Fix generic param redefined bug

When we name-resolve generic parameters their declaration gets inserted in
to the upper-most rib on the stack. Then when this is referenced we lookup
the relevant binding starting from the uppermost rib this lead to the 2nd
extern item adding the same binding into the same rib which caused this
clash. To fix this we need to have a seperate rib for each declaration
so as reusing the same names don't clash with each other.

Fixes #1131

Author: philberty

gcc/rust/resolve/rust-ast-resolve-item.h

@@ -955,6 +955,15 @@ class ResolveExternItem : public ResolverBase
 
   void visit (AST::ExternalFunctionItem &function) override
   {
+    NodeId scope_node_id = function.get_node_id ();
+    resolver->get_name_scope ().push (scope_node_id);
+    resolver->get_type_scope ().push (scope_node_id);
+    resolver->get_label_scope ().push (scope_node_id);
+    resolver->push_new_name_rib (resolver->get_name_scope ().peek ());
+    resolver->push_new_type_rib (resolver->get_type_scope ().peek ());
+    resolver->push_new_label_rib (resolver->get_type_scope ().peek ());
+
+    // resolve the generics
     if (function.has_generics ())
       {
 	for (auto &generic : function.get_generic_params ())
@@ -971,6 +980,11 @@ class ResolveExternItem : public ResolverBase
       {
 	ResolveType::go (param.get_type ().get (), param.get_node_id ());
       }
+
+    // done
+    resolver->get_name_scope ().pop ();
+    resolver->get_type_scope ().pop ();
+    resolver->get_label_scope ().pop ();
   }
 
   void visit (AST::ExternalStaticItem &item) override

gcc/testsuite/rust/compile/issue-1131.rs

@@ -0,0 +1,4 @@
+extern "rust-intrinsic" {
+    fn size_of<T>() -> usize;
+    fn offset<T>(dst: *const T, offset: isize) -> *const T;
+}