Rollup merge of #63537 - petrochenkov:novisit, r=alexcrichton

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 hide under a generic visitor call.

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

cc https://github.com/rust-lang/rust/pull/63468#discussion_r313504119

2 files changed, 20 insertions, 16 deletions

diff --git a/src/libsyntax_ext/proc_macro_harness.rs b/src/libsyntax_ext/proc_macro_harness.rs
index 7913a7442ed..70325539f30 100644
--- a/src/libsyntax_ext/proc_macro_harness.rs
+++ b/src/libsyntax_ext/proc_macro_harness.rs
@@ -1,18 +1,17 @@
 use std::mem;
 
+use smallvec::smallvec;
 use syntax::ast::{self, Ident};
 use syntax::attr;
 use syntax::source_map::{ExpnInfo, ExpnKind, respan};
 use syntax::ext::base::{ExtCtxt, MacroKind};
-use syntax::ext::expand::ExpansionConfig;
+use syntax::ext::expand::{AstFragment, ExpansionConfig};
 use syntax::ext::hygiene::ExpnId;
 use syntax::ext::proc_macro::is_proc_macro_attr;
-use syntax::mut_visit::MutVisitor;
 use syntax::parse::ParseSess;
 use syntax::ptr::P;
 use syntax::symbol::{kw, sym};
 use syntax::visit::{self, Visitor};
-
 use syntax_pos::{Span, DUMMY_SP};
 
 struct ProcMacroDerive {
@@ -409,5 +408,7 @@ fn mk_decls(
         i
     });
 
-    cx.monotonic_expander().flat_map_item(module).pop().unwrap()
+    // Integrate the new module into existing module structures.
+    let module = AstFragment::Items(smallvec![module]);
+    cx.monotonic_expander().fully_expand_fragment(module).make_items().pop().unwrap()
 }
diff --git a/src/libsyntax_ext/test_harness.rs b/src/libsyntax_ext/test_harness.rs
index eec8a3f8023..0267637e540 100644
--- a/src/libsyntax_ext/test_harness.rs
+++ b/src/libsyntax_ext/test_harness.rs
@@ -6,7 +6,7 @@ use syntax::ast::{self, Ident};
 use syntax::attr;
 use syntax::entry::{self, EntryPointType};
 use syntax::ext::base::{ExtCtxt, Resolver};
-use syntax::ext::expand::ExpansionConfig;
+use syntax::ext::expand::{AstFragment, ExpansionConfig};
 use syntax::ext::hygiene::{ExpnId, MacroKind};
 use syntax::feature_gate::Features;
 use syntax::mut_visit::{*, ExpectOne};
@@ -74,12 +74,7 @@ impl<'a> MutVisitor for TestHarnessGenerator<'a> {
         noop_visit_crate(c, self);
 
         // Create a main function to run our tests
-        let test_main = {
-            let unresolved = mk_main(&mut self.cx);
-            self.cx.ext_cx.monotonic_expander().flat_map_item(unresolved).pop().unwrap()
-        };
-
-        c.module.items.push(test_main);
+        c.module.items.push(mk_main(&mut self.cx));
     }
 
     fn flat_map_item(&mut self, i: P<ast::Item>) -> SmallVec<[P<ast::Item>; 1]> {
@@ -216,7 +211,7 @@ fn mk_reexport_mod(cx: &mut TestCtxt<'_>,
     let name = Ident::from_str("__test_reexports").gensym();
     let parent = if parent == ast::DUMMY_NODE_ID { ast::CRATE_NODE_ID } else { parent };
     cx.ext_cx.current_expansion.id = cx.ext_cx.resolver.get_module_scope(parent);
-    let it = cx.ext_cx.monotonic_expander().flat_map_item(P(ast::Item {
+    let module = P(ast::Item {
         ident: name,
         attrs: Vec::new(),
         id: ast::DUMMY_NODE_ID,
@@ -224,9 +219,14 @@ fn mk_reexport_mod(cx: &mut TestCtxt<'_>,
         vis: dummy_spanned(ast::VisibilityKind::Public),
         span: DUMMY_SP,
         tokens: None,
-    })).pop().unwrap();
+    });
 
-    (it, name)
+    // Integrate the new module into existing module structures.
+    let module = AstFragment::Items(smallvec![module]);
+    let module =
+        cx.ext_cx.monotonic_expander().fully_expand_fragment(module).make_items().pop().unwrap();
+
+    (module, name)
 }
 
 /// Crawl over the crate, inserting test reexports and the test main function
@@ -321,7 +321,7 @@ fn mk_main(cx: &mut TestCtxt<'_>) -> P<ast::Item> {
         None => Ident::from_str_and_span("main", sp).gensym(),
     };
 
-    P(ast::Item {
+    let main = P(ast::Item {
         ident: main_id,
         attrs: vec![main_attr],
         id: ast::DUMMY_NODE_ID,
@@ -329,8 +329,11 @@ fn mk_main(cx: &mut TestCtxt<'_>) -> P<ast::Item> {
         vis: dummy_spanned(ast::VisibilityKind::Public),
         span: sp,
         tokens: None,
-    })
+    });
 
+    // Integrate the new item into existing module structures.
+    let main = AstFragment::Items(smallvec![main]);
+    cx.ext_cx.monotonic_expander().fully_expand_fragment(main).make_items().pop().unwrap()
 }
 
 fn path_name_i(idents: &[Ident]) -> String {