Fix qualified name lookup for Rust

In https://github.com/rust-lang/rust/pull/46457, "m4b" pointed out
that the Rust support in gdb doesn't properly handle the lookup of
qualified names.

In particular, as shown in the test case in this patch, something like
"::NAME" should be found in the global scope, but is not.

This turns out to happen because rust_lookup_symbol_nonlocal does not
search the global scope unless the name in question is unqualified.
However, lookup_symbol_aux does not search the global scope, and
appears to search the static scope only as a fallback (I wonder if
this is needed?).

This patch fixes the problem by changing rust_lookup_symbol_nonlocal
to search the static and global blocks in more cases.

Regression tested against various versions of the rust compiler on
Fedora 26 x86-64.  (Note that there are unrelated failures with newer
versions of rustc; I will be addressing those separately.)

2018-01-19  Tom Tromey  <tom@tromey.com>

	* rust-lang.c (rust_lookup_symbol_nonlocal): Look up qualified
	symbols in the static and global blocks.

2018-01-19  Tom Tromey  <tom@tromey.com>

	* gdb.rust/modules.rs (TWENTY_THREE): New global.
	* gdb.rust/modules.exp: Add ::-qualified lookup test.

1 files changed, 12 insertions, 6 deletions

diff --git a/gdb/rust-lang.c b/gdb/rust-lang.c
index f7bec33..5ff80b2 100644
--- a/gdb/rust-lang.c
+++ b/gdb/rust-lang.c
@@ -2201,19 +2201,25 @@ rust_lookup_symbol_nonlocal (const struct language_defn *langdef,
     }
 
   /* Look up bare names in the block's scope.  */
+  std::string scopedname;
   if (name[cp_find_first_component (name)] == '\0')
     {
       const char *scope = block_scope (block);
 
       if (scope[0] != '\0')
 	{
-	  std::string scopedname = std::string (scope) + "::" + name;
-
-	  result = lookup_symbol_in_static_block (scopedname.c_str (), block,
-						  domain);
-	  if (result.symbol == NULL)
-	    result = lookup_global_symbol (scopedname.c_str (), block, domain);
+	  scopedname = std::string (scope) + "::" + name;
+	  name = scopedname.c_str ();
 	}
+      else
+	name = NULL;
+    }
+
+  if (name != NULL)
+    {
+      result = lookup_symbol_in_static_block (name, block, domain);
+      if (result.symbol == NULL)
+	result = lookup_global_symbol (name, block, domain);
     }
   return result;
 }