rustc_trans: Pass /LTCG to the MSVC linker

This flag, the link-time code generation flag, informs the linker that it can
call the compiler to perform whole program optimization.  We don't actually take
advantage of this for Rust code itself (as we have LLVM as our backend), but
native libraries typically compile for whole program optimization and then
require this flag to be passed to the linker so they can be successfully linked
together.

This shouldn't slow the linker down for Rust code itself, just help  perform
LTCG on native code being linked in which requires it.

Author: alexcrichton

src/librustc_trans/back/linker.rs

@@ -233,7 +233,16 @@ impl<'a> Linker for MsvcLinker<'a> {
         self.link_staticlib(lib);
     }
     fn optimize(&mut self) {
-        // Needs more investigation of `/OPT` arguments
+        // This flag, the link-time code generation flag, informs the linker
+        // that it can call the compiler to perform whole program optimization.
+        // We don't actually take advantage of this for Rust code itself (as we
+        // have LLVM as our backend), but native libraries typically compile for
+        // whole program optimization and then require this flag to be passed to
+        // the linker so they can be successfully linked together.
+        //
+        // This shouldn't slow the linker down for Rust code itself, just help
+        // perform LTCG on native code being linked in which requires it.
+        self.cmd.arg("/LTCG");
     }
     fn whole_archives(&mut self) {
         // hints not supported?