expand: Unimplement `MutVisitor` on `MacroExpander`

Each call to `fully_expand_fragment` is something unique, interesting, and requiring attention.
It represents a "root" of expansion and its use means that something unusual is happening, like eager expansion or expansion performed outside of the primary expansion pass.
So, it shouldn't be hide under a generic visitor call.

Also, from all the implemented visitor methods only two were actually used.

2 files changed, 11 insertions, 17 deletions

diff --git a/src/libsyntax/ext/base.rs b/src/libsyntax/ext/base.rs
index 7f4feff6be6..532de05eea2 100644
--- a/src/libsyntax/ext/base.rs
+++ b/src/libsyntax/ext/base.rs
@@ -947,8 +947,10 @@ pub fn expr_to_spanned_string<'a>(
     // Update `expr.span`'s ctxt now in case expr is an `include!` macro invocation.
     expr.span = expr.span.apply_mark(cx.current_expansion.id);
 
-    // we want to be able to handle e.g., `concat!("foo", "bar")`
-    cx.expander().visit_expr(&mut expr);
+    // Perform eager expansion on the expression.
+    // We want to be able to handle e.g., `concat!("foo", "bar")`.
+    let expr = cx.expander().fully_expand_fragment(AstFragment::Expr(expr)).make_expr();
+
     Err(match expr.node {
         ast::ExprKind::Lit(ref l) => match l.node {
             ast::LitKind::Str(s, style) => return Ok(respan(expr.span, (s, style))),
@@ -1013,8 +1015,12 @@ pub fn get_exprs_from_tts(cx: &mut ExtCtxt<'_>,
     let mut p = cx.new_parser_from_tts(tts);
     let mut es = Vec::new();
     while p.token != token::Eof {
-        let mut expr = panictry!(p.parse_expr());
-        cx.expander().visit_expr(&mut expr);
+        let expr = panictry!(p.parse_expr());
+
+        // Perform eager expansion on the expression.
+        // We want to be able to handle e.g., `concat!("foo", "bar")`.
+        let expr = cx.expander().fully_expand_fragment(AstFragment::Expr(expr)).make_expr();
+
         es.push(expr);
         if p.eat(&token::Comma) {
             continue;
diff --git a/src/libsyntax/ext/expand.rs b/src/libsyntax/ext/expand.rs
index 21cf232ecc3..402b42dfbc8 100644
--- a/src/libsyntax/ext/expand.rs
+++ b/src/libsyntax/ext/expand.rs
@@ -116,18 +116,6 @@ macro_rules! ast_fragments {
             }
         }
 
-        impl<'a, 'b> MutVisitor for MacroExpander<'a, 'b> {
-            fn filter_map_expr(&mut self, expr: P<ast::Expr>) -> Option<P<ast::Expr>> {
-                self.fully_expand_fragment(AstFragment::OptExpr(Some(expr))).make_opt_expr()
-            }
-            $($(fn $mut_visit_ast(&mut self, ast: &mut $AstTy) {
-                visit_clobber(ast, |ast| self.fully_expand_fragment(AstFragment::$Kind(ast)).$make_ast());
-            })?)*
-            $($(fn $flat_map_ast_elt(&mut self, ast_elt: <$AstTy as IntoIterator>::Item) -> $AstTy {
-                self.fully_expand_fragment(AstFragment::$Kind(smallvec![ast_elt])).$make_ast()
-            })?)*
-        }
-
         impl<'a> MacResult for crate::ext::tt::macro_rules::ParserAnyMacro<'a> {
             $(fn $make_ast(self: Box<crate::ext::tt::macro_rules::ParserAnyMacro<'a>>)
                            -> Option<$AstTy> {
@@ -286,7 +274,7 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
     }
 
     // Recursively expand all macro invocations in this AST fragment.
-    fn fully_expand_fragment(&mut self, input_fragment: AstFragment) -> AstFragment {
+    pub fn fully_expand_fragment(&mut self, input_fragment: AstFragment) -> AstFragment {
         let orig_expansion_data = self.cx.current_expansion.clone();
         self.cx.current_expansion.depth = 0;