Force vec! to expressions only

library/alloc/src/lib.rs

@@ -140,6 +140,7 @@
 #![feature(type_alias_impl_trait)]
 #![feature(associated_type_bounds)]
 #![feature(slice_group_by)]
+#![feature(decl_macro)]
 // Allow testing this library
 
 #[cfg(test)]
@@ -193,4 +194,11 @@ mod std {
 #[unstable(feature = "liballoc_internals", issue = "none", reason = "implementation detail")]
 pub mod __export {
     pub use core::format_args;
+
+    /// Force AST node to an expression to improve diagnostics in pattern position.
+    #[rustc_macro_transparency = "semitransparent"]
+    #[unstable(feature = "liballoc_internals", issue = "none", reason = "implementation detail")]
+    pub macro force_expr($e:expr) {
+        $e
+    }
 }

library/alloc/src/macros.rs

@@ -37,16 +37,16 @@
 #[cfg(not(test))]
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[allow_internal_unstable(box_syntax)]
+#[allow_internal_unstable(box_syntax, liballoc_internals)]
 macro_rules! vec {
     () => (
-        $crate::vec::Vec::new()
+        $crate::__export::force_expr!($crate::vec::Vec::new())
     );
     ($elem:expr; $n:expr) => (
-        $crate::vec::from_elem($elem, $n)
+        $crate::__export::force_expr!($crate::vec::from_elem($elem, $n))
     );
     ($($x:expr),+ $(,)?) => (
-        <[_]>::into_vec(box [$($x),+])
+        $crate::__export::force_expr!(<[_]>::into_vec(box [$($x),+]))
     );
 }
 

src/test/ui/macros/vec-macro-in-pattern.rs

@@ -0,0 +1,10 @@
+// This is a regression test for #61933
+// Verify that the vec![] macro may not be used in patterns
+// and that the resulting diagnostic is actually helpful.
+
+fn main() {
+    match Some(vec![42]) {
+        Some(vec![43]) => {} //~ ERROR arbitrary expressions aren't allowed in patterns
+        _ => {}
+    }
+}

src/test/ui/macros/vec-macro-in-pattern.stderr

@@ -0,0 +1,10 @@
+error: arbitrary expressions aren't allowed in patterns
+  --> $DIR/vec-macro-in-pattern.rs:7:14
+   |
+LL |         Some(vec![43]) => {}
+   |              ^^^^^^^^
+   |
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to previous error
+

src/test/ui/type/ascription/issue-47666.stderr

@@ -1,4 +1,4 @@
-error: expected type, found reserved keyword `box`
+error: expected type, found `<[_]>::into_vec(box [0, 1])`
   --> $DIR/issue-47666.rs:3:25
    |
 LL |     let _ = Option:Some(vec![0, 1]);