Fix a bug in rust-lang#5152 that causes rustc/rustdoc to fail unnecessarily

Author: infinity0

src/bin/commands/rustc.rs

@@ -62,7 +62,8 @@ pub fn exec(config: &mut Config, args: &ArgMatches) -> CliResult {
     let mut compile_opts = args.compile_options_for_single_package(
         config, mode,
     )?;
-    compile_opts.target_rustc_args = Some(values(args, "args"));
+    let target_args = values(args, "args");
+    compile_opts.target_rustc_args = if target_args.is_empty() { None } else { Some(target_args) };
     ops::compile(&ws, &compile_opts)?;
     Ok(())
 }

src/bin/commands/rustdoc.rs

@@ -45,7 +45,8 @@ pub fn exec(config: &mut Config, args: &ArgMatches) -> CliResult {
     let mut compile_opts = args.compile_options_for_single_package(
         config, CompileMode::Doc { deps: false },
     )?;
-    compile_opts.target_rustdoc_args = Some(values(args, "args"));
+    let target_args = values(args, "args");
+    compile_opts.target_rustdoc_args = if target_args.is_empty() { None } else { Some(target_args) };
     let doc_opts = DocOptions {
         open_result: args.is_present("open"),
         compile_opts,