[WIP] Remove StorageDead and StorageLive statements using MIR locals of type ()

src/librustc_mir/transform/mod.rs

@@ -30,6 +30,7 @@ pub mod no_landing_pads;
 pub mod promote_consts;
 pub mod qualify_min_const_fn;
 pub mod remove_noop_landing_pads;
+pub mod remove_unit_storage;
 pub mod rustc_peek;
 pub mod simplify;
 pub mod simplify_branches;
@@ -299,6 +300,7 @@ fn run_optimization_passes<'tcx>(
             // From here on out, regions are gone.
             &erase_regions::EraseRegions,
             // Optimizations begin.
+            &remove_unit_storage::RemoveUnitStorage,
             &unreachable_prop::UnreachablePropagation,
             &uninhabited_enum_branching::UninhabitedEnumBranching,
             &simplify::SimplifyCfg::new("after-uninhabited-enum-branching"),

src/librustc_mir/transform/remove_unit_storage.rs

@@ -0,0 +1,29 @@
+//! The `RemoveUnitStorage` pass removes `StorageLive` and `StorageDead` statements
+//! which operates on locals of type `()`.
+
+use crate::transform::{MirPass, MirSource};
+use rustc::mir::*;
+use rustc::ty::TyCtxt;
+
+pub struct RemoveUnitStorage;
+
+impl<'tcx> MirPass<'tcx> for RemoveUnitStorage {
+    fn run_pass(&self, tcx: TyCtxt<'tcx>, _: MirSource<'tcx>, body: &mut BodyAndCache<'tcx>) {
+        // This pass removes UB, so only run it when optimizations are enabled.
+        if tcx.sess.opts.debugging_opts.mir_opt_level == 0 {
+            return;
+        }
+
+        let (blocks, locals) = body.basic_blocks_and_local_decls_mut();
+
+        for block in blocks {
+            for stmt in &mut block.statements {
+                if let StatementKind::StorageLive(l) | StatementKind::StorageDead(l) = stmt.kind {
+                    if locals[l].ty == tcx.types.unit {
+                        stmt.make_nop();
+                    }
+                }
+            }
+        }
+    }
+}

src/test/ui/consts/dangling-alloc-id-ice.rs

@@ -1,11 +1,14 @@
+// run-pass
 // https://github.com/rust-lang/rust/issues/55223
 
 union Foo<'a> {
+    //~^ WARN union is never used
     y: &'a (),
     long_live_the_unit: &'static (),
 }
 
-const FOO: &() = { //~ ERROR any use of this value will cause an error
+const FOO: &() = {
+    //~^ WARN constant item is never used
     let y = ();
     unsafe { Foo { y: &y }.long_live_the_unit }
 };

src/test/ui/consts/dangling-alloc-id-ice.stderr

@@ -1,13 +1,18 @@
-error: any use of this value will cause an error
-  --> $DIR/dangling-alloc-id-ice.rs:8:1
+warning: union is never used: `Foo`
+  --> $DIR/dangling-alloc-id-ice.rs:4:7
+   |
+LL | union Foo<'a> {
+   |       ^^^
+   |
+   = note: `#[warn(dead_code)]` on by default
+
+warning: constant item is never used: `FOO`
+  --> $DIR/dangling-alloc-id-ice.rs:10:1
    |
 LL | / const FOO: &() = {
+LL | |
 LL | |     let y = ();
 LL | |     unsafe { Foo { y: &y }.long_live_the_unit }
 LL | | };
-   | |__^ encountered dangling pointer in final constant
-   |
-   = note: `#[deny(const_err)]` on by default
-
-error: aborting due to previous error
+   | |__^