Restrict sysroot crate imports to those defined in this repo.

It's common to import dependencies from the sysroot via `extern crate`
rather than use an explicit cargo dependency, when it's necessary to use
the same dependency version as used by rustc itself. However, this is
dangerous for crates.io crates, since rustc may not pull in the
dependency on some targets, or may pull in multiple versions. In both
cases, the `extern crate` fails to resolve.

To address this, re-export all such dependencies from the appropriate
`rustc_*` crates, and use this alias from crates which would otherwise
need to use `extern crate`.

compiler/rustc_codegen_gcc/src/back/lto.rs

@@ -30,6 +30,7 @@ use rustc_codegen_ssa::traits::*;
 use rustc_codegen_ssa::{ModuleCodegen, ModuleKind, looks_like_rust_object_file};
 use rustc_data_structures::memmap::Mmap;
 use rustc_errors::DiagCtxtHandle;
+use rustc_log::tracing::info;
 use rustc_middle::bug;
 use rustc_middle::dep_graph::WorkProduct;
 use rustc_session::config::Lto;

compiler/rustc_codegen_gcc/src/back/write.rs

@@ -5,6 +5,7 @@ use rustc_codegen_ssa::back::link::ensure_removed;
 use rustc_codegen_ssa::back::write::{BitcodeSection, CodegenContext, EmitObj, ModuleConfig};
 use rustc_codegen_ssa::{CompiledModule, ModuleCodegen};
 use rustc_fs_util::link_or_copy;
+use rustc_log::tracing::debug;
 use rustc_session::config::OutputType;
 use rustc_target::spec::SplitDebuginfo;
 

compiler/rustc_codegen_gcc/src/consts.rs

@@ -8,6 +8,7 @@ use rustc_codegen_ssa::traits::{
 use rustc_hir::attrs::Linkage;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::LOCAL_CRATE;
+use rustc_log::tracing::trace;
 use rustc_middle::middle::codegen_fn_attrs::{CodegenFnAttrFlags, CodegenFnAttrs};
 use rustc_middle::mir::interpret::{
     self, ConstAllocation, ErrorHandled, Scalar as InterpScalar, read_target_uint,

compiler/rustc_codegen_gcc/src/gcc_util.rs

@@ -1,8 +1,8 @@
 #[cfg(feature = "master")]
 use gccjit::Context;
 use rustc_codegen_ssa::target_features;
+use rustc_data_structures::smallvec::{SmallVec, smallvec};
 use rustc_session::Session;
-use smallvec::{SmallVec, smallvec};
 
 fn gcc_features_by_flags(sess: &Session, features: &mut Vec<String>) {
     target_features::retpoline_features_by_flags(sess, features);

compiler/rustc_codegen_gcc/src/lib.rs

@@ -25,12 +25,6 @@
 #![deny(clippy::pattern_type_mismatch)]
 #![allow(clippy::needless_lifetimes, clippy::uninlined_format_args)]
 
-// These crates are pulled from the sysroot because they are part of
-// rustc's public API, so we need to ensure version compatibility.
-extern crate smallvec;
-#[macro_use]
-extern crate tracing;
-
 // The rustc crates we need
 extern crate rustc_abi;
 extern crate rustc_apfloat;
@@ -44,6 +38,7 @@ extern crate rustc_hir;
 extern crate rustc_index;
 #[cfg(feature = "master")]
 extern crate rustc_interface;
+extern crate rustc_log;
 extern crate rustc_macros;
 extern crate rustc_middle;
 extern crate rustc_session;