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Mar 11, 2024
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Add tests (and a bit of cleanup) for interior mut handling in promotion and const-checking #121893

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d9a2886
add comment and test: we do not do value-based reasoning for promotio…
RalfJung Feb 29, 2024
279465b
const-checking: add some corner case tests, and fix some nits
RalfJung Mar 2, 2024
fb802f2
promote-not: add test that distinguishes promotion from outer scope rule
RalfJung Mar 11, 2024
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8 changes: 4 additions & 4 deletions compiler/rustc_const_eval/src/const_eval/eval_queries.rs
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Expand Up @@ -412,10 +412,10 @@ pub fn const_validate_mplace<'mir, 'tcx>(
_ if cid.promoted.is_some() => CtfeValidationMode::Promoted,
Some(mutbl) => CtfeValidationMode::Static { mutbl }, // a `static`
None => {
// In normal `const` (not promoted), the outermost allocation is always only copied,
// so having `UnsafeCell` in there is okay despite them being in immutable memory.
let allow_immutable_unsafe_cell = cid.promoted.is_none() && !inner;
CtfeValidationMode::Const { allow_immutable_unsafe_cell }
// This is a normal `const` (not promoted).
// The outermost allocation is always only copied, so having `UnsafeCell` in there
// is okay despite them being in immutable memory.
CtfeValidationMode::Const { allow_immutable_unsafe_cell: !inner }
}
};
ecx.const_validate_operand(&mplace.into(), path, &mut ref_tracking, mode)?;
Expand Down
2 changes: 1 addition & 1 deletion compiler/rustc_const_eval/src/transform/check_consts/check.rs
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Expand Up @@ -30,7 +30,7 @@ type QualifResults<'mir, 'tcx, Q> =
rustc_mir_dataflow::ResultsCursor<'mir, 'tcx, FlowSensitiveAnalysis<'mir, 'mir, 'tcx, Q>>;

#[derive(Default)]
pub struct Qualifs<'mir, 'tcx> {
pub(crate) struct Qualifs<'mir, 'tcx> {
has_mut_interior: Option<QualifResults<'mir, 'tcx, HasMutInterior>>,
needs_drop: Option<QualifResults<'mir, 'tcx, NeedsDrop>>,
needs_non_const_drop: Option<QualifResults<'mir, 'tcx, NeedsNonConstDrop>>,
Expand Down
1 change: 1 addition & 0 deletions compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs
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Expand Up @@ -284,6 +284,7 @@ where
if Q::in_adt_inherently(cx, def, args) {
return true;
}
// Don't do any value-based reasoning for unions.
if def.is_union() && Q::in_any_value_of_ty(cx, rvalue.ty(cx.body, cx.tcx)) {
return true;
}
Expand Down
16 changes: 16 additions & 0 deletions tests/ui/consts/enclosing-scope-rule.rs
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@@ -0,0 +1,16 @@
//@build-pass
// Some code that looks like it might be relying on promotion, but actually this is using the
// enclosing-scope rule, meaning the reference is "extended" to outlive its block and live as long
// as the surrounding block (which in this case is the entire program). There are multiple
// allocations being interned at once.

struct Gen<T>(T);
impl<'a, T> Gen<&'a T> {
// Can't be promoted because `T` might not be `'static`.
const C: &'a [T] = &[];
}

// Can't be promoted because of `Drop`.
const V: &Vec<i32> = &Vec::new();

fn main() {}
21 changes: 20 additions & 1 deletion tests/ui/consts/promote-not.rs
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Expand Up @@ -3,6 +3,7 @@
#![allow(unconditional_panic)]

use std::cell::Cell;
use std::mem::ManuallyDrop;

// We do not promote mutable references.
static mut TEST1: Option<&mut [i32]> = Some(&mut [1, 2, 3]); //~ ERROR temporary value dropped while borrowed
Expand Down Expand Up @@ -39,7 +40,16 @@ const TEST_INTERIOR_MUT: () = {
let _val: &'static _ = &(Cell::new(1), 2).1; //~ ERROR temporary value dropped while borrowed
};

const TEST_DROP: String = String::new();
// This gets accepted by the "outer scope" rule, not promotion.
const TEST_DROP_OUTER_SCOPE: &String = &String::new();
// To demonstrate that, we can rewrite it as follows. If this was promotion it would still work.
const TEST_DROP_NOT_PROMOTE: &String = {
let x = &String::new(); //~ ERROR destructor of `String` cannot be evaluated at compile-time
// The "dropped while borrowed" error seems to be suppressed, but the key point is that this
// fails to compile.
x
};


fn main() {
// We must not promote things with interior mutability. Not even if we "project it away".
Expand All @@ -58,6 +68,7 @@ fn main() {
let _val: &'static _ = &([1,2,3][4]+1); //~ ERROR temporary value dropped while borrowed

