Fix debugger stepping for consts assigned to local vars

Author: vlad20012

compiler/rustc_mir_transform/src/single_use_consts.rs

@@ -46,6 +46,10 @@ impl<'tcx> crate::MirPass<'tcx> for SingleUseConsts {
                 continue;
             };
 
+            if finder.locals_in_debug_info.contains(local) {
+                continue;
+            }
+
             // We're only changing an operand, not the terminator kinds or successors
             let basic_blocks = body.basic_blocks.as_mut_preserves_cfg();
             let init_statement_kind = std::mem::replace(
@@ -61,12 +65,6 @@ impl<'tcx> crate::MirPass<'tcx> for SingleUseConsts {
 
             let mut replacer = LocalReplacer { tcx, local, operand: Some(operand) };
 
-            if finder.locals_in_debug_info.contains(local) {
-                for var_debug_info in &mut body.var_debug_info {
-                    replacer.visit_var_debug_info(var_debug_info);
-                }
-            }
-
             let Some(use_loc) = locations.use_loc else { continue };
 
             let use_block = &mut basic_blocks[use_loc.block];
@@ -185,21 +183,4 @@ impl<'tcx> MutVisitor<'tcx> for LocalReplacer<'tcx> {
             });
         }
     }
-
-    fn visit_var_debug_info(&mut self, var_debug_info: &mut VarDebugInfo<'tcx>) {
-        if let VarDebugInfoContents::Place(place) = &var_debug_info.value
-            && let Some(local) = place.as_local()
-            && local == self.local
-        {
-            let const_op = *self
-                .operand
-                .as_ref()
-                .unwrap_or_else(|| {
-                    bug!("the operand was already stolen");
-                })
-                .constant()
-                .unwrap();
-            var_debug_info.value = VarDebugInfoContents::Const(const_op);
-        }
-    }
 }

tests/mir-opt/single_use_consts.if_const_debug.SingleUseConsts.panic-unwind.diff

@@ -7,23 +7,20 @@
       let _2: ();
       let mut _3: bool;
       scope 1 {
--         debug my_bool => _1;
-+         debug my_bool => const <T as MyTrait>::ASSOC_BOOL;
+          debug my_bool => _1;
       }
 
       bb0: {
           StorageLive(_1);
--         _1 = const <T as MyTrait>::ASSOC_BOOL;
-+         nop;
+          _1 = const <T as MyTrait>::ASSOC_BOOL;
           StorageLive(_2);
           _2 = do_whatever() -> [return: bb1, unwind continue];
       }
 
       bb1: {
           StorageDead(_2);
           StorageLive(_3);
--         _3 = copy _1;
-+         _3 = const <T as MyTrait>::ASSOC_BOOL;
+          _3 = copy _1;
           switchInt(move _3) -> [0: bb3, otherwise: bb2];
       }
 

tests/mir-opt/single_use_consts.match_const_debug.SingleUseConsts.panic-unwind.diff

@@ -6,22 +6,19 @@
       let _1: i32;
       let _2: ();
       scope 1 {
--         debug my_int => _1;
-+         debug my_int => const <T as MyTrait>::ASSOC_INT;
+          debug my_int => _1;
       }
 
       bb0: {
           StorageLive(_1);
--         _1 = const <T as MyTrait>::ASSOC_INT;
-+         nop;
+          _1 = const <T as MyTrait>::ASSOC_INT;
           StorageLive(_2);
           _2 = do_whatever() -> [return: bb1, unwind continue];
       }
 
       bb1: {
           StorageDead(_2);
--         switchInt(copy _1) -> [7: bb4, 42: bb3, otherwise: bb2];
-+         switchInt(const <T as MyTrait>::ASSOC_INT) -> [7: bb4, 42: bb3, otherwise: bb2];
+          switchInt(copy _1) -> [7: bb4, 42: bb3, otherwise: bb2];
       }
 
       bb2: {

tests/mir-opt/single_use_consts.never_used_debug.SingleUseConsts.panic-unwind.diff

@@ -5,14 +5,12 @@
       let mut _0: ();
       let _1: i32;
       scope 1 {
--         debug my_int => _1;
-+         debug my_int => const <T as MyTrait>::ASSOC_INT;
+          debug my_int => _1;
       }
 
       bb0: {
           StorageLive(_1);
--         _1 = const <T as MyTrait>::ASSOC_INT;
-+         nop;
+          _1 = const <T as MyTrait>::ASSOC_INT;
           _0 = const ();
           StorageDead(_1);
           return;

tests/mir-opt/single_use_consts.rs

@@ -28,9 +28,9 @@ fn match_const<T: MyTrait>() -> &'static str {
 // EMIT_MIR single_use_consts.if_const_debug.SingleUseConsts.diff
 fn if_const_debug<T: MyTrait>() -> i32 {
     // CHECK-LABEL: fn if_const_debug(
-    // CHECK: my_bool => const <T as MyTrait>::ASSOC_BOOL;
-    // FIXME: `if` forces a temporary (unlike `match`), so the const isn't direct
-    // CHECK: _3 = const <T as MyTrait>::ASSOC_BOOL;
+    // Note: we must not reorder assignments for vars with debuginfo
+    // CHECK: my_bool => _1;
+    // CHECK: _3 = copy _1;
     // CHECK: switchInt(move _3)
     let my_bool = T::ASSOC_BOOL;
     do_whatever();
@@ -40,8 +40,8 @@ fn if_const_debug<T: MyTrait>() -> i32 {
 // EMIT_MIR single_use_consts.match_const_debug.SingleUseConsts.diff
 fn match_const_debug<T: MyTrait>() -> &'static str {
     // CHECK-LABEL: fn match_const_debug(
-    // CHECK: my_int => const <T as MyTrait>::ASSOC_INT;
-    // CHECK: switchInt(const <T as MyTrait>::ASSOC_INT)
+    // CHECK: my_int => _1;
+    // CHECK: switchInt(copy _1)
     let my_int = T::ASSOC_INT;
     do_whatever();
     match my_int {
@@ -55,10 +55,8 @@ fn match_const_debug<T: MyTrait>() -> &'static str {
 #[allow(unused_variables)]
 fn never_used_debug<T: MyTrait>() {
     // CHECK-LABEL: fn never_used_debug(
-    // CHECK: my_int => const <T as MyTrait>::ASSOC_INT;
-    // CHECK-NOT: ASSOC_INT
-    // CHECK: nop
-    // CHECK-NOT: ASSOC_INT
+    // CHECK: my_int => _1;
+    // CHECK: _1 = const <T as MyTrait>::ASSOC_INT
     let my_int = T::ASSOC_INT;
 }