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Rollup merge of rust-lang#126154 - RalfJung:storage-live, r=compiler-…
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…errors

StorageLive: refresh storage (instead of UB) when local is already live

Blocked on [this FCP](rust-lang#99160 (comment)), which also contains the motivation.

Fixes rust-lang#99160
Fixes rust-lang#98896 (by declaring it not-a-bug)
Fixes rust-lang#119366
Fixes rust-lang/unsafe-code-guidelines#129
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jieyouxu authored Jun 19, 2024
2 parents aad1f30 + 408c8eb commit 1381b0c
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Showing 7 changed files with 71 additions and 11 deletions.
2 changes: 0 additions & 2 deletions compiler/rustc_const_eval/messages.ftl
Original file line number Diff line number Diff line change
Expand Up @@ -73,8 +73,6 @@ const_eval_division_by_zero =
dividing by zero
const_eval_division_overflow =
overflow in signed division (dividing MIN by -1)
const_eval_double_storage_live =
StorageLive on a local that was already live
const_eval_dyn_call_not_a_method =
`dyn` call trying to call something that is not a method
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8 changes: 3 additions & 5 deletions compiler/rustc_const_eval/src/interpret/eval_context.rs
Original file line number Diff line number Diff line change
Expand Up @@ -1100,11 +1100,9 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
Operand::Immediate(Immediate::Uninit)
});

// StorageLive expects the local to be dead, and marks it live.
// If the local is already live, deallocate its old memory.
let old = mem::replace(&mut self.frame_mut().locals[local].value, local_val);
if !matches!(old, LocalValue::Dead) {
throw_ub_custom!(fluent::const_eval_double_storage_live);
}
self.deallocate_local(old)?;
Ok(())
}

Expand All @@ -1118,7 +1116,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
assert!(local != mir::RETURN_PLACE, "Cannot make return place dead");
trace!("{:?} is now dead", local);

// It is entirely okay for this local to be already dead (at least that's how we currently generate MIR)
// If the local is already dead, this is a NOP.
let old = mem::replace(&mut self.frame_mut().locals[local].value, LocalValue::Dead);
self.deallocate_local(old)?;
Ok(())
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11 changes: 7 additions & 4 deletions compiler/rustc_middle/src/mir/syntax.rs
Original file line number Diff line number Diff line change
Expand Up @@ -361,16 +361,19 @@ pub enum StatementKind<'tcx> {
/// At any point during the execution of a function, each local is either allocated or
/// unallocated. Except as noted below, all locals except function parameters are initially
/// unallocated. `StorageLive` statements cause memory to be allocated for the local while
/// `StorageDead` statements cause the memory to be freed. Using a local in any way (not only
/// reading/writing from it) while it is unallocated is UB.
/// `StorageDead` statements cause the memory to be freed. In other words,
/// `StorageLive`/`StorageDead` act like the heap operations `allocate`/`deallocate`, but for
/// stack-allocated local variables. Using a local in any way (not only reading/writing from it)
/// while it is unallocated is UB.
///
/// Some locals have no `StorageLive` or `StorageDead` statements within the entire MIR body.
/// These locals are implicitly allocated for the full duration of the function. There is a
/// convenience method at `rustc_mir_dataflow::storage::always_storage_live_locals` for
/// computing these locals.
///
/// If the local is already allocated, calling `StorageLive` again is UB. However, for an
/// unallocated local an additional `StorageDead` all is simply a nop.
/// If the local is already allocated, calling `StorageLive` again will implicitly free the
/// local and then allocate fresh uninitilized memory. If a local is already deallocated,
/// calling `StorageDead` again is a NOP.
StorageLive(Local),

/// See `StorageLive` above.
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14 changes: 14 additions & 0 deletions src/tools/miri/tests/fail/storage-live-dead-var.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,14 @@
#![feature(core_intrinsics, custom_mir)]
use std::intrinsics::mir::*;

#[custom_mir(dialect = "runtime")]
fn main() {
mir! {
let val: i32;
{
val = 42; //~ERROR: accessing a dead local variable
StorageLive(val); // too late... (but needs to be here to make `val` not implicitly live)
Return()
}
}
}
15 changes: 15 additions & 0 deletions src/tools/miri/tests/fail/storage-live-dead-var.stderr
Original file line number Diff line number Diff line change
@@ -0,0 +1,15 @@
error: Undefined Behavior: accessing a dead local variable
--> $DIR/storage-live-dead-var.rs:LL:CC
|
LL | val = 42;
| ^^^^^^^^ accessing a dead local variable
|
= help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
= help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
= note: BACKTRACE:
= note: inside `main` at $DIR/storage-live-dead-var.rs:LL:CC

note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

error: aborting due to 1 previous error

17 changes: 17 additions & 0 deletions src/tools/miri/tests/fail/storage-live-resets-var.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,17 @@
#![feature(core_intrinsics, custom_mir)]
use std::intrinsics::mir::*;

#[custom_mir(dialect = "runtime")]
fn main() {
mir! {
let val: i32;
let _val2: i32;
{
StorageLive(val);
val = 42;
StorageLive(val); // reset val to `uninit`
_val2 = val; //~ERROR: uninitialized
Return()
}
}
}
15 changes: 15 additions & 0 deletions src/tools/miri/tests/fail/storage-live-resets-var.stderr
Original file line number Diff line number Diff line change
@@ -0,0 +1,15 @@
error: Undefined Behavior: constructing invalid value: encountered uninitialized memory, but expected an integer
--> $DIR/storage-live-resets-var.rs:LL:CC
|
LL | _val2 = val;
| ^^^^^^^^^^^ constructing invalid value: encountered uninitialized memory, but expected an integer
|
= help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
= help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
= note: BACKTRACE:
= note: inside `main` at $DIR/storage-live-resets-var.rs:LL:CC

note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

error: aborting due to 1 previous error

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