// No promotion of temporaries that need to be dropped.
const TEST_DROP: String = String::new();
let _val: &'static _ = &TEST_DROP;
//~^ ERROR temporary value dropped while borrowed
let _val: &'static _ = &&TEST_DROP;
Expand All @@ -69,4 +80,12 @@ fn main() {
let _val: &'static _ = &[&TEST_DROP; 1];
//~^ ERROR temporary value dropped while borrowed
//~| ERROR temporary value dropped while borrowed

// Make sure there is no value-based reasoning for unions.
union UnionWithCell {
f1: i32,
f2: ManuallyDrop<Cell<i32>>,
}
let x: &'static _ = &UnionWithCell { f1: 0 };
//~^ ERROR temporary value dropped while borrowed
}
73 changes: 47 additions & 26 deletions tests/ui/consts/promote-not.stderr
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@@ -1,5 +1,5 @@
error[E0716]: temporary value dropped while borrowed
--> $DIR/promote-not.rs:8:50
--> $DIR/promote-not.rs:9:50
|
LL | static mut TEST1: Option<&mut [i32]> = Some(&mut [1, 2, 3]);
| ----------^^^^^^^^^-
Expand All @@ -9,7 +9,7 @@ LL | static mut TEST1: Option<&mut [i32]> = Some(&mut [1, 2, 3]);
| using this value as a static requires that borrow lasts for `'static`

error[E0716]: temporary value dropped while borrowed
--> $DIR/promote-not.rs:11:18
--> $DIR/promote-not.rs:12:18
|
LL | let x = &mut [1,2,3];
| ^^^^^^^ creates a temporary value which is freed while still in use
Expand All @@ -19,7 +19,7 @@ LL | };
| - temporary value is freed at the end of this statement

error[E0716]: temporary value dropped while borrowed
--> $DIR/promote-not.rs:33:29
--> $DIR/promote-not.rs:34:29
|
LL | let _x: &'static i32 = &unsafe { U { x: 0 }.x };
| ------------ ^^^^^^^^^^^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
Expand All @@ -29,7 +29,7 @@ LL | };
| - temporary value is freed at the end of this statement

error[E0716]: temporary value dropped while borrowed
--> $DIR/promote-not.rs:39:29
--> $DIR/promote-not.rs:40:29
|
LL | let _val: &'static _ = &(Cell::new(1), 2).1;
| ---------- ^^^^^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
Expand All @@ -38,8 +38,17 @@ LL | let _val: &'static _ = &(Cell::new(1), 2).1;
LL | };
| - temporary value is freed at the end of this statement

error[E0493]: destructor of `String` cannot be evaluated at compile-time
--> $DIR/promote-not.rs:47:14
|
LL | let x = &String::new();
| ^^^^^^^^^^^^^ the destructor for this type cannot be evaluated in constants
...
LL | };
| - value is dropped here

error[E0716]: temporary value dropped while borrowed
--> $DIR/promote-not.rs:20:32
--> $DIR/promote-not.rs:21:32
|
LL | let _x: &'static () = &foo();
| ----------- ^^^^^ creates a temporary value which is freed while still in use
Expand All @@ -49,7 +58,7 @@ LL | }
| - temporary value is freed at the end of this statement

error[E0716]: temporary value dropped while borrowed
--> $DIR/promote-not.rs:28:29
--> $DIR/promote-not.rs:29:29
|
LL | let _x: &'static i32 = &unsafe { U { x: 0 }.x };
| ------------ ^^^^^^^^^^^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
Expand All @@ -59,7 +68,7 @@ LL | }
| - temporary value is freed at the end of this statement

error[E0716]: temporary value dropped while borrowed
--> $DIR/promote-not.rs:46:29
--> $DIR/promote-not.rs:56:29
|
LL | let _val: &'static _ = &(Cell::new(1), 2).0;
| ---------- ^^^^^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
Expand All @@ -70,7 +79,7 @@ LL | }
| - temporary value is freed at the end of this statement

error[E0716]: temporary value dropped while borrowed
--> $DIR/promote-not.rs:47:29
--> $DIR/promote-not.rs:57:29
|
LL | let _val: &'static _ = &(Cell::new(1), 2).1;
| ---------- ^^^^^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
Expand All @@ -81,7 +90,7 @@ LL | }
| - temporary value is freed at the end of this statement

