make it possible to silence linker warnings with a crate-level attribute

this was slightly complicated because codegen_ssa doesn't have access to a tcx.

compiler/rustc_codegen_ssa/src/lib.rs

@@ -29,18 +29,23 @@ use rustc_ast as ast;
 use rustc_data_structures::fx::{FxHashSet, FxIndexMap};
 use rustc_data_structures::sync::Lrc;
 use rustc_data_structures::unord::UnordMap;
+use rustc_hir::CRATE_HIR_ID;
 use rustc_hir::def_id::CrateNum;
 use rustc_macros::{Decodable, Encodable, HashStable};
 use rustc_middle::dep_graph::WorkProduct;
+use rustc_middle::lint::LintLevelSource;
 use rustc_middle::middle::debugger_visualizer::DebuggerVisualizerFile;
 use rustc_middle::middle::dependency_format::Dependencies;
 use rustc_middle::middle::exported_symbols::SymbolExportKind;
+use rustc_middle::ty::TyCtxt;
 use rustc_middle::util::Providers;
 use rustc_serialize::opaque::{FileEncoder, MemDecoder};
 use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
 use rustc_session::Session;
 use rustc_session::config::{CrateType, OutputFilenames, OutputType, RUST_CGU_EXT};
 use rustc_session::cstore::{self, CrateSource};
+use rustc_session::lint::Level;
+use rustc_session::lint::builtin::LINKER_MESSAGES;
 use rustc_session::utils::NativeLibKind;
 use rustc_span::Symbol;
 
@@ -303,3 +308,19 @@ impl CodegenResults {
         Ok((codegen_results, outputs))
     }
 }
+
+/// A list of lint levels used in codegen.
+///
+/// When using `-Z link-only`, we don't have access to the tcx and must work
+/// solely from the `.rlink` file. `Lint`s are defined too early to be encodeable.
+/// Instead, encode exactly the information we need.
+#[derive(Copy, Clone, Debug, Encodable, Decodable)]
+pub struct CodegenLintLevels {
+    linker_messages: (Level, LintLevelSource),
+}
+
+impl CodegenLintLevels {
+    pub fn from_tcx(tcx: TyCtxt<'_>) -> Self {
+        Self { linker_messages: tcx.lint_level_at_node(LINKER_MESSAGES, CRATE_HIR_ID) }
+    }
+}