Support linking from a .rlink file

Flag `-Z no-link` was previously introduced, which allows creating
an `.rlink` file to perform compilation without linking.
This change enables linking from an `.rlink` file.

Cargo.lock

@@ -3541,6 +3541,7 @@ dependencies = [
  "log",
  "rustc",
  "rustc_ast_pretty",
+ "rustc_codegen_ssa",
  "rustc_codegen_utils",
  "rustc_data_structures",
  "rustc_error_codes",

src/librustc_codegen_llvm/lib.rs

@@ -39,7 +39,7 @@ use syntax::expand::allocator::AllocatorKind;
 
 use rustc::dep_graph::DepGraph;
 use rustc::middle::cstore::{EncodedMetadata, MetadataLoaderDyn};
-use rustc::session::config::{OptLevel, OutputFilenames, PrintRequest};
+use rustc::session::config::{self, OptLevel, OutputFilenames, PrintRequest};
 use rustc::session::Session;
 use rustc::ty::{self, TyCtxt};
 use rustc::util::common::ErrorReported;
@@ -308,7 +308,7 @@ impl CodegenBackend for LlvmCodegenBackend {
             let rlink_data = json::encode(&codegen_results).map_err(|err| {
                 sess.fatal(&format!("failed to encode rlink: {}", err));
             })?;
-            let rlink_file = outputs.with_extension("rlink");
+            let rlink_file = outputs.with_extension(config::RLINK_EXT);
             fs::write(&rlink_file, rlink_data).map_err(|err| {
                 sess.fatal(&format!("failed to write file {}: {}", rlink_file.display(), err));
             })?;

src/librustc_driver/Cargo.toml

@@ -26,6 +26,7 @@ rustc_mir = { path = "../librustc_mir" }
 rustc_parse = { path = "../librustc_parse" }
 rustc_plugin_impl = { path = "../librustc_plugin_impl" }
 rustc_save_analysis = { path = "../librustc_save_analysis" }
+rustc_codegen_ssa = { path = "../librustc_codegen_ssa" }
 rustc_codegen_utils = { path = "../librustc_codegen_utils" }
 rustc_error_codes = { path = "../librustc_error_codes" }
 rustc_interface = { path = "../librustc_interface" }

src/librustc_driver/lib.rs

@@ -31,25 +31,27 @@ use rustc::session::{config, DiagnosticOutput, Session};
 use rustc::session::{early_error, early_warn};
 use rustc::ty::TyCtxt;
 use rustc::util::common::ErrorReported;
+use rustc_codegen_ssa::CodegenResults;
 use rustc_codegen_utils::codegen_backend::CodegenBackend;
 use rustc_data_structures::profiling::print_time_passes_entry;
 use rustc_data_structures::sync::SeqCst;
 use rustc_errors::{registry::Registry, PResult};
 use rustc_feature::{find_gated_cfg, UnstableFeatures};
 use rustc_hir::def_id::LOCAL_CRATE;
-use rustc_interface::util::get_builtin_codegen_backend;
+use rustc_interface::util::{collect_crate_types, get_builtin_codegen_backend};
 use rustc_interface::{interface, Queries};
 use rustc_lint::LintStore;
 use rustc_metadata::locator;
 use rustc_save_analysis as save;
 use rustc_save_analysis::DumpHandler;
-use rustc_serialize::json::ToJson;
+use rustc_serialize::json::{self, ToJson};
 
 use std::borrow::Cow;
 use std::cmp::max;
 use std::default::Default;
 use std::env;
 use std::ffi::OsString;
+use std::fs;
 use std::io::{self, Read, Write};
 use std::mem;
 use std::panic::{self, catch_unwind};
@@ -286,7 +288,8 @@ pub fn run_compiler(
                 &matches,
                 compiler.input(),
             )
-        });
+        })
+        .and_then(|| RustcDefaultCalls::try_process_rlink(sess, compiler));
 
         if should_stop == Compilation::Stop {
             return sess.compile_status();
@@ -593,6 +596,34 @@ fn show_content_with_pager(content: &String) {
 }
 
