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.diff

@@ -1,44 +0,0 @@
-- // MIR for `concrete` before ConstProp
-+ // MIR for `concrete` after ConstProp
-  
-  fn concrete() -> () {
-      let mut _0: ();
-      let _1: usize;
-      scope 1 {
-          debug x => _1;
-          let _2: usize;
-          scope 2 {
-              debug y => _2;
-              let _3: usize;
-              scope 3 {
-                  debug z0 => _3;
-                  let _4: usize;
-                  scope 4 {
-                      debug z1 => _4;
-                  }
-              }
-          }
-      }
-  
-      bb0: {
-          StorageLive(_1);
--         _1 = OffsetOf(Alpha, [0]);
-+         _1 = const 4_usize;
-          StorageLive(_2);
--         _2 = OffsetOf(Alpha, [1]);
-+         _2 = const 0_usize;
-          StorageLive(_3);
--         _3 = OffsetOf(Alpha, [2, 0]);
-+         _3 = const 2_usize;
-          StorageLive(_4);
--         _4 = OffsetOf(Alpha, [2, 1]);
-+         _4 = const 3_usize;
-          _0 = const ();
-          StorageDead(_4);
-          StorageDead(_3);
-          StorageDead(_2);
-          StorageDead(_1);
-          return;
-      }
-  }
-  

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.diff

@@ -1,40 +0,0 @@
-- // MIR for `generic` before ConstProp
-+ // MIR for `generic` after ConstProp
-  
-  fn generic() -> () {
-      let mut _0: ();
-      let _1: usize;
-      scope 1 {
-          debug gx => _1;
-          let _2: usize;
-          scope 2 {
-              debug gy => _2;
-              let _3: usize;
-              scope 3 {
-                  debug dx => _3;
-                  let _4: usize;
-                  scope 4 {
-                      debug dy => _4;
-                  }
-              }
-          }
-      }
-  
-      bb0: {
-          StorageLive(_1);
-          _1 = OffsetOf(Gamma<T>, [0]);
-          StorageLive(_2);
-          _2 = OffsetOf(Gamma<T>, [1]);
-          StorageLive(_3);
-          _3 = OffsetOf(Delta<T>, [1]);
-          StorageLive(_4);
-          _4 = OffsetOf(Delta<T>, [2]);
-          _0 = const ();
-          StorageDead(_4);
-          StorageDead(_3);
-          StorageDead(_2);
-          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)]
 

tests/ui/offset-of/offset-of-dst-field.stderr

@@ -34,6 +34,20 @@ LL |     offset_of!((u8, dyn Trait), 1);
    = help: the trait `Sized` is not implemented for `dyn Trait`
    = note: this error originates in the macro `offset_of` (in Nightly builds, run with -Z macro-backtrace for more info)
 
+error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
+  --> $DIR/offset-of-dst-field.rs:44:5
+   |
+LL |     offset_of!(Delta<Alpha>, z);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: within `Alpha`, the trait `Sized` is not implemented for `[u8]`
+note: required because it appears within the type `Alpha`
+  --> $DIR/offset-of-dst-field.rs:5:8
+   |
+LL | struct Alpha {
+   |        ^^^^^
+   = note: this error originates in the macro `offset_of` (in Nightly builds, run with -Z macro-backtrace for more info)
+
 error[E0277]: the size for values of type `Extern` cannot be known at compilation time
   --> $DIR/offset-of-dst-field.rs:45:5
    |
@@ -52,20 +66,6 @@ LL |     offset_of!(Delta<dyn Trait>, z);
    = help: the trait `Sized` is not implemented for `dyn Trait`
    = note: this error originates in the macro `offset_of` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
-  --> $DIR/offset-of-dst-field.rs:44:5
-   |
-LL |     offset_of!(Delta<Alpha>, z);
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
-   |
-   = help: within `Alpha`, the trait `Sized` is not implemented for `[u8]`
-note: required because it appears within the type `Alpha`
-  --> $DIR/offset-of-dst-field.rs:5:8
-   |
-LL | struct Alpha {
-   |        ^^^^^
-   = note: this error originates in the macro `offset_of` (in Nightly builds, run with -Z macro-backtrace for more info)
-
 error[E0277]: the size for values of type `T` cannot be known at compilation time
   --> $DIR/offset-of-dst-field.rs:50:5
    |

tests/ui/offset-of/offset-of-must-use.rs

@@ -0,0 +1,9 @@
+// check-pass
+
+#![feature(offset_of)]
+#![warn(unused)]
+
+fn main() {
+    core::mem::offset_of!((String,), 0);
+    //~^ WARN unused return value of `must_use` that must be used
+}

tests/ui/offset-of/offset-of-must-use.stderr

@@ -0,0 +1,16 @@
+warning: unused return value of `must_use` that must be used
+  --> $DIR/offset-of-must-use.rs:7:5
+   |
+LL |     core::mem::offset_of!((String,), 0);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/offset-of-must-use.rs:4:9
+   |
+LL | #![warn(unused)]
+   |         ^^^^^^
+   = note: `#[warn(unused_must_use)]` implied by `#[warn(unused)]`
+   = note: this warning originates in the macro `core::mem::offset_of` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+warning: 1 warning emitted
+

tests/ui/offset-of/offset-of-output-type.stderr

@@ -2,9 +2,7 @@ error[E0308]: mismatched types
   --> $DIR/offset-of-output-type.rs:12:17
    |
 LL |     let _: u8 = offset_of!(S, v);
-   |            --   ^^^^^^^^^^^^^^^^ expected `u8`, found `usize`
+   |                 ^^^^^^^^^^^^^^^^ expected `u8`, found `usize`
-   |            |
-   |            expected due to this
    |
    = note: this error originates in the macro `offset_of` (in Nightly builds, run with -Z macro-backtrace for more info)
 
