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

src/librustc_driver/driver.rs

@@ -755,8 +755,6 @@ pub fn phase_2_configure_and_expand<'a, F>(sess: &Session,
          || ast_validation::check_crate(sess, &krate));
 
     time(sess.time_passes(), "name resolution", || -> CompileResult {
-        resolver.resolve_imports();
-
         // Since import resolution will eventually happen in expansion,
         // don't perform `after_expand` until after import resolution.
         after_expand(&krate)?;

src/librustc_resolve/check_unused.rs

@@ -22,7 +22,6 @@
 use std::ops::{Deref, DerefMut};
 
 use Resolver;
-use Namespace::{TypeNS, ValueNS};
 
 use rustc::lint;
 use rustc::util::nodemap::NodeMap;
@@ -56,8 +55,9 @@ impl<'a, 'b> UnusedImportCheckVisitor<'a, 'b> {
     // We have information about whether `use` (import) directives are actually
     // used now. If an import is not used at all, we signal a lint error.
     fn check_import(&mut self, item_id: ast::NodeId, id: ast::NodeId, span: Span) {
-        if !self.used_imports.contains(&(id, TypeNS)) &&
-           !self.used_imports.contains(&(id, ValueNS)) {
+        let mut used = false;
+        self.per_ns(|this, ns| used |= this.used_imports.contains(&(id, ns)));
+        if !used {
             if self.maybe_unused_trait_imports.contains(&id) {
                 // Check later.
                 return;