linker: Use --as-needed by default when linker supports it

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -1651,6 +1651,12 @@ fn linker_with_args<'a, B: ArchiveBuilder<'a>>(
         cmd.add_eh_frame_header();
     }
 
+    // NO-OPT-OUT, OBJECT-FILES-NO
+    // Avoid linking to dynamic libraries unless they satisfy some undefined symbols
+    // at the point at which they are specified on the command line.
+    // Must be passed before any dynamic libraries.
+    cmd.add_as_needed();
+
     // NO-OPT-OUT, OBJECT-FILES-NO
     if crt_objects_fallback {
         cmd.no_crt_objects();

compiler/rustc_codegen_ssa/src/back/linker.rs

@@ -130,6 +130,7 @@ pub trait Linker {
     fn group_end(&mut self);
     fn linker_plugin_lto(&mut self);
     fn add_eh_frame_header(&mut self) {}
+    fn add_as_needed(&mut self) {}
     fn finalize(&mut self);
 }
 
@@ -641,6 +642,12 @@ impl<'a> Linker for GccLinker<'a> {
     fn add_eh_frame_header(&mut self) {
         self.linker_arg("--eh-frame-hdr");
     }
+
+    fn add_as_needed(&mut self) {
+        if self.sess.target.linker_is_gnu {
+            self.linker_arg("--as-needed");
+        }
+    }
 }
 
 pub struct MsvcLinker<'a> {