1 files changed, 100 insertions, 0 deletions
diff --git a/compiler/rustc_mir_transform/src/const_debuginfo.rs b/compiler/rustc_mir_transform/src/const_debuginfo.rs
new file mode 100644
index 000000000..6f0ae4f07
--- /dev/null
+++ b/compiler/rustc_mir_transform/src/const_debuginfo.rs
@@ -0,0 +1,100 @@
+//! Finds locals which are assigned once to a const and unused except for debuginfo and converts
+//! their debuginfo to use the const directly, allowing the local to be removed.
+
+use rustc_middle::{
+    mir::{
+        visit::{PlaceContext, Visitor},
+        Body, Constant, Local, Location, Operand, Rvalue, StatementKind, VarDebugInfoContents,
+    },
+    ty::TyCtxt,
+};
+
+use crate::MirPass;
+use rustc_index::{bit_set::BitSet, vec::IndexVec};
+
+pub struct ConstDebugInfo;
+
+impl<'tcx> MirPass<'tcx> for ConstDebugInfo {
+    fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
+        sess.opts.unstable_opts.unsound_mir_opts && sess.mir_opt_level() > 0
+    }
+
+    fn run_pass(&self, _tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        trace!("running ConstDebugInfo on {:?}", body.source);
+
+        for (local, constant) in find_optimization_oportunities(body) {
+            for debuginfo in &mut body.var_debug_info {
+                if let VarDebugInfoContents::Place(p) = debuginfo.value {
+                    if p.local == local && p.projection.is_empty() {
+                        trace!(
+                            "changing debug info for {:?} from place {:?} to constant {:?}",
+                            debuginfo.name,
+                            p,
+                            constant
+                        );
+                        debuginfo.value = VarDebugInfoContents::Const(constant);
+                    }
+                }
+            }
+        }
+    }
+}
+
+struct LocalUseVisitor {
+    local_mutating_uses: IndexVec<Local, u8>,
+    local_assignment_locations: IndexVec<Local, Option<Location>>,
+}
+
+fn find_optimization_oportunities<'tcx>(body: &Body<'tcx>) -> Vec<(Local, Constant<'tcx>)> {
+    let mut visitor = LocalUseVisitor {
+        local_mutating_uses: IndexVec::from_elem(0, &body.local_decls),
+        local_assignment_locations: IndexVec::from_elem(None, &body.local_decls),
+    };
+
+    visitor.visit_body(body);
+
+    let mut locals_to_debuginfo = BitSet::new_empty(body.local_decls.len());
+    for debuginfo in &body.var_debug_info {
+        if let VarDebugInfoContents::Place(p) = debuginfo.value && let Some(l) = p.as_local() {
+            locals_to_debuginfo.insert(l);
+        }
+    }
+
+    let mut eligible_locals = Vec::new();
+    for (local, mutating_uses) in visitor.local_mutating_uses.drain_enumerated(..) {
+        if mutating_uses != 1 || !locals_to_debuginfo.contains(local) {
+            continue;
+        }
+
+        if let Some(location) = visitor.local_assignment_locations[local] {
+            let bb = &body[location.block];
+
+            // The value is assigned as the result of a call, not a constant
+            if bb.statements.len() == location.statement_index {
+                continue;
+            }
+
+            if let StatementKind::Assign(box (p, Rvalue::Use(Operand::Constant(box c)))) =
+                &bb.statements[location.statement_index].kind
+            {
+                if let Some(local) = p.as_local() {
+                    eligible_locals.push((local, *c));
+                }
+            }
+        }
+    }
+
+    eligible_locals
+}
+
+impl Visitor<'_> for LocalUseVisitor {
+    fn visit_local(&mut self, local: Local, context: PlaceContext, location: Location) {
+        if context.is_mutating_use() {
+            self.local_mutating_uses[local] = self.local_mutating_uses[local].saturating_add(1);
+
+            if context.is_place_assignment() {
+                self.local_assignment_locations[local] = Some(location);
+            }
+        }
+    }
+}