Add allow-by-default lint UNUSED_EXTERN_OPTIONS

This lint fires for any unused crates in the Rust 2018 extern prelude
(e.g. passed in via `--extern`)

For now, it is allow-by-default to prevent false positives in
multi-target crate. See rust-lang#69203 (comment)
for more details, and rust-lang/cargo#1982 for
the tracking issue.

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: FxHashSet<Symbol>,
 }
 
 #[derive(Clone, Copy, PartialEq, Eq, Debug, HashStable)]

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.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<()> {

src/librustc_metadata/creader.rs

@@ -26,6 +26,7 @@ use rustc_target::spec::{PanicStrategy, TargetTriple};
 
 use log::{debug, info, log_enabled};
 use proc_macro::bridge::client::ProcMacro;
+use rustc_data_structures::fx::FxHashSet;
 use std::path::Path;
 use std::{cmp, fs};
 
@@ -47,6 +48,7 @@ pub struct CrateLoader<'a> {
     local_crate_name: Symbol,
     // Mutable output.
     cstore: CStore,
+    loaded_crates: Option<FxHashSet<Symbol>>,
 }
 
 pub enum LoadedMacro {
@@ -197,6 +199,7 @@ impl<'a> CrateLoader<'a> {
                 allocator_kind: None,
                 has_global_allocator: false,
             },
+            loaded_crates: Some(Default::default()),
         }
     }
 
@@ -495,6 +498,7 @@ impl<'a> CrateLoader<'a> {
                 Ok(cnum)
             }
             (LoadResult::Loaded(library), host_library) => {
+                self.loaded_crates.as_mut().unwrap().insert(name);
                 Ok(self.register_crate(host_library, root, span, library, dep_kind, name))
             }
             _ => panic!(),
@@ -830,14 +834,15 @@ impl<'a> CrateLoader<'a> {
         });
     }
 
-    pub fn postprocess(&mut self, krate: &ast::Crate) {
+    pub fn postprocess(&mut self, krate: &ast::Crate) -> FxHashSet<Symbol> {
         self.inject_profiler_runtime();
         self.inject_allocator_crate(krate);
         self.inject_panic_runtime(krate);
 
         if log_enabled!(log::Level::Info) {
             dump_crates(&self.cstore);
         }
+        self.loaded_crates.take().unwrap()
     }
 
     pub fn process_extern_crate(

src/librustc_resolve/lib.rs

@@ -47,7 +47,7 @@ use rustc_session::Session;
 use rustc_span::hygiene::{ExpnId, ExpnKind, MacroKind, SyntaxContext, Transparency};
 use rustc_span::source_map::Spanned;
 use rustc_span::symbol::{kw, sym};
-use rustc_span::{Span, DUMMY_SP};
+use rustc_span::{Span, Symbol, DUMMY_SP};
 
 use log::debug;
 use std::cell::{Cell, RefCell};
@@ -962,6 +962,8 @@ pub struct Resolver<'a> {
     lint_buffer: LintBuffer,
 
     next_node_id: NodeId,
+
+    used_crates: FxHashSet<Symbol>,
 }
 
 /// Nothing really interesting here; it just provides memory for the rest of the crate.
@@ -1239,6 +1241,7 @@ impl<'a> Resolver<'a> {
             variant_vis: Default::default(),
             lint_buffer: LintBuffer::default(),
             next_node_id: NodeId::from_u32(1),
+            used_crates: Default::default(),
         }
     }
 
@@ -1275,6 +1278,7 @@ impl<'a> Resolver<'a> {
                 .iter()
                 .map(|(ident, entry)| (ident.name, entry.introduced_by_item))
                 .collect(),
+            used_crates: self.used_crates,
         }
     }
 
@@ -1293,6 +1297,7 @@ impl<'a> Resolver<'a> {
                 .iter()
                 .map(|(ident, entry)| (ident.name, entry.introduced_by_item))
                 .collect(),
+            used_crates: self.used_crates.clone(),
         }
     }
 
@@ -1343,7 +1348,7 @@ impl<'a> Resolver<'a> {
 
         self.check_unused(krate);
         self.report_errors(krate);
-        self.crate_loader.postprocess(krate);
+        self.used_crates = self.crate_loader.postprocess(krate);
     }
 
     fn new_module(

src/librustc_session/lint/builtin.rs

@@ -71,6 +71,12 @@ declare_lint! {
     "extern crates that are never used"
 }
 
+declare_lint! {
+    pub UNUSED_EXTERN_OPTIONS,
+    Allow,
+    "extern path crates that are never used"
+}
+
 declare_lint! {
     pub UNUSED_QUALIFICATIONS,
     Allow,
@@ -505,6 +511,7 @@ declare_lint_pass! {
         UNCONDITIONAL_PANIC,
         UNUSED_IMPORTS,
         UNUSED_EXTERN_CRATES,
+        UNUSED_EXTERN_OPTIONS,
         UNUSED_QUALIFICATIONS,
         UNKNOWN_LINTS,
         UNUSED_VARIABLES,

src/test/ui/lint/auxiliary/unused_crate.rs

@@ -0,0 +1 @@
+// Deliberately empty

src/test/ui/lint/unused_extern_options.rs

@@ -0,0 +1,5 @@
+// aux-crate:unused_crate=unused_crate.rs
+
+#![deny(unused_extern_options)]
+
+fn main() {}

src/test/ui/lint/unused_extern_options.stderr

@@ -0,0 +1,11 @@
+error: crate `unused_crate` is unused in crate `unused_extern_options`
+   |
+note: the lint level is defined here
+  --> $DIR/unused_extern_options.rs:3:9
+   |
+LL | #![deny(unused_extern_options)]
+   |         ^^^^^^^^^^^^^^^^^^^^^
+   = help: try removing `unused_crate` from your `Cargo.toml`
+
+error: aborting due to previous error
+