rustc_mir_transform/
remove_unneeded_drops.rs

1//! This pass replaces a drop of a type that does not need dropping, with a goto.
2//!
3//! When the MIR is built, we check `needs_drop` before emitting a `Drop` for a place. This pass is
4//! useful because (unlike MIR building) it runs after type checking, so it can make use of
5//! `TypingMode::PostAnalysis` to provide more precise type information, especially about opaque
6//! types.
7//!
8//! When we're optimizing, we also remove calls to `drop_in_place<T>` when `T` isn't `needs_drop`,
9//! as those are essentially equivalent to `Drop` terminators. While the compiler doesn't insert
10//! them automatically, preferring the built-in instead, they're common in generic code (such as
11//! `Vec::truncate`) so removing them from things like inlined `Vec<u8>` is helpful.
12
13use rustc_hir::LangItem;
14use rustc_middle::mir::*;
15use rustc_middle::ty::TyCtxt;
16use tracing::{debug, trace};
17
18use super::simplify::simplify_cfg;
19
20pub(super) struct RemoveUnneededDrops;
21
22impl<'tcx> crate::MirPass<'tcx> for RemoveUnneededDrops {
23    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
24        trace!("Running RemoveUnneededDrops on {:?}", body.source);
25
26        let typing_env = body.typing_env(tcx);
27        let mut should_simplify = false;
28        for block in body.basic_blocks.as_mut() {
29            let terminator = block.terminator_mut();
30            let (ty, target) = match terminator.kind {
31                TerminatorKind::Drop { place, target, .. } => {
32                    (place.ty(&body.local_decls, tcx).ty, target)
33                }
34                TerminatorKind::Call { ref func, target: Some(target), .. }
35                    if tcx.sess.mir_opt_level() > 0
36                        && let Some((def_id, generics)) = func.const_fn_def()
37                        && tcx.is_lang_item(def_id, LangItem::DropInPlace) =>
38                {
39                    (generics.type_at(0), target)
40                }
41                _ => continue,
42            };
43
44            if ty.needs_drop(tcx, typing_env) {
45                continue;
46            }
47            debug!("SUCCESS: replacing `drop` with goto({:?})", target);
48            terminator.kind = TerminatorKind::Goto { target };
49            should_simplify = true;
50        }
51
52        // if we applied optimizations, we potentially have some cfg to cleanup to
53        // make it easier for further passes
54        if should_simplify {
55            simplify_cfg(tcx, body);
56        }
57    }
58
59    fn is_required(&self) -> bool {
60        true
61    }
62}
rustc_mir_transform/remove_unneeded_drops.rs

rustc_mir_transform/
remove_unneeded_drops.rs
