Rollup merge of #60597 - wesleywiser:const_prop, r=oli-obk

Do some simple constant propagation in the ConstProp pass

r? @oli-obk

I added a few test cases. Let me know if you think there should be more.

4 files changed, 125 insertions, 0 deletions

diff --git a/src/test/mir-opt/const_prop/array_index.rs b/src/test/mir-opt/const_prop/array_index.rs
new file mode 100644
index 00000000000..4b97af68ff0
--- /dev/null
+++ b/src/test/mir-opt/const_prop/array_index.rs
@@ -0,0 +1,33 @@
+fn main() {
+    let x: u32 = [0, 1, 2, 3][2];
+}
+
+// END RUST SOURCE
+// START rustc.main.ConstProp.before.mir
+//  bb0: {
+//      ...
+//      _2 = [const 0u32, const 1u32, const 2u32, const 3u32];
+//      ...
+//      _3 = const 2usize;
+//      _4 = const 4usize;
+//      _5 = Lt(_3, _4);
+//      assert(move _5, "index out of bounds: the len is move _4 but the index is _3") -> bb1;
+//  }
+//  bb1: {
+//      _1 = _2[_3];
+//      ...
+//      return;
+//  }
+// END rustc.main.ConstProp.before.mir
+// START rustc.main.ConstProp.after.mir
+//  bb0: {
+//      ...
+//      _5 = const true;
+//      assert(move _5, "index out of bounds: the len is move _4 but the index is _3") -> bb1;
+//  }
+//  bb1: {
+//      _1 = _2[_3];
+//      ...
+//      return;
+//  }
+// END rustc.main.ConstProp.after.mir
diff --git a/src/test/mir-opt/const_prop/checked_add.rs b/src/test/mir-opt/const_prop/checked_add.rs
new file mode 100644
index 00000000000..0718316307c
--- /dev/null
+++ b/src/test/mir-opt/const_prop/checked_add.rs
@@ -0,0 +1,21 @@
+// compile-flags: -C overflow-checks=on
+
+fn main() {
+    let x: u32 = 1 + 1;
+}
+
+// END RUST SOURCE
+// START rustc.main.ConstProp.before.mir
+//  bb0: {
+//      ...
+//      _2 = CheckedAdd(const 1u32, const 1u32);
+//      assert(!move (_2.1: bool), "attempt to add with overflow") -> bb1;
+//  }
+// END rustc.main.ConstProp.before.mir
+// START rustc.main.ConstProp.after.mir
+//  bb0: {
+//      ...
+//      _2 = (const 2u32, const false);
+//      assert(!move (_2.1: bool), "attempt to add with overflow") -> bb1;
+//  }
+// END rustc.main.ConstProp.after.mir
diff --git a/src/test/mir-opt/const_prop/const_prop_fails_gracefully.rs b/src/test/mir-opt/const_prop/const_prop_fails_gracefully.rs
new file mode 100644
index 00000000000..97d3abdcc6c
--- /dev/null
+++ b/src/test/mir-opt/const_prop/const_prop_fails_gracefully.rs
@@ -0,0 +1,34 @@
+#[inline(never)]
+fn read(_: usize) { }
+
+fn main() {
+    const FOO: &i32 = &1;
+    let x = FOO as *const i32 as usize;
+    read(x);
+}
+
+// END RUST SOURCE
+// START rustc.main.ConstProp.before.mir
+//  bb0: {
+//      ...
+//      _3 = _4;
+//      _2 = move _3 as *const i32 (Misc);
+//      ...
+//      _1 = move _2 as usize (Misc);
+//      ...
+//      _6 = _1;
+//      _5 = const read(move _6) -> bb1;
+//  }
+// END rustc.main.ConstProp.before.mir
+// START rustc.main.ConstProp.after.mir
+//  bb0: {
+//      ...
+//      _3 = _4;
+//      _2 = move _3 as *const i32 (Misc);
+//      ...
+//      _1 = move _2 as usize (Misc);
+//      ...
+//      _6 = _1;
+//      _5 = const read(move _6) -> bb1;
+//  }
+// END rustc.main.ConstProp.after.mir
diff --git a/src/test/mir-opt/const_prop/slice_len.rs b/src/test/mir-opt/const_prop/slice_len.rs
new file mode 100644
index 00000000000..3435ca07f4c
--- /dev/null
+++ b/src/test/mir-opt/const_prop/slice_len.rs
@@ -0,0 +1,37 @@
+fn test() -> &'static [u32] {
+    &[1, 2]
+}
+
+fn main() {
+    let x = test()[0];
+}
+
+// END RUST SOURCE
+// START rustc.main.ConstProp.before.mir
+//  bb1: {
+//      ...
+//      _3 = const 0usize;
+//      _4 = Len((*_2));
+//      _5 = Lt(_3, _4);
+//      assert(move _5, "index out of bounds: the len is move _4 but the index is _3") -> bb2;
+//  }
+//  bb2: {
+//      _1 = (*_2)[_3];
+//      ...
+//      return;
+//  }
+// END rustc.main.ConstProp.before.mir
+// START rustc.main.ConstProp.after.mir
+//  bb0: {
+//      ...
+//      _3 = const 0usize;
+//      _4 = Len((*_2));
+//      _5 = Lt(_3, _4);
+//      assert(move _5, "index out of bounds: the len is move _4 but the index is _3") -> bb2;
+//  }
+//  bb2: {
+//      _1 = (*_2)[_3];
+//      ...
+//      return;
+//  }
+// END rustc.main.ConstProp.after.mir