Implement unused crate lint for 2018 edition

Cargo.lock

@@ -3505,7 +3505,6 @@ version = "0.0.0"
 dependencies = [
  "log",
  "rustc_ast",
- "rustc_data_structures",
  "rustc_span",
 ]
 
@@ -3522,7 +3521,6 @@ dependencies = [
  "rustc_session",
  "rustc_span",
  "serialize",
- "smallvec 1.0.0",
 ]
 
 [[package]]
@@ -3781,7 +3779,6 @@ dependencies = [
  "rustc_ast",
  "rustc_attr",
  "rustc_data_structures",
- "rustc_error_codes",
  "rustc_errors",
  "rustc_hir",
  "rustc_index",
@@ -3948,7 +3945,6 @@ dependencies = [
  "rustc_hir",
  "rustc_index",
  "rustc_infer",
- "rustc_macros",
  "rustc_session",
  "rustc_span",
  "rustc_target",
@@ -3985,7 +3981,6 @@ dependencies = [
  "rustc_attr",
  "rustc_data_structures",
  "rustc_errors",
- "rustc_feature",
  "rustc_hir",
  "rustc_index",
  "rustc_infer",
@@ -4039,7 +4034,6 @@ dependencies = [
  "rustc_expand",
  "rustc_feature",
  "rustc_hir",
- "rustc_infer",
  "rustc_metadata",
  "rustc_session",
  "rustc_span",
@@ -4075,7 +4069,6 @@ dependencies = [
  "rustc_errors",
  "rustc_feature",
  "rustc_fs_util",
- "rustc_index",
  "rustc_span",
  "rustc_target",
  "serialize",
@@ -4129,9 +4122,7 @@ dependencies = [
  "rustc_data_structures",
  "rustc_hir",
  "rustc_infer",
- "rustc_macros",
  "rustc_span",
- "rustc_target",
  "smallvec 1.0.0",
 ]
 

src/bootstrap/bootstrap.py

@@ -691,6 +691,7 @@ def build_bootstrap(self):
         target_linker = self.get_toml("linker", build_section)
         if target_linker is not None:
             env["RUSTFLAGS"] += " -C linker=" + target_linker
+        # After the next cfg(bootstrap) bump, add "-Wunused_extern_options" to RUSTFLAGS
         env["RUSTFLAGS"] += " -Wrust_2018_idioms -Wunused_lifetimes"
         if self.get_toml("deny-warnings", "rust") != "false":
             env["RUSTFLAGS"] += " -Dwarnings"

src/bootstrap/builder.rs

@@ -1067,6 +1067,10 @@ impl<'a> Builder<'a> {
             // some code doesn't go through this `rustc` wrapper.
             rustflags.arg("-Wrust_2018_idioms");
             rustflags.arg("-Wunused_lifetimes");
+            // Remove this after the next cfg(bootstrap) bump
+            if stage != 0 {
+                rustflags.arg("-Wunused_extern_options");
+            }
 
             if self.config.deny_warnings {
                 rustflags.arg("-Dwarnings");

src/liballoc/lib.rs

@@ -174,3 +174,7 @@ mod std {
 pub mod __export {
     pub use core::format_args;
 }
+
+// Suppress warning: this is only used in benchmarks
+#[cfg(test)]
+use rand_xorshift as _;

src/liballoc/tests/lib.rs

@@ -17,6 +17,13 @@
 use std::collections::hash_map::DefaultHasher;
 use std::hash::{Hash, Hasher};
 
