crates that provide a panic_handler are exempt from unused_extern_crates

fixes the *first* false positive reported in #53964

src/librustc/dep_graph/dep_node.rs

@@ -574,6 +574,7 @@ define_dep_nodes!( <'tcx>
     [] IsPanicRuntime(CrateNum),
     [] IsCompilerBuiltins(CrateNum),
     [] HasGlobalAllocator(CrateNum),
+    [] HasPanicHandler(CrateNum),
     [input] ExternCrate(DefId),
     [eval_always] LintLevels,
     [] Specializes { impl1: DefId, impl2: DefId },

src/librustc/session/mod.rs

@@ -159,6 +159,9 @@ pub struct Session {
     /// Metadata about the allocators for the current crate being compiled
     pub has_global_allocator: Once<bool>,
 
+    /// Metadata about the panic handlers for the current crate being compiled
+    pub has_panic_handler: Once<bool>,
+
     /// Cap lint level specified by a driver specifically.
     pub driver_lint_caps: FxHashMap<lint::LintId, lint::Level>,
 }
@@ -1160,6 +1163,7 @@ pub fn build_session_(
             (*GLOBAL_JOBSERVER).clone()
         },
         has_global_allocator: Once::new(),
+        has_panic_handler: Once::new(),
         driver_lint_caps: FxHashMap(),
     };
 

src/librustc/ty/query/config.rs

@@ -464,6 +464,12 @@ impl<'tcx> QueryDescription<'tcx> for queries::has_global_allocator<'tcx> {
     }
 }
 
+impl<'tcx> QueryDescription<'tcx> for queries::has_panic_handler<'tcx> {
+    fn describe(_: TyCtxt, _: CrateNum) -> String {
+        "checking if the crate has_panic_handler".to_string()
+    }
+}
+
 impl<'tcx> QueryDescription<'tcx> for queries::extern_crate<'tcx> {
     fn describe(_: TyCtxt, _: DefId) -> String {
         "getting crate's ExternCrateData".to_string()

src/librustc/ty/query/mod.rs

@@ -381,6 +381,7 @@ define_queries! { <'tcx>
         [fatal_cycle] fn is_panic_runtime: IsPanicRuntime(CrateNum) -> bool,
         [fatal_cycle] fn is_compiler_builtins: IsCompilerBuiltins(CrateNum) -> bool,
         [fatal_cycle] fn has_global_allocator: HasGlobalAllocator(CrateNum) -> bool,
+        [fatal_cycle] fn has_panic_handler: HasPanicHandler(CrateNum) -> bool,
         [fatal_cycle] fn is_sanitizer_runtime: IsSanitizerRuntime(CrateNum) -> bool,
         [fatal_cycle] fn is_profiler_runtime: IsProfilerRuntime(CrateNum) -> bool,
         [fatal_cycle] fn panic_strategy: GetPanicStrategy(CrateNum) -> PanicStrategy,

src/librustc/ty/query/plumbing.rs

@@ -1168,6 +1168,7 @@ pub fn force_from_dep_node<'a, 'gcx, 'lcx>(tcx: TyCtxt<'a, 'gcx, 'lcx>,
         DepKind::IsPanicRuntime => { force!(is_panic_runtime, krate!()); }
         DepKind::IsCompilerBuiltins => { force!(is_compiler_builtins, krate!()); }
         DepKind::HasGlobalAllocator => { force!(has_global_allocator, krate!()); }
+        DepKind::HasPanicHandler => { force!(has_panic_handler, krate!()); }
         DepKind::ExternCrate => { force!(extern_crate, def_id!()); }
         DepKind::LintLevels => { force!(lint_levels, LOCAL_CRATE); }
         DepKind::InScopeTraits => { force!(in_scope_traits_map, def_id!().index); }