@@ -12,9 +10,7 @@ error[E0308]: mismatched types
   --> $DIR/offset-of-output-type.rs:13:18
    |
 LL |     let _: u16 = offset_of!(S, v);
-   |            ---   ^^^^^^^^^^^^^^^^ expected `u16`, found `usize`
+   |                  ^^^^^^^^^^^^^^^^ expected `u16`, found `usize`
-   |            |
-   |            expected due to this
    |
    = note: this error originates in the macro `offset_of` (in Nightly builds, run with -Z macro-backtrace for more info)
 
@@ -22,9 +18,7 @@ error[E0308]: mismatched types
   --> $DIR/offset-of-output-type.rs:14:18
    |
 LL |     let _: u32 = offset_of!(S, v);
-   |            ---   ^^^^^^^^^^^^^^^^ expected `u32`, found `usize`
+   |                  ^^^^^^^^^^^^^^^^ expected `u32`, found `usize`
-   |            |
-   |            expected due to this
    |
    = note: this error originates in the macro `offset_of` (in Nightly builds, run with -Z macro-backtrace for more info)
 
@@ -32,9 +26,7 @@ error[E0308]: mismatched types
   --> $DIR/offset-of-output-type.rs:15:18
    |
 LL |     let _: u64 = offset_of!(S, v);
-   |            ---   ^^^^^^^^^^^^^^^^ expected `u64`, found `usize`
+   |                  ^^^^^^^^^^^^^^^^ expected `u64`, found `usize`
-   |            |
-   |            expected due to this
    |
    = note: this error originates in the macro `offset_of` (in Nightly builds, run with -Z macro-backtrace for more info)
 
@@ -42,9 +34,7 @@ error[E0308]: mismatched types
   --> $DIR/offset-of-output-type.rs:16:20
    |
 LL |     let _: isize = offset_of!(S, v);
-   |            -----   ^^^^^^^^^^^^^^^^ expected `isize`, found `usize`
+   |                    ^^^^^^^^^^^^^^^^ expected `isize`, found `usize`
-   |            |
-   |            expected due to this
    |
    = note: this error originates in the macro `offset_of` (in Nightly builds, run with -Z macro-backtrace for more info)
 

tests/ui/offset-of/offset-of-tuple.stderr

@@ -90,10 +90,7 @@ error: expected identifier, found `)`
   --> $DIR/offset-of-tuple.rs:14:5
    |
 LL |     offset_of!((u8, u8), 1.);
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^ expected identifier
-   |     |
-   |     expected identifier
-   |     in this macro invocation
    |
    = note: this error originates in the macro `offset_of` (in Nightly builds, run with -Z macro-backtrace for more info)
 
@@ -110,10 +107,7 @@ error: expected identifier, found `)`
   --> $DIR/offset-of-tuple.rs:36:5
    |
 LL |     offset_of!(ComplexTup, 0.0.1.);
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected identifier
-   |     |
-   |     expected identifier
-   |     in this macro invocation
    |
    = note: this error originates in the macro `offset_of` (in Nightly builds, run with -Z macro-backtrace for more info)
 
@@ -147,10 +141,7 @@ error: expected identifier, found `)`
   --> $DIR/offset-of-tuple.rs:40:5
    |
 LL |     offset_of!(ComplexTup, 0.0 .1.);
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected identifier
-   |     |
-   |     expected identifier
-   |     in this macro invocation
    |
    = note: this error originates in the macro `offset_of` (in Nightly builds, run with -Z macro-backtrace for more info)
 
@@ -158,10 +149,7 @@ error: expected identifier, found `)`
   --> $DIR/offset-of-tuple.rs:41:5
    |
 LL |     offset_of!(ComplexTup, 0.0 . 1.);
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected identifier
-   |     |
-   |     expected identifier
-   |     in this macro invocation
    |
    = note: this error originates in the macro `offset_of` (in Nightly builds, run with -Z macro-backtrace for more info)
 
@@ -169,10 +157,7 @@ error: expected identifier, found `)`
   --> $DIR/offset-of-tuple.rs:42:5
    |
 LL |     offset_of!(ComplexTup, 0.0. 1.);
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected identifier
-   |     |
-   |     expected identifier
-   |     in this macro invocation
    |
    = note: this error originates in the macro `offset_of` (in Nightly builds, run with -Z macro-backtrace for more info)