Auto merge of #42588 - ishitatsuyuki:patch-1, r=petrochenkov

Make unused-extern-crate warn-by-default

Apart from enabling the lint, this pull request also removes existing unused crates in the codebase, and fix some amount of false positives on crates with special purposes.

Now that all false positive issues are closed, it should be possible to make it available to wider users.

Quote:
> Now that macro modularization is implemented, this is true today! *#30849 (comment)

Concerns: can break some `#[deny(warnings)]`.

Close #42591

Author: bors

src/Cargo.lock

@@ -10,7 +10,6 @@
 
 #![deny(warnings)]
 
-#![feature(alloc)]
 #![feature(attr_literals)]
 #![feature(box_syntax)]
 #![feature(inclusive_range_syntax)]
@@ -27,14 +26,10 @@
 #![feature(splice)]
 #![feature(str_escape)]
 #![feature(string_retain)]
-#![feature(test)]
 #![feature(unboxed_closures)]
 #![feature(unicode)]
 
-extern crate alloc;
-extern crate test;
 extern crate std_unicode;
-extern crate core;
 
 use std::hash::{Hash, Hasher};
 use std::collections::hash_map::DefaultHasher;

src/liballoc/tests/lib.rs

@@ -19,7 +19,7 @@
 #![feature(libc)]
 #![feature(linkage)]
 #![feature(staged_api)]
-#![cfg_attr(dummy_jemalloc, allow(dead_code))]
+#![cfg_attr(dummy_jemalloc, allow(dead_code, unused_extern_crates))]
 #![cfg_attr(not(dummy_jemalloc), feature(allocator_api))]
 
 extern crate alloc;

src/liballoc_jemalloc/lib.rs

@@ -26,7 +26,6 @@
 #![feature(inclusive_range)]
 #![feature(inclusive_range_syntax)]
 #![feature(iter_rfind)]
-#![feature(libc)]
 #![feature(nonzero)]
 #![feature(ord_max_min)]
 #![feature(rand)]
@@ -41,13 +40,10 @@
 #![feature(test)]
 #![feature(trusted_len)]
 #![feature(try_from)]
-#![feature(unicode)]
 #![feature(unique)]
 
 extern crate core;
 extern crate test;
-extern crate libc;
-extern crate std_unicode;
 extern crate rand;
 
 mod any;

src/libcore/tests/lib.rs

@@ -34,7 +34,9 @@
 #![feature(core_intrinsics)]
 #![feature(lang_items)]
 #![feature(libc)]
-#![feature(panic_unwind)]
+#![cfg_attr(not(any(target_env = "msvc",
+                    all(windows, target_arch = "x86_64", target_env = "gnu"))),
+            feature(panic_unwind))]
 #![feature(raw)]
 #![feature(staged_api)]
 #![feature(unwind_attributes)]
@@ -45,6 +47,7 @@
 
 extern crate alloc;
 extern crate libc;
+#[cfg(not(any(target_env = "msvc", all(windows, target_arch = "x86_64", target_env = "gnu"))))]
 extern crate unwind;
 
 use core::intrinsics;

src/libpanic_unwind/lib.rs

@@ -522,6 +522,8 @@ define_dep_nodes!( <'tcx>
     [] DylibDepFormats(DefId),
     [] IsAllocator(DefId),
     [] IsPanicRuntime(DefId),
+    [] IsCompilerBuiltins(DefId),
+    [] HasGlobalAllocator(DefId),
     [] ExternCrate(DefId),
     [] LintLevels,
 );

src/librustc/dep_graph/dep_node.rs

@@ -26,7 +26,7 @@
 #![feature(core_intrinsics)]
 #![feature(discriminant_value)]
 #![feature(i128_type)]
-#![feature(libc)]
+#![cfg_attr(windows, feature(libc))]
 #![feature(never_type)]
 #![feature(nonzero)]
 #![feature(quote)]
@@ -45,6 +45,7 @@ extern crate core;
 extern crate fmt_macros;
 extern crate getopts;
 extern crate graphviz;
+#[cfg(windows)]
 extern crate libc;
 extern crate owning_ref;
 extern crate rustc_back;
@@ -62,7 +63,9 @@ extern crate serialize as rustc_serialize; // used by deriving
 
 // Note that librustc doesn't actually depend on these crates, see the note in
 // `Cargo.toml` for this crate about why these are here.
+#[allow(unused_extern_crates)]
 extern crate flate2;
+#[allow(unused_extern_crates)]
 extern crate test;
 
 #[macro_use]

src/librustc/lib.rs

@@ -30,7 +30,7 @@ declare_lint! {
 
 declare_lint! {
     pub UNUSED_EXTERN_CRATES,
-    Allow,
+    Warn,
     "extern crates that are never used"
 }
 

src/librustc/lint/builtin.rs

@@ -797,6 +797,8 @@ pub struct GlobalCtxt<'tcx> {
 
     pub maybe_unused_trait_imports: NodeSet,
 
+    pub maybe_unused_extern_crates: Vec<(NodeId, Span)>,
+
     // Internal cache for metadata decoding. No need to track deps on this.
     pub rcache: RefCell<FxHashMap<ty::CReaderCacheKey, Ty<'tcx>>>,
 
@@ -1038,6 +1040,7 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
             mir_passes,
             freevars: RefCell::new(resolutions.freevars),
             maybe_unused_trait_imports: resolutions.maybe_unused_trait_imports,
+            maybe_unused_extern_crates: resolutions.maybe_unused_extern_crates,
             rcache: RefCell::new(FxHashMap()),
             normalized_cache: RefCell::new(FxHashMap()),
             inhabitedness_cache: RefCell::new(FxHashMap()),

src/librustc/ty/context.rs

@@ -516,6 +516,18 @@ impl<'tcx> QueryDescription for queries::is_panic_runtime<'tcx> {
     }
 }
 
+impl<'tcx> QueryDescription for queries::is_compiler_builtins<'tcx> {
+    fn describe(_: TyCtxt, _: DefId) -> String {
+        "checking if the crate is_compiler_builtins".to_string()
+    }
+}
+
+impl<'tcx> QueryDescription for queries::has_global_allocator<'tcx> {
+    fn describe(_: TyCtxt, _: DefId) -> String {
+        "checking if the crate has_global_allocator".to_string()
+    }
+}
+
 impl<'tcx> QueryDescription for queries::extern_crate<'tcx> {
     fn describe(_: TyCtxt, _: DefId) -> String {
         "getting crate's ExternCrateData".to_string()
@@ -1079,6 +1091,8 @@ define_maps! { <'tcx>
 
     [] is_allocator: IsAllocator(DefId) -> bool,
     [] is_panic_runtime: IsPanicRuntime(DefId) -> bool,
+    [] is_compiler_builtins: IsCompilerBuiltins(DefId) -> bool,
+    [] has_global_allocator: HasGlobalAllocator(DefId) -> bool,
 
     [] extern_crate: ExternCrate(DefId) -> Rc<Option<ExternCrate>>,