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Auto merge of rust-lang#134486 - compiler-errors:drop-for-lint, r=nik…
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…omatsakis

Make sure we handle `backwards_incompatible_lint` drops appropriately in drop elaboration

In rust-lang#131326, a new kind of scheduled drop (`drop_kind: DropKind::Value` + `backwards_incompatible_lint: true`) was added so that we could insert a new kind of no-op MIR statement (`backward incompatible drop`) for linting purposes.

These drops were intended to have *no side-effects*, but drop elaboration code forgot to handle these drops specially and they were handled otherwise as normal drops in most of the code. This ends up being **unsound** since we insert more than one drop call for some values, which means that `Drop::drop` could be called more than once.

This PR fixes this by splitting out the `DropKind::ForLint` and adjusting the code. I'm not totally certain if all of the places I've adjusted are either reachable or correct, but I'm pretty certain that it's *more* correct than it was previously.

cc `@dingxiangfei2009`
r? nikomatsakis

Fixes rust-lang#134482
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bors committed Dec 19, 2024
2 parents 3bf62cc + 6564403 commit 11663cd
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Showing 4 changed files with 461 additions and 41 deletions.
148 changes: 107 additions & 41 deletions compiler/rustc_mir_build/src/builder/scope.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -151,15 +151,13 @@ struct DropData {

/// Whether this is a value Drop or a StorageDead.
kind: DropKind,

/// Whether this is a backwards-incompatible drop lint
backwards_incompatible_lint: bool,
}

#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub(crate) enum DropKind {
Value,
Storage,
ForLint,
}

#[derive(Debug)]
Expand Down Expand Up @@ -248,7 +246,7 @@ impl Scope {
/// use of optimizations in the MIR coroutine transform.
fn needs_cleanup(&self) -> bool {
self.drops.iter().any(|drop| match drop.kind {
DropKind::Value => true,
DropKind::Value | DropKind::ForLint => true,
DropKind::Storage => false,
})
}
Expand Down Expand Up @@ -277,12 +275,8 @@ impl DropTree {
// represents the block in the tree that should be jumped to once all
// of the required drops have been performed.
let fake_source_info = SourceInfo::outermost(DUMMY_SP);
let fake_data = DropData {
source_info: fake_source_info,
local: Local::MAX,
kind: DropKind::Storage,
backwards_incompatible_lint: false,
};
let fake_data =
DropData { source_info: fake_source_info, local: Local::MAX, kind: DropKind::Storage };
let drops = IndexVec::from_raw(vec![DropNode { data: fake_data, next: DropIdx::MAX }]);
Self { drops, entry_points: Vec::new(), existing_drops_map: FxHashMap::default() }
}
Expand Down Expand Up @@ -411,6 +405,27 @@ impl DropTree {
};
cfg.terminate(block, drop_node.data.source_info, terminator);
}
DropKind::ForLint => {
let stmt = Statement {
source_info: drop_node.data.source_info,
kind: StatementKind::BackwardIncompatibleDropHint {
place: Box::new(drop_node.data.local.into()),
reason: BackwardIncompatibleDropReason::Edition2024,
},
};
cfg.push(block, stmt);
let target = blocks[drop_node.next].unwrap();
if target != block {
// Diagnostics don't use this `Span` but debuginfo
// might. Since we don't want breakpoints to be placed
// here, especially when this is on an unwind path, we
// use `DUMMY_SP`.
let source_info =
SourceInfo { span: DUMMY_SP, ..drop_node.data.source_info };
let terminator = TerminatorKind::Goto { target };
cfg.terminate(block, source_info, terminator);
}
}
// Root nodes don't correspond to a drop.
DropKind::Storage if drop_idx == ROOT_NODE => {}
DropKind::Storage => {
Expand Down Expand Up @@ -770,12 +785,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
let local =
place.as_local().unwrap_or_else(|| bug!("projection in tail call args"));

Some(DropData {
source_info,
local,
kind: DropKind::Value,
backwards_incompatible_lint: false,
})
Some(DropData { source_info, local, kind: DropKind::Value })
}
Operand::Constant(_) => None,
})
Expand Down Expand Up @@ -822,6 +832,15 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
});
block = next;
}
DropKind::ForLint => {
self.cfg.push(block, Statement {
source_info,
kind: StatementKind::BackwardIncompatibleDropHint {
place: Box::new(local.into()),
reason: BackwardIncompatibleDropReason::Edition2024,
},
});
}
DropKind::Storage => {
// Only temps and vars need their storage dead.
assert!(local.index() > self.arg_count);
Expand Down Expand Up @@ -1021,7 +1040,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
drop_kind: DropKind,
) {
let needs_drop = match drop_kind {
DropKind::Value => {
DropKind::Value | DropKind::ForLint => {
if !self.local_decls[local].ty.needs_drop(self.tcx, self.typing_env()) {
return;
}
Expand Down Expand Up @@ -1101,7 +1120,6 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
source_info: SourceInfo { span: scope_end, scope: scope.source_scope },
local,
kind: drop_kind,
backwards_incompatible_lint: false,
});

return;
Expand Down Expand Up @@ -1132,8 +1150,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
scope.drops.push(DropData {
source_info: SourceInfo { span: scope_end, scope: scope.source_scope },
local,
kind: DropKind::Value,
backwards_incompatible_lint: true,
kind: DropKind::ForLint,
});

return;
Expand Down Expand Up @@ -1376,12 +1393,23 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
}

