Mark some macros with must_use hint

Author: lukas

library/alloc/src/macros.rs

@@ -38,21 +38,21 @@
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_diagnostic_item = "vec_macro"]
-#[allow_internal_unstable(rustc_attrs, liballoc_internals)]
+#[allow_internal_unstable(rustc_attrs, liballoc_internals, hint_must_use)]
 macro_rules! vec {
     () => (
-        $crate::vec::Vec::new()
+        $crate::__export::must_use($crate::vec::Vec::new())
     );
     ($elem:expr; $n:expr) => (
-        $crate::vec::from_elem($elem, $n)
+        $crate::__export::must_use($crate::vec::from_elem($elem, $n))
     );
     ($($x:expr),+ $(,)?) => (
-        <[_]>::into_vec(
+        $crate::__export::must_use(<[_]>::into_vec(
             // This rustc_box is not required, but it produces a dramatic improvement in compile
             // time when constructing arrays with many elements.
             #[rustc_box]
             $crate::boxed::Box::new([$($x),+])
-        )
+        ))
     );
 }
 

library/core/src/lib.rs

@@ -467,4 +467,10 @@ pub mod simd {
     pub use crate::core_simd::simd::*;
 }
 
+#[doc(hidden)]
+#[unstable(feature = "liballoc_internals", issue = "none", reason = "implementation detail")]
+pub mod __export {
+    pub use core::hint::must_use;
+}
+
 include!("primitive_docs.rs");

library/core/src/macros/mod.rs

@@ -451,13 +451,16 @@ pub macro debug_assert_matches($($arg:tt)*) {
 /// ```
 #[macro_export]
 #[stable(feature = "matches_macro", since = "1.42.0")]
+#[allow_internal_unstable(hint_must_use, liballoc_internals)]
 #[cfg_attr(not(test), rustc_diagnostic_item = "matches_macro")]
 macro_rules! matches {
     ($expression:expr, $pattern:pat $(if $guard:expr)? $(,)?) => {
-        match $expression {
-            $pattern $(if $guard)? => true,
-            _ => false
-        }
+        $crate::__export::must_use({
+            match $expression {
+                $pattern $(if $guard)? => true,
+                _ => false
+            }
+        })
     };
 }
 
@@ -1010,8 +1013,12 @@ pub(crate) mod builtin {
     #[rustc_builtin_macro]
     #[macro_export]
     macro_rules! format_args {
-        ($fmt:expr) => {{ /* compiler built-in */ }};
-        ($fmt:expr, $($args:tt)*) => {{ /* compiler built-in */ }};
+        ($fmt:expr) => {
+            $crate::__export::must_use({ /* compiler built-in */ })
+        };
+        ($fmt:expr, $($args:tt)*) => {
+            $crate::__export::must_use({ /* compiler built-in */ })
+        };
     }
 
     /// Same as [`format_args`], but can be used in some const contexts.
@@ -1081,8 +1088,12 @@ pub(crate) mod builtin {
     #[macro_export]
     #[rustc_diagnostic_item = "env_macro"] // useful for external lints
     macro_rules! env {
-        ($name:expr $(,)?) => {{ /* compiler built-in */ }};
-        ($name:expr, $error_msg:expr $(,)?) => {{ /* compiler built-in */ }};
+        ($name:expr $(,)?) => {
+            $crate::__export::must_use({ /* compiler built-in */ })
+        };
+        ($name:expr, $error_msg:expr $(,)?) => {
+            $crate::__export::must_use({ /* compiler built-in */ })
+        };
     }
 
     /// Optionally inspects an environment variable at compile time.
@@ -1112,7 +1123,9 @@ pub(crate) mod builtin {
     #[macro_export]
     #[rustc_diagnostic_item = "option_env_macro"] // useful for external lints
     macro_rules! option_env {
-        ($name:expr $(,)?) => {{ /* compiler built-in */ }};
+        ($name:expr $(,)?) => {
+            $crate::__export::must_use({ /* compiler built-in */ })
+        };
     }
 
     /// Concatenates identifiers into one identifier.
@@ -1174,7 +1187,9 @@ pub(crate) mod builtin {
     #[rustc_builtin_macro]
     #[macro_export]
     macro_rules! concat_bytes {
-        ($($e:literal),+ $(,)?) => {{ /* compiler built-in */ }};
+        ($($e:literal),+ $(,)?) => {
+            $crate::__export::must_use({ /* compiler built-in */ })
+        };
     }
 
