Add -Z extra-link-arg

This hides the new rustc-bin-link-arg and rustc-link-arg build script
configuration items behind an unstable flag.

Author: npmccallum

src/cargo/core/compiler/mod.rs

@@ -215,6 +215,7 @@ fn rustc(cx: &mut Context<'_, '_>, unit: &Unit, exec: &Arc<dyn Executor>) -> Car
     // don't pass the `-l` flags.
     let pass_l_flag = unit.target.is_lib() || !unit.pkg.targets().iter().any(|t| t.is_lib());
     let link_type = unit.target.into();
+    let extra_link_arg = cx.bcx.config.cli_unstable().extra_link_arg;
 
     let dep_info_name = match cx.files().metadata(unit) {
         Some(metadata) => format!("{}-{}.d", unit.target.crate_name(), metadata),
@@ -263,6 +264,7 @@ fn rustc(cx: &mut Context<'_, '_>, unit: &Unit, exec: &Arc<dyn Executor>) -> Car
                     &build_scripts,
                     pass_l_flag,
                     link_type,
+                    extra_link_arg,
                     current_id,
                 )?;
                 add_plugin_deps(&mut rustc, &script_outputs, &build_scripts, &root_output)?;
@@ -345,6 +347,7 @@ fn rustc(cx: &mut Context<'_, '_>, unit: &Unit, exec: &Arc<dyn Executor>) -> Car
         build_scripts: &BuildScripts,
         pass_l_flag: bool,
         link_type: Option<LinkType>,
+        extra_link_arg: bool,
         current_id: PackageId,
     ) -> CargoResult<()> {
         for key in build_scripts.to_link.iter() {
@@ -366,17 +369,15 @@ fn rustc(cx: &mut Context<'_, '_>, unit: &Unit, exec: &Arc<dyn Executor>) -> Car
                         rustc.arg("-l").arg(name);
                     }
                 }
-
                 if link_type.is_some() {
-                    for arg in output
+                    output
                         .linker_args
                         .iter()
                         .filter(|x| x.0.is_none() || x.0 == link_type)
-                        .map(|x| &x.1)
-                    {
-                        let link_arg = format!("link-arg={}", arg);
-                        rustc.arg("-C").arg(link_arg);
-                    }
+                        .filter(|x| x.0 == Some(LinkType::Cdylib) || extra_link_arg)
+                        .for_each(|x| {
+                            rustc.arg("-C").arg(format!("link-arg={}", x.1));
+                        });
                 }
             }
         }

src/cargo/core/features.rs

@@ -359,6 +359,7 @@ pub struct CliUnstable {
     pub terminal_width: Option<Option<usize>>,
     pub namespaced_features: bool,
     pub weak_dep_features: bool,
+    pub extra_link_arg: bool,
 }
 
 fn deserialize_build_std<'de, D>(deserializer: D) -> Result<Option<Vec<String>>, D::Error>
@@ -466,6 +467,7 @@ impl CliUnstable {
             "terminal-width" => self.terminal_width = Some(parse_usize_opt(v)?),
             "namespaced-features" => self.namespaced_features = parse_empty(k, v)?,
             "weak-dep-features" => self.weak_dep_features = parse_empty(k, v)?,
+            "extra-link-arg" => self.extra_link_arg = parse_empty(k, v)?,
             _ => bail!("unknown `-Z` flag specified: {}", k),
         }