Improve SimplifyLocals pass so it can remove unused consts

The `ConstProp` can cause many locals to be initialized to a constant
value and then never read from. `ConstProp` can also evaluate ZSTs into
constant values. Previously, many of these would be removed by other
parts of the MIR optimization pipeline. However, evaluating ZSTs
(especially `()`) into constant values defeated those parts of the
optimizer and so in a2e3ed5c05, I added a
hack to `ConstProp` that skips evaluating ZSTs to avoid that regression.

This commit changes `SimplifyLocals` so that it doesn't consider writes
of const values to a local to be a use of that local. In doing so,
`SimplifyLocals` is able to remove otherwise unused locals left behind
by other optimization passes (`ConstProp` in particular).

src/test/incremental/hashes/for_loops.rs

@@ -25,7 +25,7 @@ pub fn change_loop_body() {
 }
 
 #[cfg(not(cfail1))]
-#[rustc_clean(cfg="cfail2", except="HirBody, mir_built, optimized_mir")]
+#[rustc_clean(cfg="cfail2", except="HirBody, mir_built")]
 #[rustc_clean(cfg="cfail3")]
 pub fn change_loop_body() {
     let mut _x = 0;

src/test/incremental/hashes/let_expressions.rs

@@ -22,7 +22,7 @@ pub fn change_name() {
 
 #[cfg(not(cfail1))]
 #[rustc_clean(cfg="cfail2",
-    except="HirBody,mir_built,optimized_mir")]
+    except="HirBody,mir_built")]
 #[rustc_clean(cfg="cfail3")]
 pub fn change_name() {
     let _y = 2u64;
@@ -86,7 +86,7 @@ pub fn change_mutability_of_slot() {
 
 #[cfg(not(cfail1))]
 #[rustc_clean(cfg="cfail2",
-    except="HirBody,typeck_tables_of,mir_built,optimized_mir")]
+    except="HirBody,typeck_tables_of,mir_built")]
 #[rustc_clean(cfg="cfail3")]
 pub fn change_mutability_of_slot() {
     let _x: u64 = 0;
@@ -182,7 +182,7 @@ pub fn add_initializer() {
 
 #[cfg(not(cfail1))]
 #[rustc_clean(cfg="cfail2",
-    except="HirBody,typeck_tables_of,mir_built,optimized_mir")]
+    except="HirBody,typeck_tables_of,mir_built")]
 #[rustc_clean(cfg="cfail3")]
 pub fn add_initializer() {
     let _x: i16 = 3i16;
@@ -198,7 +198,7 @@ pub fn change_initializer() {
 
 #[cfg(not(cfail1))]
 #[rustc_clean(cfg="cfail2",
-    except="HirBody,mir_built,optimized_mir")]
+    except="HirBody,mir_built")]
 #[rustc_clean(cfg="cfail3")]
 pub fn change_initializer() {
     let _x = 5u16;

src/test/incremental/hashes/loop_expressions.rs

@@ -25,7 +25,7 @@ pub fn change_loop_body() {
 }
 
 #[cfg(not(cfail1))]
-#[rustc_clean(cfg="cfail2", except="HirBody, mir_built, optimized_mir")]
+#[rustc_clean(cfg="cfail2", except="HirBody, mir_built")]
 #[rustc_clean(cfg="cfail3")]
 pub fn change_loop_body() {
     let mut _x = 0;

src/test/incremental/hashes/while_let_loops.rs

@@ -25,7 +25,7 @@ pub fn change_loop_body() {
 }
 
 #[cfg(not(cfail1))]
-#[rustc_clean(cfg="cfail2", except="HirBody, mir_built, optimized_mir")]
+#[rustc_clean(cfg="cfail2", except="HirBody, mir_built")]
 #[rustc_clean(cfg="cfail3")]
 pub fn change_loop_body() {
     let mut _x = 0;

src/test/incremental/hashes/while_loops.rs

@@ -25,7 +25,7 @@ pub fn change_loop_body() {
 }
 
 #[cfg(not(cfail1))]
-#[rustc_clean(cfg="cfail2", except="HirBody, mir_built, optimized_mir")]
+#[rustc_clean(cfg="cfail2", except="HirBody, mir_built")]
 #[rustc_clean(cfg="cfail3")]
 pub fn change_loop_body() {
     let mut _x = 0;

src/test/mir-opt/simplify-locals-removes-unused-consts.rs

@@ -0,0 +1,89 @@
+// compile-flags: -C overflow-checks=no
+
+fn use_zst(_: ((), ())) { }
+
+struct Temp {
+    x: u8
+}
+
+fn use_u8(_: u8) { }
+
+fn main() {
+    let ((), ()) = ((), ());
+    use_zst(((), ()));
+
+    use_u8((Temp { x : 40 }).x + 2);
+}
+
+// END RUST SOURCE
+
+// START rustc.main.SimplifyLocals.before.mir
+// let mut _0: ();
+// let mut _1: ((), ());
+// let mut _2: ();
+// let mut _3: ();
+// let _4: ();
+// let mut _5: ((), ());
+// let mut _6: ();
+// let mut _7: ();
+// let _8: ();
+// let mut _9: u8;
+// let mut _10: u8;
+// let mut _11: Temp;
+// scope 1 {
+// }
+// bb0: {
+//   StorageLive(_1);
+//   StorageLive(_2);
+//   _2 = const Scalar(<ZST>) : ();
+//   StorageLive(_3);
+//   _3 = const Scalar(<ZST>) : ();
+//   _1 = const Scalar(<ZST>) : ((), ());
+//   StorageDead(_3);
+//   StorageDead(_2);
+//   StorageDead(_1);
+//   StorageLive(_4);
+//   StorageLive(_6);
+//   _6 = const Scalar(<ZST>) : ();
+//   StorageLive(_7);
+//   _7 = const Scalar(<ZST>) : ();
+//   StorageDead(_7);
+//   StorageDead(_6);
+//   _4 = const use_zst(const Scalar(<ZST>) : ((), ())) -> bb1;
+// }
+// bb1: {
+//   StorageDead(_4);
+//   StorageLive(_8);
+//   StorageLive(_10);
+//   StorageLive(_11);
+//   _11 = const Scalar(0x28) : Temp;
+//   _10 = const 40u8;
+//   StorageDead(_10);
+//   _8 = const use_u8(const 42u8) -> bb2;
+// }
+// bb2: {
+//   StorageDead(_11);
+//   StorageDead(_8);
+//   return;
+// }
+// END rustc.main.SimplifyLocals.before.mir
+// START rustc.main.SimplifyLocals.after.mir
+// let mut _0: ();
+// let _1: ();
+// let _2: ();
+// scope 1 {
+// }
+// bb0: {
+//   StorageLive(_1);
+//   _1 = const use_zst(const Scalar(<ZST>) : ((), ())) -> bb1;
+// }
+// bb1: {
+//   StorageDead(_1);
+//   StorageLive(_2);
+//   _2 = const use_u8(const 42u8) -> bb2;
+// }
+// bb2: {
+//   StorageDead(_2);
+//   return;
+// }
+// END rustc.main.SimplifyLocals.after.mir

src/test/mir-opt/slice-drop-shim.rs

@@ -1,5 +1,7 @@
+// compile-flags: -Zmir-opt-level=0
+
 fn main() {
-    std::ptr::drop_in_place::<[String]> as unsafe fn(_);
+    let _fn = std::ptr::drop_in_place::<[String]> as unsafe fn(_);
 }
 
 // END RUST SOURCE