Update rustfmt for ast::ExprKind::FormatArgs.

Rustfmt doesn't expand macros, so that's easy: FormatArgs nodes do not
occur in the unexpanded AST.

src/tools/rustfmt/src/expr.rs

@@ -399,7 +399,10 @@ pub(crate) fn format_expr(
             }
         }
         ast::ExprKind::Underscore => Some("_".to_owned()),
-        ast::ExprKind::IncludedBytes(..) => unreachable!(),
+        ast::ExprKind::FormatArgs(..) | ast::ExprKind::IncludedBytes(..) => {
+            // These do not occur in the AST because macros aren't expanded.
+            unreachable!()
+        }
         ast::ExprKind::Err => None,
     };
 

src/tools/rustfmt/src/utils.rs

@@ -462,6 +462,7 @@ pub(crate) fn first_line_ends_with(s: &str, c: char) -> bool {
 pub(crate) fn is_block_expr(context: &RewriteContext<'_>, expr: &ast::Expr, repr: &str) -> bool {
     match expr.kind {
         ast::ExprKind::MacCall(..)
+        | ast::ExprKind::FormatArgs(..)
         | ast::ExprKind::Call(..)
         | ast::ExprKind::MethodCall(..)
         | ast::ExprKind::Array(..)