/// Builds drops for `pop_scope` and `leave_top_scope`.
///
/// # Parameters
///
/// * `unwind_drops`, the drop tree data structure storing what needs to be cleaned up if unwind occurs
/// * `scope`, describes the drops that will occur on exiting the scope in regular execution
/// * `block`, the block to branch to once drops are complete (assuming no unwind occurs)
/// * `unwind_to`, describes the drops that would occur at this point in the code if a
/// panic occurred (a subset of the drops in `scope`, since we sometimes elide StorageDead and other
/// instructions on unwinding)
/// * `storage_dead_on_unwind`, if true, then we should emit `StorageDead` even when unwinding
/// * `arg_count`, number of MIR local variables corresponding to fn arguments (used to assert that we don't drop those)
fn build_scope_drops<'tcx>(
cfg: &mut CFG<'tcx>,
unwind_drops: &mut DropTree,
scope: &Scope,
mut block: BasicBlock,
mut unwind_to: DropIdx,
block: BasicBlock,
unwind_to: DropIdx,
storage_dead_on_unwind: bool,
arg_count: usize,
) -> BlockAnd<()> {
Expand All @@ -1406,6 +1434,18 @@ fn build_scope_drops<'tcx>(
// drops for the unwind path should have already been generated by
// `diverge_cleanup_gen`.

// `unwind_to` indicates what needs to be dropped should unwinding occur.
// This is a subset of what needs to be dropped when exiting the scope.
// As we unwind the scope, we will also move `unwind_to` backwards to match,
// so that we can use it should a destructor panic.
let mut unwind_to = unwind_to;

// The block that we should jump to after drops complete. We start by building the final drop (`drops[n]`
// in the diagram above) and then build the drops (e.g., `drop[1]`, `drop[0]`) that come before it.
// block begins as the successor of `drops[n]` and then becomes `drops[n]` so that `drops[n-1]`
// will branch to `drops[n]`.
let mut block = block;

for drop_data in scope.drops.iter().rev() {
let source_info = drop_data.source_info;
let local = drop_data.local;
Expand All @@ -1415,6 +1455,9 @@ fn build_scope_drops<'tcx>(
// `unwind_to` should drop the value that we're about to
// schedule. If dropping this value panics, then we continue
// with the *next* value on the unwind path.
//
// We adjust this BEFORE we create the drop (e.g., `drops[n]`)
// because `drops[n]` should unwind to `drops[n-1]`.
debug_assert_eq!(unwind_drops.drops[unwind_to].data.local, drop_data.local);
debug_assert_eq!(unwind_drops.drops[unwind_to].data.kind, drop_data.kind);
unwind_to = unwind_drops.drops[unwind_to].next;
Expand All @@ -1427,27 +1470,50 @@ fn build_scope_drops<'tcx>(
continue;
}

if drop_data.backwards_incompatible_lint {
cfg.push(block, Statement {
source_info,
kind: StatementKind::BackwardIncompatibleDropHint {
place: Box::new(local.into()),
reason: BackwardIncompatibleDropReason::Edition2024,
},
});
} else {
unwind_drops.add_entry_point(block, unwind_to);
let next = cfg.start_new_block();
cfg.terminate(block, source_info, TerminatorKind::Drop {
place: local.into(),
target: next,
unwind: UnwindAction::Continue,
replace: false,
});
block = next;
unwind_drops.add_entry_point(block, unwind_to);
let next = cfg.start_new_block();
cfg.terminate(block, source_info, TerminatorKind::Drop {
place: local.into(),
target: next,
unwind: UnwindAction::Continue,
replace: false,
});
block = next;
}
DropKind::ForLint => {
// If the operand has been moved, and we are not on an unwind
// path, then don't generate the drop. (We only take this into
// account for non-unwind paths so as not to disturb the
// caching mechanism.)
if scope.moved_locals.iter().any(|&o| o == local) {
continue;
}

// As in the `DropKind::Storage` case below:
// normally lint-related drops are not emitted for unwind,
// so we can just leave `unwind_to` unmodified, but in some
// cases we emit things ALSO on the unwind path, so we need to adjust
// `unwind_to` in that case.
if storage_dead_on_unwind {
debug_assert_eq!(unwind_drops.drops[unwind_to].data.local, drop_data.local);
debug_assert_eq!(unwind_drops.drops[unwind_to].data.kind, drop_data.kind);
unwind_to = unwind_drops.drops[unwind_to].next;
}

cfg.push(block, Statement {
source_info,
kind: StatementKind::BackwardIncompatibleDropHint {
place: Box::new(local.into()),
reason: BackwardIncompatibleDropReason::Edition2024,
},
});
}
DropKind::Storage => {
// Ordinarily, storage-dead nodes are not emitted on unwind, so we don't
// need to adjust `unwind_to` on this path. However, in some specific cases
// we *do* emit storage-dead nodes on the unwind path, and in that case now that
// the storage-dead has completed, we need to adjust the `unwind_to` pointer
// so that any future drops we emit will not register storage-dead.
if storage_dead_on_unwind {
debug_assert_eq!(unwind_drops.drops[unwind_to].data.local, drop_data.local);
debug_assert_eq!(unwind_drops.drops[unwind_to].data.kind, drop_data.kind);
Expand Down Expand Up @@ -1497,7 +1563,7 @@ impl<'a, 'tcx: 'a> Builder<'a, 'tcx> {
unwind_indices.push(unwind_indices[drop_node.next]);
}
}
DropKind::Value => {
DropKind::Value | DropKind::ForLint => {
let unwind_drop = self
.scopes
.unwind_drops
Expand Down
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