Do not attempt to const-prop unsize.

Author: cjgillot

compiler/rustc_mir_transform/src/const_prop.rs

@@ -532,6 +532,8 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
 
                 return None;
             }
+            // We do not track pointers, and unsized lengths are handled by `NormalizeArrayLen`.
+            Rvalue::Cast(CastKind::Pointer(_), ..) => return None,
 
             // There's no other checking to do at this time.
             Rvalue::Aggregate(..)

tests/mir-opt/const_prop/slice_len.main.ConstProp.32bit.diff

@@ -28,17 +28,15 @@
           StorageDead(_3);                 // scope 0 at $DIR/slice_len.rs:+1:18: +1:19
           StorageLive(_6);                 // scope 0 at $DIR/slice_len.rs:+1:31: +1:32
           _6 = const 1_usize;              // scope 0 at $DIR/slice_len.rs:+1:31: +1:32
--         _7 = Len((*_2));                 // scope 0 at $DIR/slice_len.rs:+1:5: +1:33
+          _7 = const 3_usize;              // scope 0 at $DIR/slice_len.rs:+1:5: +1:33
 -         _8 = Lt(_6, _7);                 // scope 0 at $DIR/slice_len.rs:+1:5: +1:33
 -         assert(move _8, "index out of bounds: the length is {} but the index is {}", move _7, _6) -> bb1; // scope 0 at $DIR/slice_len.rs:+1:5: +1:33
-+         _7 = const 3_usize;              // scope 0 at $DIR/slice_len.rs:+1:5: +1:33
 +         _8 = const true;                 // scope 0 at $DIR/slice_len.rs:+1:5: +1:33
 +         assert(const true, "index out of bounds: the length is {} but the index is {}", move _7, _6) -> bb1; // scope 0 at $DIR/slice_len.rs:+1:5: +1:33
       }
 
       bb1: {
--         _1 = (*_2)[_6];                  // scope 0 at $DIR/slice_len.rs:+1:5: +1:33
-+         _1 = const 2_u32;                // scope 0 at $DIR/slice_len.rs:+1:5: +1:33
+          _1 = (*_2)[_6];                  // scope 0 at $DIR/slice_len.rs:+1:5: +1:33
           StorageDead(_6);                 // scope 0 at $DIR/slice_len.rs:+1:33: +1:34
           StorageDead(_4);                 // scope 0 at $DIR/slice_len.rs:+1:33: +1:34
           StorageDead(_2);                 // scope 0 at $DIR/slice_len.rs:+1:33: +1:34

tests/mir-opt/const_prop/slice_len.main.ConstProp.64bit.diff

@@ -28,17 +28,15 @@
           StorageDead(_3);                 // scope 0 at $DIR/slice_len.rs:+1:18: +1:19
           StorageLive(_6);                 // scope 0 at $DIR/slice_len.rs:+1:31: +1:32
           _6 = const 1_usize;              // scope 0 at $DIR/slice_len.rs:+1:31: +1:32
--         _7 = Len((*_2));                 // scope 0 at $DIR/slice_len.rs:+1:5: +1:33
+          _7 = const 3_usize;              // scope 0 at $DIR/slice_len.rs:+1:5: +1:33
 -         _8 = Lt(_6, _7);                 // scope 0 at $DIR/slice_len.rs:+1:5: +1:33
 -         assert(move _8, "index out of bounds: the length is {} but the index is {}", move _7, _6) -> bb1; // scope 0 at $DIR/slice_len.rs:+1:5: +1:33
-+         _7 = const 3_usize;              // scope 0 at $DIR/slice_len.rs:+1:5: +1:33
 +         _8 = const true;                 // scope 0 at $DIR/slice_len.rs:+1:5: +1:33
 +         assert(const true, "index out of bounds: the length is {} but the index is {}", move _7, _6) -> bb1; // scope 0 at $DIR/slice_len.rs:+1:5: +1:33
       }
 
       bb1: {
--         _1 = (*_2)[_6];                  // scope 0 at $DIR/slice_len.rs:+1:5: +1:33
-+         _1 = const 2_u32;                // scope 0 at $DIR/slice_len.rs:+1:5: +1:33
+          _1 = (*_2)[_6];                  // scope 0 at $DIR/slice_len.rs:+1:5: +1:33
           StorageDead(_6);                 // scope 0 at $DIR/slice_len.rs:+1:33: +1:34
           StorageDead(_4);                 // scope 0 at $DIR/slice_len.rs:+1:33: +1:34
           StorageDead(_2);                 // scope 0 at $DIR/slice_len.rs:+1:33: +1:34

tests/mir-opt/const_prop/slice_len.rs

@@ -1,5 +1,5 @@
 // unit-test: ConstProp
-// compile-flags: -Zmir-enable-passes=+InstCombine
+// compile-flags: -Zmir-enable-passes=+InstCombine,+NormalizeArrayLen
 // EMIT_MIR_FOR_EACH_BIT_WIDTH
 
 // EMIT_MIR slice_len.main.ConstProp.diff