trans: Disambiguate generic instance symbol names by instantiating crate.

Two crates will often instantiate the same generic functions. Since
we don't make any attempt to re-use these instances cross-crate, we
would run into symbol conflicts for anything with external linkage.

In order to avoid this, this commit makes the compiler incorporate
the ID of the instantiating crate into the symbol hash. This way
equal generic instances will have different symbols names when
used in different crates.

Author: michaelwoerister

src/librustc_trans/back/symbol_names.rs

@@ -152,6 +152,15 @@ fn get_symbol_hash<'a, 'tcx>(scx: &SharedCrateContext<'a, 'tcx>,
             assert!(!substs.has_erasable_regions());
             assert!(!substs.needs_subst());
             substs.visit_with(&mut hasher);
+
+            // If this is an instance of a generic function, we also hash in
+            // the ID of the instantiating crate. This avoids symbol conflicts
+            // in case the same instances is emitted in two crates of the same
+            // project.
+            if substs.types().next().is_some() {
+                hasher.hash(scx.tcx().crate_name.as_str());
+                hasher.hash(scx.sess().local_crate_disambiguator().as_str());
+            }
         }
     });