rustc: filter out empty linker args

This is inspired by a mystifying linker failure when using `pkg-config` to
generate the linker args: `pkg-config` produces output that ends in a
space, thus resulting in an empty linker argument.

Also added some updates to the concerning error messages that helped
spotting this bug.

src/librustc/back/link.rs

@@ -362,12 +362,12 @@ pub mod write {
             ~"-o", object.as_str().unwrap().to_owned(),
             assembly.as_str().unwrap().to_owned()];
 
-        debug!("{} {}", cc, args.connect(" "));
+        debug!("{} '{}'", cc, args.connect("' '"));
         let prog = run::process_output(cc, args);
 
         if !prog.status.success() {
             sess.err(format!("linking with `{}` failed: {}", cc, prog.status));
-            sess.note(format!("{} arguments: {}", cc, args.connect(" ")));
+            sess.note(format!("{} arguments: '{}'", cc, args.connect("' '")));
             sess.note(str::from_utf8(prog.error + prog.output));
             sess.abort_if_errors();
         }
@@ -1061,7 +1061,7 @@ fn link_natively(sess: Session, dylib: bool, obj_filename: &Path,
     let mut cc_args = sess.targ_cfg.target_strs.cc_args.clone();
     cc_args.push_all_move(link_args(sess, dylib, obj_filename, out_filename));
     if (sess.opts.debugging_opts & session::print_link_args) != 0 {
-        println!("{} link args: {}", cc_prog, cc_args.connect(" "));
+        println!("{} link args: '{}'", cc_prog, cc_args.connect("' '"));
     }
 
     // May have not found libraries in the right formats.
@@ -1073,7 +1073,7 @@ fn link_natively(sess: Session, dylib: bool, obj_filename: &Path,
 
     if !prog.status.success() {
         sess.err(format!("linking with `{}` failed: {}", cc_prog, prog.status));
-        sess.note(format!("{} arguments: {}", cc_prog, cc_args.connect(" ")));
+        sess.note(format!("{} arguments: '{}'", cc_prog, cc_args.connect("' '")));
         sess.note(str::from_utf8(prog.error + prog.output));
         sess.abort_if_errors();
     }

src/librustc/driver/driver.rs

@@ -762,7 +762,13 @@ pub fn build_session_options(binary: @str,
     let ar = matches.opt_str("ar");
     let linker = matches.opt_str("linker");
     let linker_args = matches.opt_strs("link-args").flat_map( |a| {
-        a.split(' ').map(|arg| arg.to_owned()).collect()
+        a.split(' ').filter_map(|arg| {
+            if arg.is_empty() {
+                None
+            } else {
+                Some(arg.to_owned())
+            }
+        }).collect()
     });
 
     let cfg = parse_cfgspecs(matches.opt_strs("cfg"), demitter);

src/test/run-make/prune-link-args/Makefile

@@ -0,0 +1,6 @@
+-include ../tools.mk
+# Notice the space in the end, this emulates the output of pkg-config
+RUSTC_FLAGS = --link-args "-lc "
+
+all:
+	$(RUSTC) $(RUSTC_FLAGS) empty.rs

src/test/run-make/prune-link-args/empty.rs

@@ -0,0 +1 @@
+fn main() { }