Auto merge of #111684 - ChayimFriedman2:unused-offset-of, r=WaffleLapkin

Warn on unused `offset_of!()` result

The usage of `core::hint::must_use()` means that we don't get a specialized message. I figured out that since there are plenty of other methods that just have `#[must_use]` with no message it'll be fine, but it is a bit unfortunate that the error mentions `must_use` and not `offset_of!`.

Fixes #111669.

library/core/src/mem/mod.rs

@@ -1317,7 +1317,8 @@ impl<T> SizedTypeProperties for T {}
 /// assert_eq!(mem::offset_of!(NestedA, b.0), 0);
 /// ```
 #[unstable(feature = "offset_of", issue = "106655")]
-#[allow_internal_unstable(builtin_syntax)]
+#[allow_internal_unstable(builtin_syntax, hint_must_use)]
 pub macro offset_of($Container:ty, $($fields:tt).+ $(,)?) {
-    builtin # offset_of($Container, $($fields).+)
+    // The `{}` is for better error messages
+    crate::hint::must_use({builtin # offset_of($Container, $($fields).+)})
 }

tests/mir-opt/const_prop/offset_of.concrete.ConstProp.panic-abort.diff

@@ -0,0 +1,72 @@
+- // MIR for `concrete` before ConstProp
++ // MIR for `concrete` after ConstProp
+
+  fn concrete() -> () {
+      let mut _0: ();
+      let _1: usize;
+      let mut _2: usize;
+      let mut _4: usize;
+      let mut _6: usize;
+      let mut _8: usize;
+      scope 1 {
+          debug x => _1;
+          let _3: usize;
+          scope 2 {
+              debug y => _3;
+              let _5: usize;
+              scope 3 {
+                  debug z0 => _5;
+                  let _7: usize;
+                  scope 4 {
+                      debug z1 => _7;
+                  }
+              }
+          }
+      }
+
+      bb0: {
+          StorageLive(_1);
+          StorageLive(_2);
+-         _2 = OffsetOf(Alpha, [0]);
++         _2 = const 4_usize;
+          _1 = must_use::<usize>(move _2) -> [return: bb1, unwind unreachable];
+      }
+
+      bb1: {
+          StorageDead(_2);
+          StorageLive(_3);
+          StorageLive(_4);
+-         _4 = OffsetOf(Alpha, [1]);
++         _4 = const 0_usize;
+          _3 = must_use::<usize>(move _4) -> [return: bb2, unwind unreachable];
+      }
+
+      bb2: {
+          StorageDead(_4);
+          StorageLive(_5);
+          StorageLive(_6);
+-         _6 = OffsetOf(Alpha, [2, 0]);
++         _6 = const 2_usize;
+          _5 = must_use::<usize>(move _6) -> [return: bb3, unwind unreachable];
+      }
+
+      bb3: {
+          StorageDead(_6);
+          StorageLive(_7);
+          StorageLive(_8);
+-         _8 = OffsetOf(Alpha, [2, 1]);
++         _8 = const 3_usize;
+          _7 = must_use::<usize>(move _8) -> [return: bb4, unwind unreachable];
+      }
+
+      bb4: {
+          StorageDead(_8);
+          _0 = const ();
+          StorageDead(_7);
+          StorageDead(_5);
+          StorageDead(_3);
+          StorageDead(_1);
+          return;
+      }
+  }
+

tests/mir-opt/const_prop/offset_of.concrete.ConstProp.panic-unwind.diff

@@ -0,0 +1,72 @@
+- // MIR for `concrete` before ConstProp
++ // MIR for `concrete` after ConstProp
+
+  fn concrete() -> () {
+      let mut _0: ();
+      let _1: usize;
+      let mut _2: usize;
+      let mut _4: usize;
+      let mut _6: usize;
+      let mut _8: usize;
+      scope 1 {
+          debug x => _1;
+          let _3: usize;
+          scope 2 {
+              debug y => _3;
+              let _5: usize;
+              scope 3 {
+                  debug z0 => _5;
+                  let _7: usize;
+                  scope 4 {
+                      debug z1 => _7;
+                  }
+              }
+          }
+      }
+
+      bb0: {
+          StorageLive(_1);
+          StorageLive(_2);
+-         _2 = OffsetOf(Alpha, [0]);
++         _2 = const 4_usize;
+          _1 = must_use::<usize>(move _2) -> bb1;
+      }
+
+      bb1: {
+          StorageDead(_2);
+          StorageLive(_3);
+          StorageLive(_4);
+-         _4 = OffsetOf(Alpha, [1]);
++         _4 = const 0_usize;
+          _3 = must_use::<usize>(move _4) -> bb2;
+      }
+
+      bb2: {
+          StorageDead(_4);
+          StorageLive(_5);
+          StorageLive(_6);
+-         _6 = OffsetOf(Alpha, [2, 0]);
++         _6 = const 2_usize;
+          _5 = must_use::<usize>(move _6) -> bb3;
+      }
+
+      bb3: {
+          StorageDead(_6);
+          StorageLive(_7);
+          StorageLive(_8);
+-         _8 = OffsetOf(Alpha, [2, 1]);
++         _8 = const 3_usize;
+          _7 = must_use::<usize>(move _8) -> bb4;
+      }
+
+      bb4: {
+          StorageDead(_8);
+          _0 = const ();
+          StorageDead(_7);
+          StorageDead(_5);
+          StorageDead(_3);
+          StorageDead(_1);
+          return;
+      }
+  }
+

