[mir-opt] Allow debuginfo to be generated for a constant or a Place

Prior to this commit, debuginfo was always generated by mapping a name
to a Place. This has the side-effect that `SimplifyLocals` cannot remove
locals that are only used for debuginfo because their other uses have
been const-propagated.

To allow these locals to be removed, we now allow debuginfo to point to
a constant value. The `ConstProp` pass detects when debuginfo points to
a local with a known constant value and replaces it with the value. This
allows the later `SimplifyLocals` pass to remove the local.

6 files changed, 143 insertions, 28 deletions

diff --git a/compiler/rustc_mir/src/borrow_check/mod.rs b/compiler/rustc_mir/src/borrow_check/mod.rs
index de54c5582e0..3a6316b1ed3 100644
--- a/compiler/rustc_mir/src/borrow_check/mod.rs
+++ b/compiler/rustc_mir/src/borrow_check/mod.rs
@@ -11,7 +11,7 @@ use rustc_index::vec::IndexVec;
 use rustc_infer::infer::{InferCtxt, TyCtxtInferExt};
 use rustc_middle::mir::{
     traversal, Body, ClearCrossCrate, Local, Location, Mutability, Operand, Place, PlaceElem,
-    PlaceRef,
+    PlaceRef, VarDebugInfoContents,
 };
 use rustc_middle::mir::{AggregateKind, BasicBlock, BorrowCheckResult, BorrowKind};
 use rustc_middle::mir::{Field, ProjectionElem, Promoted, Rvalue, Statement, StatementKind};
@@ -133,19 +133,21 @@ fn do_mir_borrowck<'a, 'tcx>(
 
     let mut local_names = IndexVec::from_elem(None, &input_body.local_decls);
     for var_debug_info in &input_body.var_debug_info {
-        if let Some(local) = var_debug_info.place.as_local() {
-            if let Some(prev_name) = local_names[local] {
-                if var_debug_info.name != prev_name {
-                    span_bug!(
-                        var_debug_info.source_info.span,
-                        "local {:?} has many names (`{}` vs `{}`)",
-                        local,
-                        prev_name,
-                        var_debug_info.name
-                    );
+        if let VarDebugInfoContents::Place(place) = var_debug_info.value {
+            if let Some(local) = place.as_local() {
+                if let Some(prev_name) = local_names[local] {
+                    if var_debug_info.name != prev_name {
+                        span_bug!(
+                            var_debug_info.source_info.span,
+                            "local {:?} has many names (`{}` vs `{}`)",
+                            local,
+                            prev_name,
+                            var_debug_info.name
+                        );
+                    }
                 }
+                local_names[local] = Some(var_debug_info.name);
             }
-            local_names[local] = Some(var_debug_info.name);
         }
     }
 
diff --git a/compiler/rustc_mir/src/transform/const_debuginfo.rs b/compiler/rustc_mir/src/transform/const_debuginfo.rs
new file mode 100644
index 00000000000..ed2c48b3568
--- /dev/null
+++ b/compiler/rustc_mir/src/transform/const_debuginfo.rs
@@ -0,0 +1,98 @@
+//! 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::transform::MirPass;
+use rustc_index::{bit_set::BitSet, vec::IndexVec};
+
+pub struct ConstDebugInfo;
+
+impl<'tcx> MirPass<'tcx> for ConstDebugInfo {
+    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 {
+            if let Some(l) = p.as_local() {
+                locals_to_debuginfo.insert(l);
+            }
+        }
+    }
+
+    let mut eligable_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() {
+                    eligable_locals.push((local, *c));
+                }
+            }
+        }
+    }
+
+    eligable_locals
+}
+
+impl<'tcx> Visitor<'tcx> 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);
+            }
+        }
+    }
+}
diff --git a/compiler/rustc_mir/src/transform/mod.rs b/compiler/rustc_mir/src/transform/mod.rs
index e86d11e248f..809e29fb982 100644
--- a/compiler/rustc_mir/src/transform/mod.rs
+++ b/compiler/rustc_mir/src/transform/mod.rs
@@ -21,6 +21,7 @@ pub mod check_consts;
 pub mod check_packed_ref;
 pub mod check_unsafety;
 pub mod cleanup_post_borrowck;
+pub mod const_debuginfo;
 pub mod const_prop;
 pub mod coverage;
 pub mod deaggregator;
@@ -408,6 +409,7 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         &remove_noop_landing_pads::RemoveNoopLandingPads,
         &simplify::SimplifyCfg::new("final"),
         &nrvo::RenameReturnPlace,
+        &const_debuginfo::ConstDebugInfo,
         &simplify::SimplifyLocals,
         &multiple_return_terminators::MultipleReturnTerminators,
     ];
diff --git a/compiler/rustc_mir/src/transform/simplify_try.rs b/compiler/rustc_mir/src/transform/simplify_try.rs
index 27bb1def726..bea95bf43d2 100644
--- a/compiler/rustc_mir/src/transform/simplify_try.rs
+++ b/compiler/rustc_mir/src/transform/simplify_try.rs
@@ -246,14 +246,19 @@ fn get_arm_identity_info<'a, 'tcx>(
         tmp_assigned_vars.insert(*r);
     }
 
