Address comments by reviewers.

src/librustc/session/config.rs

@@ -258,13 +258,11 @@ top_level_options!(
  lint_cap: Option<lint::Level> [TRACKED],
  describe_lints: bool [UNTRACKED],
  output_types: OutputTypes [TRACKED],
- // FIXME(mw): I'm not entirely sure if this can have any influence on
- // incremental compilation apart from what is already handled
- // by crate metadata hashes. Better track it.
+ // FIXME(mw): We track this for now but it actually doesn't make too
+ // much sense: The search path can stay the same while the
+ // things discovered there might have changed on disk.
  search_paths: SearchPaths [TRACKED],
- // FIXME(mw): Might not need to do dep-tracking for `libs`?
  libs: Vec<(String, cstore::NativeLibraryKind)> [TRACKED],
- // FIXME(mw): Might not need to do dep-tracking for `maybe_sysroot`?
  maybe_sysroot: Option<PathBuf> [TRACKED],
 
  target_triple: String [TRACKED],
@@ -280,8 +278,9 @@ top_level_options!(
  debugging_opts: DebuggingOptions [TRACKED],
  prints: Vec<PrintRequest> [UNTRACKED],
  cg: CodegenOptions [TRACKED],
- // FIXME(mw): `externs` might not need to be tracked but let's err on
- // the side of caution for now.
+ // FIXME(mw): We track this for now but it actually doesn't make too
+ // much sense: The value of this option can stay the same
+ // while the files they refer to might have changed on disk.
  externs: Externs [TRACKED],
  crate_name: Option<String> [TRACKED],
  // An optional name to use as the crate for std during std injection,
@@ -1692,11 +1691,12 @@ mod dep_tracking {
  ($t:ty) => (
  impl DepTrackingHash for Vec<$t> {
  fn hash(&self, hasher: &mut SipHasher, error_format: ErrorOutputType) {
- let mut elems = self.clone();
+ let mut elems: Vec<&$t> = self.iter().collect();
  elems.sort();
- for (i, e) in elems.iter().enumerate() {
- Hash::hash(&i, hasher);
- DepTrackingHash::hash(e, hasher, error_format);
+ Hash::hash(&elems.len(), hasher);
+ for (index, elem) in elems.iter().enumerate() {
+ Hash::hash(&index, hasher);
+ DepTrackingHash::hash(*elem, hasher, error_format);
  }
  }
  }