Move rustc_mir::transform to rustc_mir_transform.

compiler/rustc_mir_transform/src/remove_unneeded_drops.rs

@@ -0,0 +1,42 @@
+//! This pass replaces a drop of a type that does not need dropping, with a goto
+
+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(did);
+        let mut should_simplify = false;
+
+        let (basic_blocks, local_decls) = body.basic_blocks_and_local_decls_mut();
+        for block in basic_blocks {
+            let terminator = block.terminator_mut();
+            if let TerminatorKind::Drop { place, target, .. } = terminator.kind {
+                let ty = place.ty(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);
+        }
+    }
+}