-    let dbg_info_to_adjust: Vec<_> =
-        debug_info
-            .iter()
-            .enumerate()
-            .filter_map(|(i, var_info)| {
-                if tmp_assigned_vars.contains(var_info.place.local) { Some(i) } else { None }
-            })
-            .collect();
+    let dbg_info_to_adjust: Vec<_> = debug_info
+        .iter()
+        .enumerate()
+        .filter_map(|(i, var_info)| {
+            if let VarDebugInfoContents::Place(p) = var_info.value {
+                if tmp_assigned_vars.contains(p.local) {
+                    return Some(i);
+                }
+            }
+
+            None
+        })
+        .collect();
 
     Some(ArmIdentityInfo {
         local_temp_0: local_tmp_s0,
@@ -340,9 +345,11 @@ fn optimization_applies<'tcx>(
     // Check that debug info only points to full Locals and not projections.
     for dbg_idx in &opt_info.dbg_info_to_adjust {
         let dbg_info = &var_debug_info[*dbg_idx];
-        if !dbg_info.place.projection.is_empty() {
-            trace!("NO: debug info for {:?} had a projection {:?}", dbg_info.name, dbg_info.place);
-            return false;
+        if let VarDebugInfoContents::Place(p) = dbg_info.value {
+            if !p.projection.is_empty() {
+                trace!("NO: debug info for {:?} had a projection {:?}", dbg_info.name, p);
+                return false;
+            }
         }
     }
 
@@ -423,9 +430,15 @@ impl<'tcx> MirPass<'tcx> for SimplifyArmIdentity {
                 // Fix the debug info to point to the right local
                 for dbg_index in opt_info.dbg_info_to_adjust {
                     let dbg_info = &mut debug_info[dbg_index];
-                    assert!(dbg_info.place.projection.is_empty());
-                    dbg_info.place.local = opt_info.local_0;
-                    dbg_info.place.projection = opt_info.dbg_projection;
+                    assert!(
+                        matches!(dbg_info.value, VarDebugInfoContents::Place(_)),
+                        "value was not a Place"
+                    );
+                    if let VarDebugInfoContents::Place(p) = &mut dbg_info.value {
+                        assert!(p.projection.is_empty());
+                        p.local = opt_info.local_0;
+                        p.projection = opt_info.dbg_projection;
+                    }
                 }
 
                 trace!("block is now {:?}", bb.statements);
diff --git a/compiler/rustc_mir/src/util/graphviz.rs b/compiler/rustc_mir/src/util/graphviz.rs
index 625f1a3e684..370010d65f0 100644
--- a/compiler/rustc_mir/src/util/graphviz.rs
+++ b/compiler/rustc_mir/src/util/graphviz.rs
@@ -220,7 +220,7 @@ fn write_graph_label<'tcx, W: Write>(
             w,
             r#"debug {} =&gt; {};<br align="left"/>"#,
             var_debug_info.name,
-            escape(&var_debug_info.place)
+            escape(&var_debug_info.value),
         )?;
     }
 
diff --git a/compiler/rustc_mir/src/util/pretty.rs b/compiler/rustc_mir/src/util/pretty.rs
index cd60602b088..b6a1b652cf6 100644
--- a/compiler/rustc_mir/src/util/pretty.rs
+++ b/compiler/rustc_mir/src/util/pretty.rs
@@ -495,7 +495,7 @@ fn write_scope_tree(
 
         let indented_debug_info = format!(
             "{0:1$}debug {2} => {3:?};",
-            INDENT, indent, var_debug_info.name, var_debug_info.place,
+            INDENT, indent, var_debug_info.name, var_debug_info.value,
         );
 
         writeln!(