auto merge of #14472 : huonw/rust/native-lib-warnings, r=alexcrichton

rustc: clarify warning about native deps for a staticlib.

This adjusts the "unlinked native library" warning one receives when
compiling with `crate_type="staticlib"`. The warning is just trying to
tell the user that they need to link against these libraries, but the
old text wasn't making this obvious, the new text says this explicitly.

Author: bors

src/librustc/back/link.rs

@@ -1019,6 +1019,8 @@ fn link_staticlib(sess: &Session, obj_filename: &Path, out_filename: &Path) {
     a.add_native_library("compiler-rt").unwrap();
 
     let crates = sess.cstore.get_used_crates(cstore::RequireStatic);
+    let mut all_native_libs = vec![];
+
     for &(cnum, ref path) in crates.iter() {
         let name = sess.cstore.get_crate_data(cnum).name.clone();
         let p = match *path {
@@ -1029,17 +1031,25 @@ fn link_staticlib(sess: &Session, obj_filename: &Path, out_filename: &Path) {
             }
         };
         a.add_rlib(&p, name.as_slice(), sess.lto()).unwrap();
+
         let native_libs = csearch::get_native_libraries(&sess.cstore, cnum);
-        for &(kind, ref lib) in native_libs.iter() {
-            let name = match kind {
-                cstore::NativeStatic => "static library",
-                cstore::NativeUnknown => "library",
-                cstore::NativeFramework => "framework",
-            };
-            sess.warn(format!("unlinked native {}: {}",
-                              name,
-                              *lib).as_slice());
-        }
+        all_native_libs.extend(native_libs.move_iter());
+    }
+
+    if !all_native_libs.is_empty() {
+        sess.warn("link against the following native artifacts when linking against \
+                  this static library");
+        sess.note("the order and any duplication can be significant on some platforms, \
+                  and so may need to be preserved");
+    }
+
+    for &(kind, ref lib) in all_native_libs.iter() {
+        let name = match kind {
+            cstore::NativeStatic => "static library",
+            cstore::NativeUnknown => "library",
+            cstore::NativeFramework => "framework",
+        };
+        sess.note(format!("{}: {}", name, *lib).as_slice());
     }
 }