Add an InstCombine for redundant casts

tests/mir-opt/casts.redundant.InstCombine.diff

@@ -0,0 +1,25 @@
+- // MIR for `redundant` before InstCombine
++ // MIR for `redundant` after InstCombine
+  
+  fn redundant(_1: *const &u8) -> *const &u8 {
+      debug x => _1;                       // in scope 0 at $DIR/casts.rs:+0:30: +0:31
+      let mut _0: *const &u8;              // return place in scope 0 at $DIR/casts.rs:+0:51: +0:64
+      let mut _2: *const &u8;              // in scope 0 at $DIR/casts.rs:+1:5: +1:55
+      let mut _3: *const &u8;              // in scope 0 at $DIR/casts.rs:+1:36: +1:37
+      scope 1 (inlined generic_cast::<&u8, &u8>) { // at $DIR/casts.rs:6:5: 6:38
+          debug x => _3;                   // in scope 1 at $DIR/casts.rs:10:23: 10:24
+      }
+  
+      bb0: {
+          StorageLive(_2);                 // scope 0 at $DIR/casts.rs:+1:5: +1:55
+          StorageLive(_3);                 // scope 0 at $DIR/casts.rs:+1:36: +1:37
+          _3 = _1;                         // scope 0 at $DIR/casts.rs:+1:36: +1:37
+-         _2 = _3 as *const &u8 (PtrToPtr); // scope 1 at $DIR/casts.rs:11:5: 11:18
++         _2 = _3;                         // scope 1 at $DIR/casts.rs:11:5: 11:18
+          StorageDead(_3);                 // scope 0 at $DIR/casts.rs:+1:37: +1:38
+          _0 = _2;                         // scope 0 at $DIR/casts.rs:+1:5: +1:55
+          StorageDead(_2);                 // scope 0 at $DIR/casts.rs:+2:1: +2:2
+          return;                          // scope 0 at $DIR/casts.rs:+2:2: +2:2
+      }
+  }
+  

tests/mir-opt/casts.redundant.PreCodegen.after.mir

@@ -0,0 +1,14 @@
+// MIR for `redundant` after PreCodegen
+
+fn redundant(_1: *const &u8) -> *const &u8 {
+    debug x => _1;                       // in scope 0 at $DIR/casts.rs:+0:30: +0:31
+    let mut _0: *const &u8;              // return place in scope 0 at $DIR/casts.rs:+0:51: +0:64
+    scope 1 (inlined generic_cast::<&u8, &u8>) { // at $DIR/casts.rs:6:5: 6:38
+        debug x => _1;                   // in scope 1 at $DIR/casts.rs:10:23: 10:24
+    }
+
+    bb0: {
+        _0 = _1;                         // scope 0 at $DIR/casts.rs:+1:5: +1:55
+        return;                          // scope 0 at $DIR/casts.rs:+2:2: +2:2
+    }
+}

tests/mir-opt/casts.roundtrip.PreCodegen.after.mir

@@ -0,0 +1,15 @@
+// MIR for `roundtrip` after PreCodegen
+
+fn roundtrip(_1: *const u8) -> *const u8 {
+    debug x => _1;                       // in scope 0 at $DIR/casts.rs:+0:18: +0:19
+    let mut _0: *const u8;               // return place in scope 0 at $DIR/casts.rs:+0:35: +0:44
+    let mut _2: *mut u8;                 // in scope 0 at $DIR/casts.rs:+1:5: +1:17
+
+    bb0: {
+        StorageLive(_2);                 // scope 0 at $DIR/casts.rs:+1:5: +1:17
+        _2 = _1 as *mut u8 (PtrToPtr);   // scope 0 at $DIR/casts.rs:+1:5: +1:17
+        _0 = move _2 as *const u8 (Pointer(MutToConstPointer)); // scope 0 at $DIR/casts.rs:+1:5: +1:17
+        StorageDead(_2);                 // scope 0 at $DIR/casts.rs:+1:16: +1:17
+        return;                          // scope 0 at $DIR/casts.rs:+2:2: +2:2
+    }
+}

tests/mir-opt/casts.rs

@@ -0,0 +1,17 @@
+#![crate_type = "lib"]
+
+// EMIT_MIR casts.redundant.InstCombine.diff
+// EMIT_MIR casts.redundant.PreCodegen.after.mir
+pub fn redundant<'a, 'b: 'a>(x: *const &'a u8) -> *const &'a u8 {
+    generic_cast::<&'a u8, &'b u8>(x) as *const &'a u8
+}
+
+#[inline]
+fn generic_cast<T, U>(x: *const T) -> *const U {
+    x as *const U
+}
+
+// EMIT_MIR casts.roundtrip.PreCodegen.after.mir
+pub fn roundtrip(x: *const u8) -> *const u8 {
+    x as *mut u8 as *const u8
+}