src/librustc_metadata/cstore_impl.rs

@@ -173,6 +173,7 @@ provide! { <'tcx> tcx, def_id, other, cdata,
     is_panic_runtime => { cdata.root.panic_runtime }
     is_compiler_builtins => { cdata.root.compiler_builtins }
     has_global_allocator => { cdata.root.has_global_allocator }
+    has_panic_handler => { cdata.root.has_panic_handler }
     is_sanitizer_runtime => { cdata.root.sanitizer_runtime }
     is_profiler_runtime => { cdata.root.profiler_runtime }
     panic_strategy => { cdata.root.panic_strategy }

src/librustc_metadata/encoder.rs

@@ -484,6 +484,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
         let is_proc_macro = tcx.sess.crate_types.borrow().contains(&CrateType::ProcMacro);
         let has_default_lib_allocator = attr::contains_name(&attrs, "default_lib_allocator");
         let has_global_allocator = *tcx.sess.has_global_allocator.get();
+        let has_panic_handler = *tcx.sess.has_panic_handler.try_get().unwrap_or(&false);
 
         let root = self.lazy(&CrateRoot {
             name: tcx.crate_name(LOCAL_CRATE),
@@ -494,6 +495,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
             panic_strategy: tcx.sess.panic_strategy(),
             edition: hygiene::default_edition(),
             has_global_allocator: has_global_allocator,
+            has_panic_handler: has_panic_handler,
             has_default_lib_allocator: has_default_lib_allocator,
             plugin_registrar_fn: tcx.sess
                 .plugin_registrar_fn

src/librustc_metadata/schema.rs

@@ -193,6 +193,7 @@ pub struct CrateRoot {
     pub panic_strategy: PanicStrategy,
     pub edition: Edition,
     pub has_global_allocator: bool,
+    pub has_panic_handler: bool,
     pub has_default_lib_allocator: bool,
     pub plugin_registrar_fn: Option<DefIndex>,
     pub macro_derive_registrar: Option<DefIndex>,

src/librustc_typeck/check/mod.rs

@@ -1138,6 +1138,11 @@ fn check_fn<'a, 'gcx, 'tcx>(inherited: &'a Inherited<'a, 'gcx, 'tcx>,
     if let Some(panic_impl_did) = fcx.tcx.lang_items().panic_impl() {
         if panic_impl_did == fcx.tcx.hir.local_def_id(fn_id) {
             if let Some(panic_info_did) = fcx.tcx.lang_items().panic_info() {
+                // at this point we don't care if there are duplicate handlers or if the handler has
+                // the wrong signature as this value we'll be used when writing metadata and that
+                // only happens if compilation succeeded
+                fcx.tcx.sess.has_panic_handler.try_set_same(true);
+
                 if declared_ret_ty.sty != ty::Never {
                     fcx.tcx.sess.span_err(
                         decl.output.span(),

src/librustc_typeck/check_unused.rs

@@ -117,6 +117,7 @@ fn unused_crates_lint<'tcx>(tcx: TyCtxt<'_, 'tcx, 'tcx>) {
             !tcx.is_compiler_builtins(cnum)
                 && !tcx.is_panic_runtime(cnum)
                 && !tcx.has_global_allocator(cnum)
+                && !tcx.has_panic_handler(cnum)
         })
         .cloned()
         .collect();

src/test/run-make-fulldeps/issue-53964/Makefile

@@ -0,0 +1,5 @@
+-include ../tools.mk
+
+all:
+	$(RUSTC) panic.rs
+	$(RUSTC) -C panic=abort --emit=obj app.rs -L $(TMPDIR)

src/test/run-make-fulldeps/issue-53964/app.rs

@@ -0,0 +1,8 @@
+#![crate_type = "bin"]
+#![no_main]
+#![no_std]
+
+#![deny(unused_extern_crates)]
+
+// `panic` provides a `panic_handler` so it shouldn't trip the `unused_extern_crates` lint
+extern crate panic;

src/test/run-make-fulldeps/issue-53964/panic.rs

@@ -0,0 +1,20 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![crate_type = "lib"]
+#![feature(panic_handler)]
+#![no_std]
+
+use core::panic::PanicInfo;
+
+#[panic_handler]
+fn panic(_: &PanicInfo) -> ! {
+    loop {}
+}