+// Supress warning: this crate is only used in benchmarks
+use rand_xorshift as _;
+
+// Supress warning: these crates are used indirectly
+use alloc as _;
+use compiler_builtins as _;
+
 mod arc;
 mod binary_heap;
 mod boxed;

src/librustc/lib.rs

@@ -91,3 +91,8 @@ pub mod util {
 
 // Allows macros to refer to this crate as `::rustc`
 extern crate self as rustc;
+
+// Suppress warning: these crates will be unused when cfg(parallel_compiler) is not enabled
+use jobserver as _;
+use rustc_rayon as _;
+use rustc_rayon_core as _;

src/librustc/ty/mod.rs

@@ -30,8 +30,8 @@ use rustc_ast::ast::{self, Ident, Name};
 use rustc_ast::node_id::{NodeId, NodeMap, NodeSet};
 use rustc_attr as attr;
 use rustc_data_structures::captures::Captures;
-use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::fx::FxIndexMap;
+use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::sorted_map::SortedIndexMultiMap;
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_data_structures::sync::{self, par_iter, Lrc, ParallelIterator};
@@ -136,6 +136,9 @@ pub struct ResolverOutputs {
     /// Extern prelude entries. The value is `true` if the entry was introduced
     /// via `extern crate` item and not `--extern` option or compiler built-in.
     pub extern_prelude: FxHashMap<Name, bool>,
+    /// All crates that ended up getting loaded.
+    /// Used to determine which `--extern` entries are unused
+    pub used_crates: Option<FxHashSet<Symbol>>,
 }
 
 #[derive(Clone, Copy, PartialEq, Eq, Debug, HashStable)]

src/librustc/ty/print/pretty.rs

@@ -383,7 +383,7 @@ pub trait PrettyPrinter<'tcx>:
                     .tcx()
                     .item_children(visible_parent)
                     .iter()
-                    .find(|child| child.res.def_id() == def_id)
+                    .find(|child| child.res.opt_def_id() == Some(def_id))
                     .map(|child| child.ident.name);
                 if let Some(reexport) = reexport {
                     *name = reexport;

src/librustc_ast_pretty/Cargo.toml

@@ -12,5 +12,4 @@ doctest = false
 [dependencies]
 log = "0.4"
 rustc_span = { path = "../librustc_span" }
-rustc_data_structures = { path = "../librustc_data_structures" }
 rustc_ast = { path = "../librustc_ast" }

src/librustc_attr/Cargo.toml

@@ -17,6 +17,5 @@ rustc_span = { path = "../librustc_span" }
 rustc_data_structures = { path = "../librustc_data_structures" }
 rustc_feature = { path = "../librustc_feature" }
 rustc_macros = { path = "../librustc_macros" }
-smallvec = { version = "1.0", features = ["union", "may_dangle"] }
 rustc_session = { path = "../librustc_session" }
 rustc_ast = { path = "../librustc_ast" }

src/librustc_data_structures/lib.rs

@@ -24,6 +24,11 @@
 #![feature(thread_id_value)]
 #![allow(rustc::default_hash_types)]
 
+// Suppress warning: these crates will be unused when cfg(parallel_compiler) is not enabled
+use crossbeam_utils as _;
+use rayon as _;
+use rayon_core as _;
+
 #[macro_use]
 extern crate log;
 #[cfg(unix)]

src/librustc_infer/Cargo.toml

@@ -17,7 +17,6 @@ rustc_attr = { path = "../librustc_attr" }
 rustc = { path = "../librustc" }
 rustc_data_structures = { path = "../librustc_data_structures" }
 rustc_errors = { path = "../librustc_errors" }
-rustc_error_codes = { path = "../librustc_error_codes" }
 rustc_hir = { path = "../librustc_hir" }
 rustc_index = { path = "../librustc_index" }
 rustc_macros = { path = "../librustc_macros" }

src/librustc_interface/lib.rs

@@ -21,3 +21,6 @@ pub use queries::Queries;
 
 #[cfg(test)]
 mod tests;
+
+// Suppress warning: this crate will be unused when cfg(parallel_compiler) is not enable
+use rayon as _;

