Add link-flags-ld emit switch.

Generate an unix-style ldflags line when building with
--crate-type staticlib --emit link-flags-ld.

If a filename is given, write the link line there for
later use by an external build system in linking the
static library. If none is given, just modify the
normal informative message to print the flags line
instead of the library list.

Issue rust-lang#31471.

Author: rillian

src/librustc/session/config.rs

@@ -69,6 +69,7 @@ pub enum OutputType {
     Object,
     Exe,
     DepInfo,
+    LinkFlagsLd,
 }
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
@@ -87,7 +88,8 @@ impl OutputType {
     fn is_compatible_with_codegen_units_and_single_output_file(&self) -> bool {
         match *self {
             OutputType::Exe |
-            OutputType::DepInfo => true,
+            OutputType::DepInfo |
+            OutputType::LinkFlagsLd => true,
             OutputType::Bitcode |
             OutputType::Assembly |
             OutputType::LlvmAssembly |
@@ -103,6 +105,7 @@ impl OutputType {
             OutputType::Object => "obj",
             OutputType::Exe => "link",
             OutputType::DepInfo => "dep-info",
+            OutputType::LinkFlagsLd => "link-flags-ld",
         }
     }
 }
@@ -210,6 +213,7 @@ impl OutputFilenames {
             OutputType::LlvmAssembly => base.with_extension("ll"),
             OutputType::Object => base.with_extension("o"),
             OutputType::DepInfo => base.with_extension("d"),
+            OutputType::LinkFlagsLd => base.with_extension("ldflags"),
             OutputType::Exe => base,
         }
     }
@@ -884,7 +888,7 @@ pub fn rustc_short_optgroups() -> Vec<RustcOptGroup> {
                "NAME"),
         opt::multi_s("", "emit", "Comma separated list of types of output for \
                               the compiler to emit",
-                 "[asm|llvm-bc|llvm-ir|obj|link|dep-info]"),
+                 "[asm|llvm-bc|llvm-ir|obj|link|link-flags-ld|dep-info]"),
         opt::multi_s("", "print", "Comma separated list of compiler information to \
                                print on stdout",
                  "[crate-name|file-names|sysroot|target-list]"),
@@ -1059,6 +1063,7 @@ pub fn build_session_options(matches: &getopts::Matches) -> Options {
                     "llvm-bc" => OutputType::Bitcode,
                     "obj" => OutputType::Object,
                     "link" => OutputType::Exe,
+                    "link-flags-ld" => OutputType::LinkFlagsLd,
                     "dep-info" => OutputType::DepInfo,
                     part => {
                         early_error(error_format, &format!("unknown emission type: `{}`",

src/librustc_trans/back/link.rs

@@ -830,6 +830,41 @@ fn report_link_line(sess: &Session, native_libs: Vec<(NativeLibraryKind, String)
         return;
     }
 
+    // Write out link flags to a file if requested.
+    match sess.opts.output_types.get(&OutputType::LinkFlagsLd) {
+        Some(path) => {
+            let mut ldflags = String::new();
+            for &(kind, ref lib) in &native_libs {
+                let prefix = match kind {
+                    NativeLibraryKind::NativeStatic => "-l",
+                    NativeLibraryKind::NativeUnknown => "-l",
+                    NativeLibraryKind::NativeFramework => "-f ",
+                };
+                ldflags.push_str(&format!(" {}{}", prefix, *lib));
+            }
+            ldflags.push('\n');
+            match *path {
+                Some(ref path) => {
+                    match fs::File::create(&path).and_then(|mut f| {
+                        f.write_all(ldflags.trim_left().as_bytes())
+                    }) {
+                        Ok(..) => {}
+                        Err(e) => sess.fatal(
+                            &format!("failed to write {}: {}",
+                                     path.display(), e))
+                    }
+                },
+                None => sess.note_without_error(
+                    &format!("ldflags: {}", ldflags.trim()))
+            };
+            return;
+        },
+        None => {
+            // Link flag output not requested, continue.
+        },
+    };
+
+    // Otherwise, warn about needed link lines in the build output.
     sess.note_without_error(
         "link against the following native artifacts when linking against \
                              this static library");

src/librustc_trans/back/write.rs

@@ -676,6 +676,7 @@ pub fn run_passes(sess: &Session,
                 modules_config.emit_obj = true;
                 metadata_config.emit_obj = true;
             },
+            OutputType::LinkFlagsLd |
             OutputType::DepInfo => {}
         }
     }
@@ -780,6 +781,7 @@ pub fn run_passes(sess: &Session,
                 copy_if_one_unit("0.o", OutputType::Object, true);
             }
             OutputType::Exe |
+            OutputType::LinkFlagsLd |
             OutputType::DepInfo => {}
         }
     }