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do not apply DerefMut on union field #75584

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Sep 5, 2020
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do not apply DerefMut on union field #75584

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ec0924f
do not apply DerefMut on union field
RalfJung Aug 16, 2020
44defae
also detect DerefMut in nested union fields
RalfJung Aug 16, 2020
97974e3
only emit error for ManuallyDrop derefs
RalfJung Aug 16, 2020
66b340f
test more ways of mutably accessing a place
RalfJung Aug 30, 2020
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test more ways of mutably accessing a place
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@RalfJung
RalfJung committed Aug 31, 2020
commit 66b340f5003910bd8b46f6675cd9a491809aa9fe
8 changes: 8 additions & 0 deletions src/test/ui/union/union-deref.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -13,8 +13,16 @@ fn main() {
let mut u : U1<Vec<i32>> = U1 { x: () };
unsafe { (*u.f).0 = Vec::new() }; // explicit deref, this compiles
unsafe { u.f.0 = Vec::new() }; //~ERROR not automatically applying `DerefMut` on `ManuallyDrop` union field
unsafe { &mut (*u.f).0 }; // explicit deref, this compiles
unsafe { &mut u.f.0 }; //~ERROR not automatically applying `DerefMut` on `ManuallyDrop` union field
unsafe { (*u.f).0.push(0) }; // explicit deref, this compiles
unsafe { u.f.0.push(0) }; //~ERROR not automatically applying `DerefMut` on `ManuallyDrop` union field

let mut u : U2<Vec<i32>> = U2 { x: () };
unsafe { (*u.f.0).0 = Vec::new() }; // explicit deref, this compiles
unsafe { u.f.0.0 = Vec::new() }; //~ERROR not automatically applying `DerefMut` on `ManuallyDrop` union field
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unsafe { &mut (*u.f.0).0 }; // explicit deref, this compiles
unsafe { &mut u.f.0.0 }; //~ERROR not automatically applying `DerefMut` on `ManuallyDrop` union field
unsafe { (*u.f.0).0.push(0) }; // explicit deref, this compiles
unsafe { u.f.0.0.push(0) }; //~ERROR not automatically applying `DerefMut` on `ManuallyDrop` union field
}
38 changes: 37 additions & 1 deletion src/test/ui/union/union-deref.stderr
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Expand Up @@ -7,14 +7,50 @@ LL | unsafe { u.f.0 = Vec::new() };
= help: writing to this reference calls the destructor for the old value
= help: add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor

error: not automatically applying `DerefMut` on `ManuallyDrop` union field
--> $DIR/union-deref.rs:17:19
|
LL | unsafe { &mut u.f.0 };
| ^^^
|
= help: writing to this reference calls the destructor for the old value
= help: add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor

error: not automatically applying `DerefMut` on `ManuallyDrop` union field
--> $DIR/union-deref.rs:19:14
|
LL | unsafe { u.f.0.push(0) };
| ^^^
|
= help: writing to this reference calls the destructor for the old value
= help: add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor

error: not automatically applying `DerefMut` on `ManuallyDrop` union field
--> $DIR/union-deref.rs:23:14
|
LL | unsafe { u.f.0.0 = Vec::new() };
| ^^^^^^^
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The span for this is slightly wrong, I'd expect it to only cover u.f.0. I don't know how this is happening, since the span is right above.

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I would guess this has to do with the special handling of multiple tuple field accesses in a row. Perhaps 0.0 is a float token.

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I didn't know there was such special handling.^^ So this would be a problem elsewhere, in the code that sets up the span for these places?

|
= help: writing to this reference calls the destructor for the old value
= help: add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor

error: aborting due to 2 previous errors
error: not automatically applying `DerefMut` on `ManuallyDrop` union field
--> $DIR/union-deref.rs:25:19
|
LL | unsafe { &mut u.f.0.0 };
| ^^^^^^^
|
= help: writing to this reference calls the destructor for the old value
= help: add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor

error: not automatically applying `DerefMut` on `ManuallyDrop` union field
--> $DIR/union-deref.rs:27:14
|
LL | unsafe { u.f.0.0.push(0) };
| ^^^^^^^
|
= help: writing to this reference calls the destructor for the old value
= help: add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor

error: aborting due to 6 previous errors