Auto merge of #47156 - petrochenkov:extpath, r=nikomatsakis

Support `extern` in paths

Implement the primary alternative to https://github.com/rust-lang/rust/pull/46613 + https://github.com/rust-lang/rust/pull/45771, achieving the same effect without requiring changes to other imports.
Both need to be experimentally evaluated before making further progress.

The PR also adds docs for all these related features into the unstable book.

cc https://github.com/rust-lang/rust/issues/44660
r? @nikomatsakis

diff --combined src/librustc_resolve/lib.rs
index 9f00bf97deba1f0301a6e7a73b8c503b39e3a8b5,53c2adeec98c268ec096c5f60d2a99d531ae8ab3..34e32dc9b640f0e6ce457a91b6c47ce460e70cce
--- 1/src/librustc_resolve/lib.rs
--- 2/src/librustc_resolve/lib.rs
+++ b/src/librustc_resolve/lib.rs
@@@ -1204,14 -1204,14 +1204,14 @@@ impl PrimitiveTypeTable 
          table.intern("char", TyChar);
          table.intern("f32", TyFloat(FloatTy::F32));
          table.intern("f64", TyFloat(FloatTy::F64));
 -        table.intern("isize", TyInt(IntTy::Is));
 +        table.intern("isize", TyInt(IntTy::Isize));
          table.intern("i8", TyInt(IntTy::I8));
          table.intern("i16", TyInt(IntTy::I16));
          table.intern("i32", TyInt(IntTy::I32));
          table.intern("i64", TyInt(IntTy::I64));
          table.intern("i128", TyInt(IntTy::I128));
          table.intern("str", TyStr);
 -        table.intern("usize", TyUint(UintTy::Us));
 +        table.intern("usize", TyUint(UintTy::Usize));
          table.intern("u8", TyUint(UintTy::U8));
          table.intern("u16", TyUint(UintTy::U16));
          table.intern("u32", TyUint(UintTy::U32));
@@@ -2982,6 -2982,8 +2982,8 @@@ impl<'a> Resolver<'a> 
                      let msg = "There are too many initial `super`s.".to_string();
                      return PathResult::Failed(ident.span, msg, false);
                  }
+             } else if i == 0 && ns == TypeNS && name == keywords::Extern.name() {
+                 continue;
              }
              allow_super = false;
  
@@@ -2996,16 -2998,19 +2998,19 @@@
                      // `$crate::a::b`
                      module = Some(self.resolve_crate_root(ident.node.ctxt));
                      continue
-                 } else if i == 1 && self.session.features.borrow().extern_absolute_paths &&
-                                     path[0].node.name == keywords::CrateRoot.name() &&
-                                     !token::Ident(ident.node).is_path_segment_keyword() {
-                     // `::extern_crate::a::b`
-                     let crate_id = self.crate_loader.resolve_crate_from_path(name, ident.span);
-                     let crate_root =
-                         self.get_module(DefId { krate: crate_id, index: CRATE_DEF_INDEX });
-                     self.populate_module_if_necessary(crate_root);
-                     module = Some(crate_root);
-                     continue
+                 } else if i == 1 && !token::Ident(ident.node).is_path_segment_keyword() {
+                     let prev_name = path[0].node.name;
+                     if prev_name == keywords::Extern.name() ||
+                        prev_name == keywords::CrateRoot.name() &&
+                        self.session.features.borrow().extern_absolute_paths {
+                         // `::extern_crate::a::b`
+                         let crate_id = self.crate_loader.resolve_crate_from_path(name, ident.span);
+                         let crate_root =
+                             self.get_module(DefId { krate: crate_id, index: CRATE_DEF_INDEX });
+                         self.populate_module_if_necessary(crate_root);
+                         module = Some(crate_root);
+                         continue
+                     }
                  }
              }
  
@@@ -3015,6 -3020,7 +3020,7 @@@
                 name == keywords::SelfValue.name() && i != 0 ||
                 name == keywords::SelfType.name() && i != 0 ||
                 name == keywords::Super.name() && i != 0 ||
+                name == keywords::Extern.name() && i != 0 ||
                 name == keywords::Crate.name() && i != 1 &&
                      path[0].node.name != keywords::CrateRoot.name() {
                  let name_str = if name == keywords::CrateRoot.name() {
@@@ -4089,5 -4095,4 +4095,5 @@@ pub enum MakeGlobMap 
      No,
  }
  
 +#[cfg(not(stage0))] // remove after the next snapshot
  __build_diagnostic_array! { librustc_resolve, DIAGNOSTICS }