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

3 files changed, 15 insertions, 2 deletions

diff --git a/src/libsyntax/ext/base.rs b/src/libsyntax/ext/base.rs
index b4ac576f57a..7f66b060052 100644
--- a/src/libsyntax/ext/base.rs
+++ b/src/libsyntax/ext/base.rs
@@ -524,6 +524,7 @@ pub trait Resolver {
     fn add_ext(&mut self, ident: ast::Ident, ext: Rc<SyntaxExtension>);
     fn add_expansions_at_stmt(&mut self, id: ast::NodeId, macros: Vec<Mark>);
 
+    fn resolve_imports(&mut self);
     fn find_attr_invoc(&mut self, attrs: &mut Vec<Attribute>) -> Option<Attribute>;
     fn resolve_macro(&mut self, scope: Mark, path: &ast::Path, force: bool)
                      -> Result<Rc<SyntaxExtension>, Determinacy>;
@@ -547,6 +548,7 @@ impl Resolver for DummyResolver {
     fn add_ext(&mut self, _ident: ast::Ident, _ext: Rc<SyntaxExtension>) {}
     fn add_expansions_at_stmt(&mut self, _id: ast::NodeId, _macros: Vec<Mark>) {}
 
+    fn resolve_imports(&mut self) {}
     fn find_attr_invoc(&mut self, _attrs: &mut Vec<Attribute>) -> Option<Attribute> { None }
     fn resolve_macro(&mut self, _scope: Mark, _path: &ast::Path, _force: bool)
                      -> Result<Rc<SyntaxExtension>, Determinacy> {
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 {
diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index 27f720b7609..ea66fdc31cf 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -314,6 +314,8 @@ declare_features! (
 
     // Allows #[link(..., cfg(..))]
     (active, link_cfg, "1.14.0", Some(37406)),
+
+    (active, use_extern_macros, "1.15.0", Some(35896)),
 );
 
 declare_features! (