mbe: Move MetaVarExprConcatElem closer to where it is used

Move this structure directly above the `parse_<expr>` functions that
return it to keep top-down flow.

This is a non-functional change.

Author: tgross35

compiler/rustc_expand/src/mbe/metavar_expr.rs

@@ -87,20 +87,6 @@ impl MetaVarExpr {
     }
 }
 
-/// Indicates what is placed in a `concat` parameter. For example, literals
-/// (`${concat("foo", "bar")}`) or adhoc identifiers (`${concat(foo, bar)}`).
-#[derive(Debug, Decodable, Encodable, PartialEq)]
-pub(crate) enum MetaVarExprConcatElem {
-    /// Identifier WITHOUT a preceding dollar sign, which means that this identifier should be
-    /// interpreted as a literal.
-    Ident(Ident),
-    /// For example, a number or a string.
-    Literal(Symbol),
-    /// Identifier WITH a preceding dollar sign, which means that this identifier should be
-    /// expanded and interpreted as a variable.
-    Var(Ident),
-}
-
 // Checks if there are any remaining tokens. For example, `${ignore(ident ... a b c ...)}`
 fn check_trailing_token<'psess>(
     iter: &mut TokenStreamIter<'_>,
@@ -117,6 +103,20 @@ fn check_trailing_token<'psess>(
     }
 }
 
+/// Indicates what is placed in a `concat` parameter. For example, literals
+/// (`${concat("foo", "bar")}`) or adhoc identifiers (`${concat(foo, bar)}`).
+#[derive(Debug, Decodable, Encodable, PartialEq)]
+pub(crate) enum MetaVarExprConcatElem {
+    /// Identifier WITHOUT a preceding dollar sign, which means that this identifier should be
+    /// interpreted as a literal.
+    Ident(Ident),
+    /// For example, a number or a string.
+    Literal(Symbol),
+    /// Identifier WITH a preceding dollar sign, which means that this identifier should be
+    /// expanded and interpreted as a variable.
+    Var(Ident),
+}
+
 /// Parse a meta-variable `concat` expression: `concat($metavar, ident, ...)`.
 fn parse_concat<'psess>(
     iter: &mut TokenStreamIter<'_>,