 impl RustcDefaultCalls {
+    fn process_rlink(sess: &Session, compiler: &interface::Compiler) -> Result<(), ErrorReported> {
+        if let Input::File(file) = compiler.input() {
+            // FIXME: #![crate_type] and #![crate_name] support not implemented yet
+            let attrs = vec![];
+            sess.crate_types.set(collect_crate_types(sess, &attrs));
+            let outputs = compiler.build_output_filenames(&sess, &attrs);
+            let rlink_data = fs::read_to_string(file).unwrap_or_else(|err| {
+                sess.fatal(&format!("failed to read rlink file: {}", err));
+            });
+            let codegen_results: CodegenResults = json::decode(&rlink_data).unwrap_or_else(|err| {
+                sess.fatal(&format!("failed to decode rlink: {}", err));
+            });
+            compiler.codegen_backend().link(&sess, Box::new(codegen_results), &outputs)
+        } else {
+            sess.fatal(&format!("rlink must be a file"))
+        }
+    }
+
+    pub fn try_process_rlink(sess: &Session, compiler: &interface::Compiler) -> Compilation {
+        if sess.opts.debugging_opts.link_only {
+            let result = RustcDefaultCalls::process_rlink(sess, compiler);
+            abort_on_err(result, sess);
+            Compilation::Stop
+        } else {
+            Compilation::Continue
+        }
+    }
+
     pub fn list_metadata(
         sess: &Session,
         metadata_loader: &dyn MetadataLoader,
@@ -668,7 +699,7 @@ impl RustcDefaultCalls {
                         println!("{}", id);
                         continue;
                     }
-                    let crate_types = rustc_interface::util::collect_crate_types(sess, attrs);
+                    let crate_types = collect_crate_types(sess, attrs);
                     for &style in &crate_types {
                         let fname = rustc_codegen_utils::link::filename_for_input(
                             sess, style, &id, &t_outputs,

src/librustc_interface/interface.rs

@@ -2,7 +2,7 @@ pub use crate::passes::BoxedResolver;
 use crate::util;
 
 use rustc::lint;
-use rustc::session::config::{self, ErrorOutputType, Input};
+use rustc::session::config::{self, ErrorOutputType, Input, OutputFilenames};
 use rustc::session::early_error;
 use rustc::session::{DiagnosticOutput, Session};
 use rustc::ty;
@@ -20,7 +20,7 @@ use rustc_span::source_map::{FileLoader, FileName, SourceMap};
 use std::path::PathBuf;
 use std::result;
 use std::sync::{Arc, Mutex};
-use syntax::ast::MetaItemKind;
+use syntax::ast::{self, MetaItemKind};
 use syntax::token;
 
 pub type Result<T> = result::Result<T, ErrorReported>;
@@ -61,6 +61,19 @@ impl Compiler {
     pub fn output_file(&self) -> &Option<PathBuf> {
         &self.output_file
     }
+    pub fn build_output_filenames(
+        &self,
+        sess: &Session,
+        attrs: &[ast::Attribute],
+    ) -> OutputFilenames {
+        util::build_output_filenames(
+            &self.input,
+            &self.output_dir,
+            &self.output_file,
+            &attrs,
+            &sess,
+        )
+    }
 }
 
 /// Converts strings provided as `--cfg [cfgspec]` into a `crate_cfg`.

src/librustc_session/config.rs

@@ -467,6 +467,7 @@ pub struct OutputFilenames {
 
 impl_stable_hash_via_hash!(OutputFilenames);
 
+pub const RLINK_EXT: &str = "rlink";
 pub const RUST_CGU_EXT: &str = "rcgu";
 
 impl OutputFilenames {

src/librustc_session/options.rs

@@ -968,4 +968,6 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         "use Windows Control Flow Guard (`disabled`, `nochecks` or `checks`)"),
     no_link: bool = (false, parse_bool, [TRACKED],
         "compile without linking"),
+    link_only: bool = (false, parse_bool, [TRACKED],
+        "link the `.rlink` file generated by `-Z no-link`"),
 }