     /// Concatenates literals into a static string slice.
@@ -1193,10 +1208,13 @@ pub(crate) mod builtin {
     /// assert_eq!(s, "test10btrue");
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[allow_internal_unstable(hint_must_use, liballoc_internals)]
     #[rustc_builtin_macro]
     #[macro_export]
     macro_rules! concat {
-        ($($e:expr),* $(,)?) => {{ /* compiler built-in */ }};
+        ($($e:expr),* $(,)?) => {
+            $crate::__export::must_use({ /* compiler built-in */ })
+        };
     }
 
     /// Expands to the line number on which it was invoked.
@@ -1218,11 +1236,12 @@ pub(crate) mod builtin {
     /// println!("defined on line: {current_line}");
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[allow_internal_unstable(hint_must_use, liballoc_internals)]
     #[rustc_builtin_macro]
     #[macro_export]
     macro_rules! line {
         () => {
-            /* compiler built-in */
+            $crate::__export::must_use({ /* compiler built-in */ })
         };
     }
 
@@ -1257,11 +1276,12 @@ pub(crate) mod builtin {
     /// assert_ne!(b, c);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[allow_internal_unstable(hint_must_use, liballoc_internals)]
     #[rustc_builtin_macro]
     #[macro_export]
     macro_rules! column {
         () => {
-            /* compiler built-in */
+            $crate::__export::must_use(/* compiler built-in */)
         };
     }
 
@@ -1282,11 +1302,12 @@ pub(crate) mod builtin {
     /// println!("defined in file: {this_file}");
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[allow_internal_unstable(hint_must_use, liballoc_internals)]
     #[rustc_builtin_macro]
     #[macro_export]
     macro_rules! file {
         () => {
-            /* compiler built-in */
+            $crate::__export::must_use(/* compiler built-in */)
         };
     }
 
@@ -1306,11 +1327,12 @@ pub(crate) mod builtin {
     /// assert_eq!(one_plus_one, "1 + 1");
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[allow_internal_unstable(hint_must_use, liballoc_internals)]
     #[rustc_builtin_macro]
     #[macro_export]
     macro_rules! stringify {
         ($($t:tt)*) => {
-            /* compiler built-in */
+            $crate::__export::must_use(/* compiler built-in */)
         };
     }
 
@@ -1351,7 +1373,9 @@ pub(crate) mod builtin {
     #[macro_export]
     #[cfg_attr(not(test), rustc_diagnostic_item = "include_str_macro")]
     macro_rules! include_str {
-        ($file:expr $(,)?) => {{ /* compiler built-in */ }};
+        ($file:expr $(,)?) => {
+            $crate::__export::must_use({ /* compiler built-in */ })
+        };
     }
 
     /// Includes a file as a reference to a byte array.
@@ -1391,7 +1415,9 @@ pub(crate) mod builtin {
     #[macro_export]
     #[cfg_attr(not(test), rustc_diagnostic_item = "include_bytes_macro")]
     macro_rules! include_bytes {
-        ($file:expr $(,)?) => {{ /* compiler built-in */ }};
+        ($file:expr $(,)?) => {
+            $crate::__export::must_use({ /* compiler built-in */ })
+        };
     }
 
     /// Expands to a string that represents the current module path.
@@ -1449,7 +1475,7 @@ pub(crate) mod builtin {
     #[macro_export]
     macro_rules! cfg {
         ($($cfg:tt)*) => {
-            /* compiler built-in */
+            $crate::__export::must_use(/* compiler built-in */)
         };
     }
 

library/core/tests/macros.rs

@@ -39,7 +39,7 @@ fn assert_ne_trailing_comma() {
 #[rustfmt::skip]
 #[test]
 fn matches_leading_pipe() {
-    matches!(1, | 1 | 2 | 3);
+    let _ = matches!(1, | 1 | 2 | 3);
 }
 
 #[test]

tests/mir-opt/issues/issue_59352.num_to_digit.PreCodegen.after.panic-abort.mir

@@ -8,12 +8,14 @@ fn num_to_digit(_1: char) -> u32 {
         let _2: std::option::Option<u32>;
         scope 2 (inlined Option::<u32>::is_some) {
             let mut _3: isize;
+            scope 3 (inlined must_use::<bool>) {
+            }
         }
     }
-    scope 3 (inlined #[track_caller] Option::<u32>::unwrap) {
+    scope 4 (inlined #[track_caller] Option::<u32>::unwrap) {
         let mut _5: isize;
         let mut _6: !;
-        scope 4 {
+        scope 5 {
         }
     }
 

