Auto merge of #37732 - jseyfried:use_extern_macros, r=nrc

Support `use`ing externally defined macros behind `#![feature(use_extern_macros)]`

With `#![feature(use_extern_macros)]`,
 - A name collision between macros from different upstream crates is much less of an issue since we can `use` the macros in different submodules or rename with `as`.
 - We can reexport macros with `pub use`, so `#![feature(macro_reexport)]` is no longer needed.
 - These reexports are allowed in any module, so crates can expose a macro-modular interface.

If a macro invocation can resolve to both a `use` import and a `macro_rules!` or `#[macro_use]`, it is an ambiguity error.

r? @nrc

1 files changed, 11 insertions, 2 deletions

diff --git a/src/libsyntax/ext/expand.rs b/src/libsyntax/ext/expand.rs
index 877b312539f..8e0c3ce8448 100644
--- a/src/libsyntax/ext/expand.rs
+++ b/src/libsyntax/ext/expand.rs
@@ -222,6 +222,7 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
         self.cx.current_expansion.depth = 0;
 
         let (expansion, mut invocations) = self.collect_invocations(expansion);
+        self.resolve_imports();
         invocations.reverse();
 
         let mut expansions = Vec::new();
@@ -230,9 +231,9 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
         loop {
             let invoc = if let Some(invoc) = invocations.pop() {
                 invoc
-            } else if undetermined_invocations.is_empty() {
-                break
             } else {
+                self.resolve_imports();
+                if undetermined_invocations.is_empty() { break }
                 invocations = mem::replace(&mut undetermined_invocations, Vec::new());
                 force = !mem::replace(&mut progress, false);
                 continue
@@ -292,6 +293,14 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
         expansion.fold_with(&mut placeholder_expander)
     }
 
+    fn resolve_imports(&mut self) {
+        if self.monotonic {
+            let err_count = self.cx.parse_sess.span_diagnostic.err_count();
+            self.cx.resolver.resolve_imports();
+            self.cx.resolve_err_count += self.cx.parse_sess.span_diagnostic.err_count() - err_count;
+        }
+    }
+
     fn collect_invocations(&mut self, expansion: Expansion) -> (Expansion, Vec<Invocation>) {
         let result = {
             let mut collector = InvocationCollector {