error[E0716]: temporary value dropped while borrowed
--> $DIR/promote-not.rs:50:29
--> $DIR/promote-not.rs:60:29
|
LL | let _val: &'static _ = &(1/0);
| ---------- ^^^^^ creates a temporary value which is freed while still in use
Expand All @@ -92,7 +101,7 @@ LL | }
| - temporary value is freed at the end of this statement

error[E0716]: temporary value dropped while borrowed
--> $DIR/promote-not.rs:51:29
--> $DIR/promote-not.rs:61:29
|
LL | let _val: &'static _ = &(1/(1-1));
| ---------- ^^^^^^^^^ creates a temporary value which is freed while still in use
Expand All @@ -103,7 +112,7 @@ LL | }
| - temporary value is freed at the end of this statement

error[E0716]: temporary value dropped while borrowed
--> $DIR/promote-not.rs:52:29
--> $DIR/promote-not.rs:62:29
|
LL | let _val: &'static _ = &((1+1)/(1-1));
| ---------- ^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
Expand All @@ -114,7 +123,7 @@ LL | }
| - temporary value is freed at the end of this statement

error[E0716]: temporary value dropped while borrowed
--> $DIR/promote-not.rs:53:29
--> $DIR/promote-not.rs:63:29
|
LL | let _val: &'static _ = &(i32::MIN/-1);
| ---------- ^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
Expand All @@ -125,7 +134,7 @@ LL | }
| - temporary value is freed at the end of this statement

error[E0716]: temporary value dropped while borrowed
--> $DIR/promote-not.rs:54:29
--> $DIR/promote-not.rs:64:29
|
LL | let _val: &'static _ = &(i32::MIN/(0-1));
| ---------- ^^^^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
Expand All @@ -136,7 +145,7 @@ LL | }
| - temporary value is freed at the end of this statement

error[E0716]: temporary value dropped while borrowed
--> $DIR/promote-not.rs:55:29
--> $DIR/promote-not.rs:65:29
|
LL | let _val: &'static _ = &(-128i8/-1);
| ---------- ^^^^^^^^^^^ creates a temporary value which is freed while still in use
Expand All @@ -147,7 +156,7 @@ LL | }
| - temporary value is freed at the end of this statement

error[E0716]: temporary value dropped while borrowed
--> $DIR/promote-not.rs:56:29
--> $DIR/promote-not.rs:66:29
|
LL | let _val: &'static _ = &(1%0);
| ---------- ^^^^^ creates a temporary value which is freed while still in use
Expand All @@ -158,7 +167,7 @@ LL | }
| - temporary value is freed at the end of this statement

error[E0716]: temporary value dropped while borrowed
--> $DIR/promote-not.rs:57:29
--> $DIR/promote-not.rs:67:29
|
LL | let _val: &'static _ = &(1%(1-1));
| ---------- ^^^^^^^^^ creates a temporary value which is freed while still in use
Expand All @@ -169,7 +178,7 @@ LL | }
| - temporary value is freed at the end of this statement

error[E0716]: temporary value dropped while borrowed
--> $DIR/promote-not.rs:58:29
--> $DIR/promote-not.rs:68:29
|
LL | let _val: &'static _ = &([1,2,3][4]+1);
| ---------- ^^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
Expand All @@ -180,7 +189,7 @@ LL | }
| - temporary value is freed at the end of this statement

error[E0716]: temporary value dropped while borrowed
--> $DIR/promote-not.rs:61:29
--> $DIR/promote-not.rs:72:29
|
LL | let _val: &'static _ = &TEST_DROP;
| ---------- ^^^^^^^^^ creates a temporary value which is freed while still in use
Expand All @@ -191,7 +200,7 @@ LL | }
| - temporary value is freed at the end of this statement

error[E0716]: temporary value dropped while borrowed
--> $DIR/promote-not.rs:63:29
--> $DIR/promote-not.rs:74:29
|
LL | let _val: &'static _ = &&TEST_DROP;
| ---------- ^^^^^^^^^^ creates a temporary value which is freed while still in use
Expand All @@ -202,7 +211,7 @@ LL | }
| - temporary value is freed at the end of this statement

error[E0716]: temporary value dropped while borrowed
--> $DIR/promote-not.rs:63:30
--> $DIR/promote-not.rs:74:30
|
LL | let _val: &'static _ = &&TEST_DROP;
| ---------- ^^^^^^^^^ creates a temporary value which is freed while still in use
Expand All @@ -213,7 +222,7 @@ LL | }
| - temporary value is freed at the end of this statement

