Auto merge of rust-lang#110824 - cjgillot:const-prop-index, r=JakobDegen,oli-obk

ConstProp into PlaceElem::Index.

Noticed this while looking at keccak output MIR.

This pass aims to replace `ProjectionElem::Index` with `ProjectionElem::ConstantIndex` during ConstProp.

r? `@ghost`

Author: bors

compiler/rustc_mir_transform/src/const_prop.rs

@@ -806,6 +806,24 @@ impl<'tcx> MutVisitor<'tcx> for ConstPropagator<'_, 'tcx> {
         }
     }
 
+    fn process_projection_elem(
+        &mut self,
+        elem: PlaceElem<'tcx>,
+        _: Location,
+    ) -> Option<PlaceElem<'tcx>> {
+        if let PlaceElem::Index(local) = elem
+            && let Some(value) = self.get_const(local.into())
+            && self.should_const_prop(&value)
+            && let interpret::Operand::Immediate(interpret::Immediate::Scalar(scalar)) = *value
+            && let Ok(offset) = scalar.to_target_usize(&self.tcx)
+            && let Some(min_length) = offset.checked_add(1)
+        {
+            Some(PlaceElem::ConstantIndex { offset, min_length, from_end: false })
+        } else {
+            None
+        }
+    }
+
     fn visit_assign(
         &mut self,
         place: &mut Place<'tcx>,

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

@@ -45,7 +45,8 @@
       }
 
       bb1: {
-          _5 = (*_1)[_6];                  // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:+3:18: +3:25
+-         _5 = (*_1)[_6];                  // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:+3:18: +3:25
++         _5 = (*_1)[3 of 4];              // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:+3:18: +3:25
           StorageDead(_6);                 // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:+3:25: +3:26
           _0 = const ();                   // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:+2:5: +4:6
           StorageDead(_5);                 // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:+4:5: +4:6

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

@@ -45,7 +45,8 @@
       }
 
       bb1: {
-          _5 = (*_1)[_6];                  // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:+3:18: +3:25
+-         _5 = (*_1)[_6];                  // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:+3:18: +3:25
++         _5 = (*_1)[3 of 4];              // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:+3:18: +3:25
           StorageDead(_6);                 // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:+3:25: +3:26
           _0 = const ();                   // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:+2:5: +4:6
           StorageDead(_5);                 // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:+4:5: +4:6

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

@@ -27,7 +27,8 @@
       }
 
       bb1: {
-          _1 = _2[_3];                     // scope 0 at $DIR/large_array_index.rs:+2:17: +2:32
+-         _1 = _2[_3];                     // scope 0 at $DIR/large_array_index.rs:+2:17: +2:32
++         _1 = _2[2 of 3];                 // scope 0 at $DIR/large_array_index.rs:+2:17: +2:32
           StorageDead(_3);                 // scope 0 at $DIR/large_array_index.rs:+2:32: +2:33
           StorageDead(_2);                 // scope 0 at $DIR/large_array_index.rs:+2:32: +2:33
           _0 = const ();                   // scope 0 at $DIR/large_array_index.rs:+0:11: +3:2

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

@@ -27,7 +27,8 @@
       }
 
       bb1: {
-          _1 = _2[_3];                     // scope 0 at $DIR/large_array_index.rs:+2:17: +2:32
+-         _1 = _2[_3];                     // scope 0 at $DIR/large_array_index.rs:+2:17: +2:32
++         _1 = _2[2 of 3];                 // scope 0 at $DIR/large_array_index.rs:+2:17: +2:32
           StorageDead(_3);                 // scope 0 at $DIR/large_array_index.rs:+2:32: +2:33
           StorageDead(_2);                 // scope 0 at $DIR/large_array_index.rs:+2:32: +2:33
           _0 = const ();                   // scope 0 at $DIR/large_array_index.rs:+0:11: +3:2