tests/mir-opt/const_prop/offset_of.generic.ConstProp.panic-abort.diff

@@ -0,0 +1,68 @@
+- // MIR for `generic` before ConstProp
++ // MIR for `generic` after ConstProp
+
+  fn generic() -> () {
+      let mut _0: ();
+      let _1: usize;
+      let mut _2: usize;
+      let mut _4: usize;
+      let mut _6: usize;
+      let mut _8: usize;
+      scope 1 {
+          debug gx => _1;
+          let _3: usize;
+          scope 2 {
+              debug gy => _3;
+              let _5: usize;
+              scope 3 {
+                  debug dx => _5;
+                  let _7: usize;
+                  scope 4 {
+                      debug dy => _7;
+                  }
+              }
+          }
+      }
+
+      bb0: {
+          StorageLive(_1);
+          StorageLive(_2);
+          _2 = OffsetOf(Gamma<T>, [0]);
+          _1 = must_use::<usize>(move _2) -> [return: bb1, unwind unreachable];
+      }
+
+      bb1: {
+          StorageDead(_2);
+          StorageLive(_3);
+          StorageLive(_4);
+          _4 = OffsetOf(Gamma<T>, [1]);
+          _3 = must_use::<usize>(move _4) -> [return: bb2, unwind unreachable];
+      }
+
+      bb2: {
+          StorageDead(_4);
+          StorageLive(_5);
+          StorageLive(_6);
+          _6 = OffsetOf(Delta<T>, [1]);
+          _5 = must_use::<usize>(move _6) -> [return: bb3, unwind unreachable];
+      }
+
+      bb3: {
+          StorageDead(_6);
+          StorageLive(_7);
+          StorageLive(_8);
+          _8 = OffsetOf(Delta<T>, [2]);
+          _7 = must_use::<usize>(move _8) -> [return: bb4, unwind unreachable];
+      }
+
+      bb4: {
+          StorageDead(_8);
+          _0 = const ();
+          StorageDead(_7);
+          StorageDead(_5);
+          StorageDead(_3);
+          StorageDead(_1);
+          return;
+      }
+  }
+

tests/mir-opt/const_prop/offset_of.generic.ConstProp.panic-unwind.diff

@@ -0,0 +1,68 @@
+- // MIR for `generic` before ConstProp
++ // MIR for `generic` after ConstProp
+
+  fn generic() -> () {
+      let mut _0: ();
+      let _1: usize;
+      let mut _2: usize;
+      let mut _4: usize;
+      let mut _6: usize;
+      let mut _8: usize;
+      scope 1 {
+          debug gx => _1;
+          let _3: usize;
+          scope 2 {
+              debug gy => _3;
+              let _5: usize;
+              scope 3 {
+                  debug dx => _5;
+                  let _7: usize;
+                  scope 4 {
+                      debug dy => _7;
+                  }
+              }
+          }
+      }
+
+      bb0: {
+          StorageLive(_1);
+          StorageLive(_2);
+          _2 = OffsetOf(Gamma<T>, [0]);
+          _1 = must_use::<usize>(move _2) -> bb1;
+      }
+
+      bb1: {
+          StorageDead(_2);
+          StorageLive(_3);
+          StorageLive(_4);
+          _4 = OffsetOf(Gamma<T>, [1]);
+          _3 = must_use::<usize>(move _4) -> bb2;
+      }
+
+      bb2: {
+          StorageDead(_4);
+          StorageLive(_5);
+          StorageLive(_6);
+          _6 = OffsetOf(Delta<T>, [1]);
+          _5 = must_use::<usize>(move _6) -> bb3;
+      }
+
+      bb3: {
+          StorageDead(_6);
+          StorageLive(_7);
+          StorageLive(_8);
+          _8 = OffsetOf(Delta<T>, [2]);
+          _7 = must_use::<usize>(move _8) -> bb4;
+      }
+
+      bb4: {
+          StorageDead(_8);
+          _0 = const ();
+          StorageDead(_7);
+          StorageDead(_5);
+          StorageDead(_3);
+          StorageDead(_1);
+          return;
+      }
+  }
+

tests/mir-opt/const_prop/offset_of.rs

@@ -1,4 +1,5 @@
 // unit-test: ConstProp
+// EMIT_MIR_FOR_EACH_PANIC_STRATEGY
 
 #![feature(offset_of)]