diff --git a/src/librustc/session/config.rs b/src/librustc/session/config.rs
index 8b921a4b46c2..d2020ac852d0 100644
--- a/src/librustc/session/config.rs
+++ b/src/librustc/session/config.rs
@@ -490,10 +490,10 @@ pub enum Input {
 }
 
 impl Input {
-    pub fn filestem(&self) -> String {
+    pub fn filestem(&self) -> &str {
         match *self {
-            Input::File(ref ifile) => ifile.file_stem().unwrap().to_str().unwrap().to_string(),
-            Input::Str { .. } => "rust_out".to_string(),
+            Input::File(ref ifile) => ifile.file_stem().unwrap().to_str().unwrap(),
+            Input::Str { .. } => "rust_out",
         }
     }
 
@@ -1406,6 +1406,7 @@ pub fn default_configuration(sess: &Session) -> ast::CrateConfig {
     let atomic_cas = sess.target.target.options.atomic_cas;
 
     let mut ret = FxHashSet::default();
+    ret.reserve(6); // the minimum number of insertions
     // Target bindings.
     ret.insert((Symbol::intern("target_os"), Some(Symbol::intern(os))));
     if let Some(ref fam) = sess.target.target.options.target_family {
diff --git a/src/librustc/session/filesearch.rs b/src/librustc/session/filesearch.rs
index e55b82c63c3e..fb8bf8be1c24 100644
--- a/src/librustc/session/filesearch.rs
+++ b/src/librustc/session/filesearch.rs
@@ -41,7 +41,7 @@ impl<'a> FileSearch<'a> {
         F: FnMut(&Path, PathKind)
     {
         let mut visited_dirs = FxHashSet::default();
-
+        visited_dirs.reserve(self.search_paths.paths.len() + 1);
         for (path, kind) in self.search_paths.iter(self.kind) {
             f(path, kind);
             visited_dirs.insert(path.to_path_buf());
diff --git a/src/librustc/session/search_paths.rs b/src/librustc/session/search_paths.rs
index be1bfcc0894d..768d4f1e5fb6 100644
--- a/src/librustc/session/search_paths.rs
+++ b/src/librustc/session/search_paths.rs
@@ -14,7 +14,7 @@ use session::{early_error, config};
 
 #[derive(Clone, Debug)]
 pub struct SearchPaths {
-    paths: Vec<(PathKind, PathBuf)>,
+    crate paths: Vec<(PathKind, PathBuf)>,
 }
 
 pub struct Iter<'a> {
diff --git a/src/librustc_driver/driver.rs b/src/librustc_driver/driver.rs
index b4f95b915eb8..223df7cbb187 100644
--- a/src/librustc_driver/driver.rs
+++ b/src/librustc_driver/driver.rs
@@ -1657,7 +1657,7 @@ pub fn build_output_filenames(
                 .crate_name
                 .clone()
                 .or_else(|| attr::find_crate_name(attrs).map(|n| n.to_string()))
-                .unwrap_or_else(|| input.filestem());
+                .unwrap_or_else(|| input.filestem().to_owned());
 
             OutputFilenames {
                 out_directory: dirpath,