message: Only compile user-modifiable projects incrementally.
diff: @@ -904,7 +904,11 @@ impl<'a, 'cfg> Context<'a, 'cfg> {
  }
  pub fn incremental_args(&self, unit: &Unit) -> CargoResult<Vec<String>> {
  - if self.incremental_enabled {
  + // Only enable incremental compilation for sources the user can modify.
  + // For things that change infrequently, non-incremental builds yield
  + // better performance.
  + // (see also https://github.com/rust-lang/cargo/issues/3972)
  + if self.incremental_enabled && unit.pkg.package_id().source_id().is_path() {
  Ok(vec![format!("-Zincremental={}", self.layout(unit.kind).incremental().display())])
  } else {
  Ok(vec![])
  