Remove __rust_force_expr.

This was added (with a different name) to improve an error message. It
is no longer needed -- removing it changes the error message, but overall
I think the new message is no worse:
- the mention of `#` in the first line is a little worse,
- but the extra context makes it very clear what the problem is, perhaps
  even clearer than the old message,
- and the removal of the note about the `expr` fragment (an internal
  detail of `__rust_force_expr`) is an improvement.

Overall I think the error is quite clear and still far better than the
old message that prompted rust-lang#61933, which didn't even mention patterns.

The motivation for this is rust-lang#124141, which will cause pasted
metavariables to be tokenized and reparsed instead of the AST node being
cached. This change in behaviour occasionally has a non-zero perf cost,
and `__rust_force_expr` causes the tokenize/reparse step to occur twice.
Removing `__rust_force_expr` greatly reduces the extra overhead for the
`deep-vector` benchmark.

Author: nnethercote

alloc/src/macros.rs

@@ -41,18 +41,18 @@
 #[allow_internal_unstable(rustc_attrs, liballoc_internals)]
 macro_rules! vec {
     () => (
-        $crate::__rust_force_expr!($crate::vec::Vec::new())
+        $crate::vec::Vec::new()
     );
     ($elem:expr; $n:expr) => (
-        $crate::__rust_force_expr!($crate::vec::from_elem($elem, $n))
+        $crate::vec::from_elem($elem, $n)
     );
     ($($x:expr),+ $(,)?) => (
-        $crate::__rust_force_expr!(<[_]>::into_vec(
+        <[_]>::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),+])
-        ))
+        )
     );
 }
 
@@ -126,13 +126,3 @@ macro_rules! format {
         res
     }}
 }
-
-/// Force AST node to an expression to improve diagnostics in pattern position.
-#[doc(hidden)]
-#[macro_export]
-#[unstable(feature = "liballoc_internals", issue = "none", reason = "implementation detail")]
-macro_rules! __rust_force_expr {
-    ($e:expr) => {
-        $e
-    };
-}