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//! This pass replaces a drop of a type that does not need dropping, with a goto.
//!
//! When the MIR is built, we check `needs_drop` before emitting a `Drop` for a place. This pass is
//! useful because (unlike MIR building) it runs after type checking, so it can make use of
//! `Reveal::All` to provide more precise type information.
use crate::MirPass;
use rustc_middle::mir::*;
use rustc_middle::ty::TyCtxt;
use super::simplify::simplify_cfg;
pub struct RemoveUnneededDrops;
impl<'tcx> MirPass<'tcx> for RemoveUnneededDrops {
fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
trace!("Running RemoveUnneededDrops on {:?}", body.source);
let did = body.source.def_id();
let param_env = tcx.param_env_reveal_all_normalized(did);
let mut should_simplify = false;
for block in body.basic_blocks.as_mut() {
let terminator = block.terminator_mut();
if let TerminatorKind::Drop { place, target, .. } = terminator.kind {
let ty = place.ty(&body.local_decls, tcx);
if ty.ty.needs_drop(tcx, param_env) {
continue;
}
if !tcx.consider_optimizing(|| format!("RemoveUnneededDrops {:?} ", did)) {
continue;
}
debug!("SUCCESS: replacing `drop` with goto({:?})", target);
terminator.kind = TerminatorKind::Goto { target };
should_simplify = true;
}
}
// if we applied optimizations, we potentially have some cfg to cleanup to
// make it easier for further passes
if should_simplify {
simplify_cfg(tcx, body);
}
}
}
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