
static VOID2: Void = unsafe { std::mem::transmute(()) }; //~ ERROR static of uninhabited type
1313
//~| WARN: previously accepted
14-
//~| ERROR undefined behavior to use this value
14+
//~| ERROR could not evaluate static initializer
1515
//~| WARN: type `Void` does not permit zero-initialization
1616
static NEVER2: Void = unsafe { std::mem::transmute(()) }; //~ ERROR static of uninhabited type
1717
//~| WARN: previously accepted
18-
//~| ERROR undefined behavior to use this value
18+
//~| ERROR could not evaluate static initializer
1919
//~| WARN: type `Void` does not permit zero-initialization
2020

2121
fn main() {}

Diff for: ‎src/test/ui/statics/uninhabited-static.stderr

+6-12
Original file line numberDiff line numberDiff line change
@@ -43,23 +43,17 @@ LL | static NEVER2: Void = unsafe { std::mem::transmute(()) };
4343
= note: for more information, see issue #74840 <https://github.com/rust-lang/rust/issues/74840>
4444
= note: uninhabited statics cannot be initialized, and any access would be an immediate error
4545

46-
error[E0080]: it is undefined behavior to use this value
47-
--> $DIR/uninhabited-static.rs:12:1
46+
error[E0080]: could not evaluate static initializer
47+
--> $DIR/uninhabited-static.rs:12:31
4848
|
4949
LL | static VOID2: Void = unsafe { std::mem::transmute(()) };
50-
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered a value of uninhabited type Void
51-
|
52-
= note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
53-
= note: the raw bytes of the constant (size: 0, align: 1) {}
50+
| ^^^^^^^^^^^^^^^^^^^^^^^ transmuting to uninhabited type
5451

55-
error[E0080]: it is undefined behavior to use this value
56-
--> $DIR/uninhabited-static.rs:16:1
52+
error[E0080]: could not evaluate static initializer
53+
--> $DIR/uninhabited-static.rs:16:32
5754
|
5855
LL | static NEVER2: Void = unsafe { std::mem::transmute(()) };
59-
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered a value of uninhabited type Void
60-
|
61-
= note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
62-
= note: the raw bytes of the constant (size: 0, align: 1) {}
56+
| ^^^^^^^^^^^^^^^^^^^^^^^ transmuting to uninhabited type
6357

6458
warning: the type `Void` does not permit zero-initialization
6559
--> $DIR/uninhabited-static.rs:12:31

Diff for: ‎src/test/ui/uninhabited/privately-uninhabited-mir-call.rs

+29
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,29 @@
1+
// Verifies that MIR building for a call expression respects
2+
// privacy when checking if a call return type is uninhabited.
3+
4+
pub mod widget {
5+
enum Unimplemented {}
6+
pub struct Widget(Unimplemented);
7+
8+
impl Widget {
9+
pub fn new() -> Widget {
10+
todo!();
11+
}
12+
}
13+
14+
pub fn f() {
15+
let x: &mut u32;
16+
Widget::new();
17+
// Ok. Widget type returned from new is known to be uninhabited
18+
// and the following code is considered unreachable.
19+
*x = 1;
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}
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}
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fn main() {
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let y: &mut u32;
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widget::Widget::new();
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// Error. Widget type is not known to be uninhabited here,
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// so the following code is considered reachable.
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*y = 2; //~ ERROR use of possibly-uninitialized variable
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}

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