Use `-Wunused_crate_dependencies` for compiler crates.

It's very useful. There are some false positives involving integration
tests in `rustc_pattern_analysis` and `rustc_serialize`. There is also a
false positive involving `rustc_driver_impl`'s
`rustc_randomized_layouts` feature. And I removed a `rustc_span` mention
in a doc comment in `rustc_log` because it wasn't integral to the
comment but caused a dev-dependency.

1 files changed, 8 insertions, 0 deletions

diff --git a/compiler/rustc_driver_impl/src/lib.rs b/compiler/rustc_driver_impl/src/lib.rs
index 43dc2e3cf9e..8ede6e41336 100644
--- a/compiler/rustc_driver_impl/src/lib.rs
+++ b/compiler/rustc_driver_impl/src/lib.rs
@@ -44,6 +44,10 @@ use rustc_errors::emitter::stderr_destination;
 use rustc_errors::registry::Registry;
 use rustc_errors::{ColorConfig, DiagCtxt, ErrCode, FatalError, PResult, markdown};
 use rustc_feature::find_gated_cfg;
+// This avoids a false positive with `-Wunused_crate_dependencies`.
+// `rust_index` isn't used in this crate's code, but it must be named in the
+// `Cargo.toml` for the `rustc_randomized_layouts` feature.
+use rustc_index as _;
 use rustc_interface::util::{self, get_codegen_backend};
 use rustc_interface::{Linker, create_and_enter_global_ctxt, interface, passes};
 use rustc_lint::unerased_lint_store;
@@ -89,6 +93,10 @@ pub mod pretty;
 #[macro_use]
 mod print;
 mod session_diagnostics;
+
+// Keep the OS parts of this `cfg` in sync with the `cfg` on the `libc`
+// dependency in `compiler/rustc_driver/Cargo.toml`, to keep
+// `-Wunused-crated-dependencies` satisfied.
 #[cfg(all(not(miri), unix, any(target_env = "gnu", target_os = "macos")))]
 mod signal_handler;