and few more

warning: this argument is a mutable reference, but not used mutably
  --> compiler\rustc_codegen_ssa\src\back\rpath.rs:80:41
   |
80 | fn get_rpath_relative_to_output(config: &mut RPathConfig<'_>, lib: &Path) -> OsString {
   |                                         ^^^^^^^^^^^^^^^^^^^^ help: consider changing to: `&RPathConfig<'_>`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
  --> compiler\rustc_codegen_ssa\src\back\rpath.rs:76:42
   |
76 | fn get_rpaths_relative_to_output(config: &mut RPathConfig<'_>) -> Vec<OsString> {
   |                                          ^^^^^^^^^^^^^^^^^^^^ help: consider changing to: `&RPathConfig<'_>`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
  --> compiler\rustc_codegen_ssa\src\back\rpath.rs:55:23
   |
55 | fn get_rpaths(config: &mut RPathConfig<'_>) -> Vec<OsString> {
   |                       ^^^^^^^^^^^^^^^^^^^^ help: consider changing to: `&RPathConfig<'_>`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
  --> compiler\rustc_codegen_ssa\src\back\rpath.rs:15:32
   |
15 | pub fn get_rpath_flags(config: &mut RPathConfig<'_>) -> Vec<OsString> {
   |                                ^^^^^^^^^^^^^^^^^^^^ help: consider changing to: `&RPathConfig<'_>`
   |
   = warning: changing this function will impact semver compatibility
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -2089,14 +2089,14 @@ fn add_rpath_args(
                     .map(|(path, _)| &**path)
             })
             .collect::<Vec<_>>();
-        let mut rpath_config = RPathConfig {
+        let rpath_config = RPathConfig {
             libs: &*libs,
             out_filename: out_filename.to_path_buf(),
             has_rpath: sess.target.has_rpath,
             is_like_osx: sess.target.is_like_osx,
             linker_is_gnu: sess.target.linker_flavor.is_gnu(),
         };
-        cmd.args(&rpath::get_rpath_flags(&mut rpath_config));
+        cmd.args(&rpath::get_rpath_flags(&rpath_config));
     }
 }
 

compiler/rustc_codegen_ssa/src/back/rpath.rs

@@ -12,7 +12,7 @@ pub struct RPathConfig<'a> {
     pub linker_is_gnu: bool,
 }
 
-pub fn get_rpath_flags(config: &mut RPathConfig<'_>) -> Vec<OsString> {
+pub fn get_rpath_flags(config: &RPathConfig<'_>) -> Vec<OsString> {
     // No rpath on windows
     if !config.has_rpath {
         return Vec::new();
@@ -52,7 +52,7 @@ fn rpaths_to_flags(rpaths: Vec<OsString>) -> Vec<OsString> {
     ret
 }
 
-fn get_rpaths(config: &mut RPathConfig<'_>) -> Vec<OsString> {
+fn get_rpaths(config: &RPathConfig<'_>) -> Vec<OsString> {
     debug!("output: {:?}", config.out_filename.display());
     debug!("libs:");
     for libpath in config.libs {
@@ -73,11 +73,11 @@ fn get_rpaths(config: &mut RPathConfig<'_>) -> Vec<OsString> {
     minimize_rpaths(&rpaths)
 }
 
-fn get_rpaths_relative_to_output(config: &mut RPathConfig<'_>) -> Vec<OsString> {
+fn get_rpaths_relative_to_output(config: &RPathConfig<'_>) -> Vec<OsString> {
     config.libs.iter().map(|a| get_rpath_relative_to_output(config, a)).collect()
 }
 
-fn get_rpath_relative_to_output(config: &mut RPathConfig<'_>, lib: &Path) -> OsString {
+fn get_rpath_relative_to_output(config: &RPathConfig<'_>, lib: &Path) -> OsString {
     // Mac doesn't appear to support $ORIGIN
     let prefix = if config.is_like_osx { "@loader_path" } else { "$ORIGIN" };