tests/mir-opt/issues/issue_59352.num_to_digit.PreCodegen.after.panic-unwind.mir

@@ -8,12 +8,14 @@ fn num_to_digit(_1: char) -> u32 {
         let _2: std::option::Option<u32>;
         scope 2 (inlined Option::<u32>::is_some) {
             let mut _3: isize;
+            scope 3 (inlined must_use::<bool>) {
+            }
         }
     }
-    scope 3 (inlined #[track_caller] Option::<u32>::unwrap) {
+    scope 4 (inlined #[track_caller] Option::<u32>::unwrap) {
         let mut _5: isize;
         let mut _6: !;
-        scope 4 {
+        scope 5 {
         }
     }
 

tests/mir-opt/matches_reduce_branches.foo.MatchBranchSimplification.diff

@@ -5,47 +5,58 @@
       debug bar => _1;
       let mut _0: ();
       let mut _2: bool;
-      let mut _3: isize;
-+     let mut _4: isize;
+      let mut _3: bool;
+      let mut _4: isize;
++     let mut _5: isize;
 
       bb0: {
           StorageLive(_2);
-          _3 = discriminant(_1);
--         switchInt(move _3) -> [0: bb2, otherwise: bb1];
-+         StorageLive(_4);
-+         _4 = move _3;
-+         _2 = Eq(_4, const 0_isize);
-+         StorageDead(_4);
-+         switchInt(move _2) -> [0: bb2, otherwise: bb1];
+          StorageLive(_3);
+          _4 = discriminant(_1);
+-         switchInt(move _4) -> [0: bb2, otherwise: bb1];
++         StorageLive(_5);
++         _5 = move _4;
++         _3 = Eq(_5, const 0_isize);
++         StorageDead(_5);
++         _2 = must_use::<bool>(move _3) -> [return: bb1, unwind continue];
       }
 
       bb1: {
--         _2 = const false;
-+         _0 = ();
-          goto -> bb3;
+-         _3 = const false;
+-         goto -> bb3;
++         switchInt(move _2) -> [0: bb3, otherwise: bb2];
       }
 
       bb2: {
--         _2 = const true;
-+         _0 = const ();
-          goto -> bb3;
-      }
-  
-      bb3: {
--         switchInt(move _2) -> [0: bb5, otherwise: bb4];
+-         _3 = const true;
+-         goto -> bb3;
 -     }
 - 
--     bb4: {
--         _0 = ();
--         goto -> bb6;
+-     bb3: {
+-         _2 = must_use::<bool>(move _3) -> [return: bb4, unwind continue];
 -     }
 - 
--     bb5: {
--         _0 = const ();
--         goto -> bb6;
+-     bb4: {
+-         switchInt(move _2) -> [0: bb6, otherwise: bb5];
 -     }
 - 
+-     bb5: {
+          StorageDead(_3);
+          _0 = ();
+-         goto -> bb7;
++         goto -> bb4;
+      }
+  
 -     bb6: {
++     bb3: {
+          StorageDead(_3);
+          _0 = const ();
+-         goto -> bb7;
++         goto -> bb4;
+      }
+  
+-     bb7: {
++     bb4: {
           StorageDead(_2);
           return;
       }

tests/mir-opt/pre-codegen/checked_ops.step_forward.PreCodegen.after.panic-abort.mir

@@ -23,9 +23,11 @@ fn step_forward(_1: u16, _2: usize) -> u16 {
         }
         scope 7 (inlined Option::<u16>::is_none) {
             scope 8 (inlined Option::<u16>::is_some) {
+                scope 9 (inlined must_use::<bool>) {
+                }
             }
         }
-        scope 9 (inlined core::num::<impl u16>::wrapping_add) {
+        scope 10 (inlined core::num::<impl u16>::wrapping_add) {
         }
     }
 

tests/mir-opt/pre-codegen/checked_ops.step_forward.PreCodegen.after.panic-unwind.mir

@@ -23,9 +23,11 @@ fn step_forward(_1: u16, _2: usize) -> u16 {
         }
         scope 7 (inlined Option::<u16>::is_none) {
             scope 8 (inlined Option::<u16>::is_some) {
+                scope 9 (inlined must_use::<bool>) {
+                }
             }
         }
-        scope 9 (inlined core::num::<impl u16>::wrapping_add) {
+        scope 10 (inlined core::num::<impl u16>::wrapping_add) {
         }
     }
 

tests/mir-opt/pre-codegen/matches_macro.issue_77355_opt.PreCodegen.after.mir

@@ -4,6 +4,8 @@ fn issue_77355_opt(_1: Foo) -> u64 {
     debug num => _1;
     let mut _0: u64;
     let mut _2: isize;
+    scope 1 (inlined must_use::<bool>) {
+    }
 
     bb0: {
         _2 = discriminant(_1);

tests/ui/box/unit/unique-create.rs

@@ -7,5 +7,5 @@ pub fn main() {
 }
 
 fn vec() {
-    vec![0];
+    let _ = vec![0];
 }