src/librustc_interface/passes.rs

@@ -26,7 +26,7 @@ use rustc_data_structures::{box_region_allow_access, declare_box_region_type, pa
 use rustc_errors::PResult;
 use rustc_expand::base::ExtCtxt;
 use rustc_hir::def_id::{CrateNum, LOCAL_CRATE};
-use rustc_hir::Crate;
+use rustc_hir::{Crate, CRATE_HIR_ID};
 use rustc_infer::traits;
 use rustc_lint::LintStore;
 use rustc_mir as mir;
@@ -35,6 +35,7 @@ use rustc_parse::{parse_crate_from_file, parse_crate_from_source_str};
 use rustc_passes::{self, hir_stats, layout_test};
 use rustc_plugin_impl as plugin;
 use rustc_resolve::{Resolver, ResolverArenas};
+use rustc_span::symbol::sym;
 use rustc_span::symbol::Symbol;
 use rustc_span::FileName;
 use rustc_typeck as typeck;
@@ -50,6 +51,9 @@ use std::path::PathBuf;
 use std::rc::Rc;
 use std::{env, fs, iter, mem};
 
+use log::debug;
+use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+
 pub fn parse<'a>(sess: &'a Session, input: &Input) -> PResult<'a, ast::Crate> {
     let krate = sess.time("parse_crate", || match input {
         Input::File(file) => parse_crate_from_file(file, &sess.parse_sess),
@@ -733,6 +737,9 @@ pub fn create_global_ctxt<'tcx>(
         callback(sess, &mut local_providers, &mut extern_providers);
     }
 
+    let extern_prelude = resolver_outputs.extern_prelude.clone();
+    let used_crates = resolver_outputs.used_crates.as_ref().unwrap().clone();
+
     let gcx = sess.time("setup_global_ctxt", || {
         global_ctxt.init_locking(|| {
             TyCtxt::create_global_ctxt(
@@ -753,11 +760,60 @@ pub fn create_global_ctxt<'tcx>(
     // Do some initialization of the DepGraph that can only be done with the tcx available.
     ty::tls::enter_global(&gcx, |tcx| {
         tcx.sess.time("dep_graph_tcx_init", || rustc_incremental::dep_graph_tcx_init(tcx));
+        check_unused_crates(tcx, &extern_prelude, &used_crates);
     });
 
     QueryContext(gcx)
 }
 
+fn check_unused_crates(
+    tcx: TyCtxt<'_>,
+    extern_prelude: &FxHashMap<Symbol, bool>,
+    used_crates: &FxHashSet<Symbol>,
+) {
+    for (krate, introduced_by_item) in extern_prelude {
+        let krate = *krate;
+        if *introduced_by_item {
+            debug!("check_unused_crate: crate {:?} added by `extern crate`, skipping", krate);
+            continue;
+        }
+        if used_crates.contains(&krate) {
+            debug!("check_unused_crate: crate {:?} was used, skipping", krate);
+            continue;
+        }
+
+        // HACK: These should not be hardcoded. However, libstd needs
+        // both of them mentioned in its Cargo.tonl, and trying to
+        // add 'use' or 'extern crate' statements for both crates
+        // causes an error.
+        //
+        // If either of these crates is unused, we will never have loaded
+        // their metadata (otherwise, they would be used). This means that we have
+        // no way of checking the `panic_runtime` field in the metadata, and must
+        // instead rely on the crate name itself.
+        if krate.as_str() == "panic_abort" || krate.as_str() == "panic_unwind" {
+            debug!("check_unused_crate: skipping panic runtime crate {:?}", krate);
+            continue;
+        }
+
+        if krate == sym::core || krate == sym::std {
+            debug!("check_unused_crate: skipping builtin crate {:?}", krate);
+            continue;
+        }
+
+        if tcx.sess.rust_2018() && krate == sym::meta {
+            debug!("check_unused_crate: skipping `meta` crate");
+            continue;
+        }
+
+        tcx.struct_lint_node(lint::builtin::UNUSED_EXTERN_OPTIONS, CRATE_HIR_ID, |lint| {
+            lint.build(&format!("crate `{}` is unused in crate `{}`", krate, tcx.crate_name))
+                .help(&format!("try removing `{}` from your `Cargo.toml`", krate))
+                .emit();
+        });
+    }
+}
+
 /// Runs the resolution, type-checking, region checking and other
 /// miscellaneous analysis passes on the crate.
 fn analysis(tcx: TyCtxt<'_>, cnum: CrateNum) -> Result<()> {