matthiaskrgr
diff --git a/‎src/librustc/back/link.rs
+3-2 b/‎src/librustc/back/link.rs
+3-2
diff --git a/‎src/librustc/back/svh.rs
+7-1 b/‎src/librustc/back/svh.rs
+7-1
diff --git a/‎src/librustc/driver/config.rs
+23-3 b/‎src/librustc/driver/config.rs
+23-3
diff --git a/‎src/librustc/metadata/creader.rs
+27-5 b/‎src/librustc/metadata/creader.rs
+27-5
@@ -588,10 +588,11 @@ pub fn find_crate_name(sess: Option<&Session>,
     }), None)
 }
 
-pub fn build_link_meta(krate: &ast::Crate, name: String) -> LinkMeta {
+pub fn build_link_meta(sess: &Session, krate: &ast::Crate,
+                       name: String) -> LinkMeta {
     let r = LinkMeta {
         crate_name: name,
-        crate_hash: Svh::calculate(krate),
+        crate_hash: Svh::calculate(sess, krate),
     };
     info!("{}", r);
     return r;
 
@@ -53,6 +53,8 @@ use std::iter::range_step;
 use syntax::ast;
 use syntax::visit;
 
+use driver::session::Session;
+
 #[deriving(Clone, PartialEq)]
 pub struct Svh {
     hash: String,
@@ -68,7 +70,7 @@ impl Svh {
         self.hash.as_slice()
     }
 
-    pub fn calculate(krate: &ast::Crate) -> Svh {
+    pub fn calculate(sess: &Session, krate: &ast::Crate) -> Svh {
         // FIXME (#14132): This is better than it used to be, but it still not
         // ideal. We now attempt to hash only the relevant portions of the
         // Crate AST as well as the top-level crate attributes. (However,
@@ -80,6 +82,10 @@ impl Svh {
         //        avoid collisions.
         let mut state = SipState::new();
 
+        for data in sess.opts.cg.metadata.iter() {
+            data.hash(&mut state);
+        }
+
         {
             let mut visit = svh_visitor::make(&mut state);
             visit::walk_crate(&mut visit, krate, ());
 
@@ -30,7 +30,7 @@ use syntax::diagnostic::{ColorConfig, Auto, Always, Never};
 use syntax::parse;
 use syntax::parse::token::InternedString;
 
-use std::collections::HashSet;
+use std::collections::{HashSet, HashMap};
 use getopts::{optopt, optmulti, optflag, optflagopt};
 use getopts;
 use lib::llvm::llvm;
@@ -95,6 +95,7 @@ pub struct Options {
     pub print_metas: (bool, bool),
     pub cg: CodegenOptions,
     pub color: ColorConfig,
+    pub externs: HashMap<String, Vec<String>>,
 }
 
 /// Some reasonable defaults
@@ -120,6 +121,7 @@ pub fn basic_options() -> Options {
         print_metas: (false, false),
         cg: basic_codegen_options(),
         color: Auto,
+        externs: HashMap::new(),
     }
 }
 
@@ -551,7 +553,9 @@ pub fn optgroups() -> Vec<getopts::OptGroup> {
         optopt("", "color", "Configure coloring of output:
             auto   = colorize, if output goes to a tty (default);
             always = always colorize output;
-            never  = never colorize output", "auto|always|never")
+            never  = never colorize output", "auto|always|never"),
+        optmulti("", "extern", "Specify where an external rust library is located",
+                 "PATH"),
     )
 }
 
@@ -730,6 +734,21 @@ pub fn build_session_options(matches: &getopts::Matches) -> Options {
         }
     };
 
+    let mut externs = HashMap::new();
+    for arg in matches.opt_strs("extern").iter() {
+        let mut parts = arg.as_slice().splitn('=', 1);
+        let name = match parts.next() {
+            Some(s) => s,
+            None => early_error("--extern value must not be empty"),
+        };
+        let location = match parts.next() {
+            Some(s) => s,
+            None => early_error("--extern value must be of the format `foo=bar`"),
+        };
+        let locs = externs.find_or_insert(name.to_string(), Vec::new());
+        locs.push(location.to_string());
+    }
+
     Options {
         crate_types: crate_types,
         gc: gc,
@@ -750,7 +769,8 @@ pub fn build_session_options(matches: &getopts::Matches) -> Options {
         write_dependency_info: write_dependency_info,
         print_metas: print_metas,
         cg: cg,
-        color: color
+        color: color,
+        externs: externs,
     }
 }
 
 
@@ -280,12 +280,32 @@ fn existing_match(e: &Env, name: &str,
                   hash: Option<&Svh>) -> Option<ast::CrateNum> {
     let mut ret = None;
     e.sess.cstore.iter_crate_data(|cnum, data| {
-        if data.name().as_slice() == name {
-            let other_hash = data.hash();
-            match hash {
-                Some(hash) if *hash != other_hash => {}
-                Some(..) | None => { ret = Some(cnum); }
+        if data.name().as_slice() != name { return }
+
+        match hash {
+            Some(hash) if *hash == data.hash() => { ret = Some(cnum); return }
+            Some(..) => return,
+            None => {}
+        }
+
+        // When the hash is None we're dealing with a top-level dependency in
+        // which case we may have a specification on the command line for this
+        // library. Even though an upstream library may have loaded something of
+        // the same name, we have to make sure it was loaded from the exact same
+        // location as well.
+        let source = e.sess.cstore.get_used_crate_source(cnum).unwrap();
+        let dylib = source.dylib.as_ref().map(|p| p.as_vec());
+        let rlib = source.rlib.as_ref().map(|p| p.as_vec());
+        match e.sess.opts.externs.find_equiv(&name) {
+            Some(locs) => {
+                let found = locs.iter().any(|l| {
+                    Some(l.as_bytes()) == dylib || Some(l.as_bytes()) == rlib
+                });
+                if found {
+                    ret = Some(cnum);
+                }
             }
+            None => ret = Some(cnum),
         }
     });
     return ret;
@@ -361,6 +381,7 @@ fn resolve_crate<'a>(e: &mut Env,
                 root: root,
                 rejected_via_hash: vec!(),
                 rejected_via_triple: vec!(),
+                should_match_name: true,
             };
             let library = load_ctxt.load_library_crate();
             register_crate(e, root, ident, name, span, library)
@@ -422,6 +443,7 @@ impl<'a> PluginMetadataReader<'a> {
             root: &None,
             rejected_via_hash: vec!(),
             rejected_via_triple: vec!(),
+            should_match_name: true,
         };
         let library = match load_ctxt.maybe_load_library_crate() {
             Some (l) => l,