error[E0716]: temporary value dropped while borrowed
--> $DIR/promote-not.rs:66:29
--> $DIR/promote-not.rs:77:29
|
LL | let _val: &'static _ = &(&TEST_DROP,);
| ---------- ^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
Expand All @@ -224,7 +233,7 @@ LL | }
| - temporary value is freed at the end of this statement

error[E0716]: temporary value dropped while borrowed
--> $DIR/promote-not.rs:66:31
--> $DIR/promote-not.rs:77:31
|
LL | let _val: &'static _ = &(&TEST_DROP,);
| ---------- ^^^^^^^^^ creates a temporary value which is freed while still in use
Expand All @@ -235,7 +244,7 @@ LL | }
| - temporary value is freed at the end of this statement

error[E0716]: temporary value dropped while borrowed
--> $DIR/promote-not.rs:69:29
--> $DIR/promote-not.rs:80:29
|
LL | let _val: &'static _ = &[&TEST_DROP; 1];
| ---------- ^^^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
Expand All @@ -246,14 +255,26 @@ LL | }
| - temporary value is freed at the end of this statement

error[E0716]: temporary value dropped while borrowed
--> $DIR/promote-not.rs:69:31
--> $DIR/promote-not.rs:80:31
|
LL | let _val: &'static _ = &[&TEST_DROP; 1];
| ---------- ^^^^^^^^^ - temporary value is freed at the end of this statement
| | |
| | creates a temporary value which is freed while still in use
| type annotation requires that borrow lasts for `'static`

error: aborting due to 24 previous errors
error[E0716]: temporary value dropped while borrowed
--> $DIR/promote-not.rs:89:26
|
LL | let x: &'static _ = &UnionWithCell { f1: 0 };
| ---------- ^^^^^^^^^^^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
| |
| type annotation requires that borrow lasts for `'static`
LL |
LL | }
| - temporary value is freed at the end of this statement

error: aborting due to 26 previous errors

For more information about this error, try `rustc --explain E0716`.
Some errors have detailed explanations: E0493, E0716.
For more information about an error, try `rustc --explain E0493`.
23 changes: 23 additions & 0 deletions tests/ui/consts/refs-to-cell-in-final.rs
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Expand Up @@ -15,4 +15,27 @@ static RAW_SYNC_S: SyncPtr<Cell<i32>> = SyncPtr { x: &Cell::new(42) };
const RAW_SYNC_C: SyncPtr<Cell<i32>> = SyncPtr { x: &Cell::new(42) };
//~^ ERROR: cannot refer to interior mutable data

// This one does not get promoted because of `Drop`, and then enters interesting codepaths because
// as a value it has no interior mutability, but as a type it does. See
// <https://github.com/rust-lang/rust/issues/121610>. Value-based reasoning for interior mutability
// is questionable (https://github.com/rust-lang/unsafe-code-guidelines/issues/493) so for now we
// reject this, though not with a great error message.
pub enum JsValue {
Undefined,
Object(Cell<bool>),
}
impl Drop for JsValue {
fn drop(&mut self) {}
}
const UNDEFINED: &JsValue = &JsValue::Undefined;
//~^ERROR: mutable pointer in final value of constant

// In contrast, this one works since it is being promoted.
const NONE: &'static Option<Cell<i32>> = &None;
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Maybe show off the promotion somewhere other than the trailing expression?

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Good point, I didn't even realize that's possible -- I will add

// Making it clear that this is promotion, not "outer scope".
const NONE_EXPLICIT_PROMOTED: &'static Option<Cell<i32>> = {
    let x = &None;
    x
};

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Interestingly, the behavior of this example changed between stable and nightly. On nightly it just says "can't drop String in const", on stable it additionally says "temporary value dropped while borrowed". I hope that's just suppressing two errors from the same location and not secretly actually doing promotion...

// Making it clear that this is promotion, not "outer scope".
const NONE_EXPLICIT_PROMOTED: &'static Option<Cell<i32>> = {
let x = &None;
x
};

fn main() {}
8 changes: 7 additions & 1 deletion tests/ui/consts/refs-to-cell-in-final.stderr
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Expand Up @@ -12,6 +12,12 @@ error[E0492]: constants cannot refer to interior mutable data
LL | const RAW_SYNC_C: SyncPtr<Cell<i32>> = SyncPtr { x: &Cell::new(42) };
| ^^^^^^^^^^^^^^ this borrow of an interior mutable value may end up in the final value

error: aborting due to 2 previous errors
error: encountered mutable pointer in final value of constant
--> $DIR/refs-to-cell-in-final.rs:30:1
|
LL | const UNDEFINED: &JsValue = &JsValue::Undefined;
| ^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0492`.