print option to dump target spec as JSON

src/librustc/session/config.rs

@@ -308,6 +308,7 @@ pub enum PrintRequest {
     TargetFeatures,
     RelocationModels,
     CodeModels,
+    TargetSpec,
 }
 
 pub enum Input {
@@ -1138,6 +1139,13 @@ mod opt {
 /// including metadata for each option, such as whether the option is
 /// part of the stable long-term interface for rustc.
 pub fn rustc_short_optgroups() -> Vec<RustcOptGroup> {
+    let mut print_opts = vec!["crate-name", "file-names", "sysroot", "cfg",
+                              "target-list", "target-cpus", "target-features",
+                              "relocation-models", "code-models"];
+    if nightly_options::is_nightly_build() {
+        print_opts.push("target-spec-json");
+    }
+
     vec![
         opt::flag_s("h", "help", "Display this message"),
         opt::multi_s("", "cfg", "Configure the compilation environment", "SPEC"),
@@ -1157,9 +1165,7 @@ pub fn rustc_short_optgroups() -> Vec<RustcOptGroup> {
                               the compiler to emit",
                  "[asm|llvm-bc|llvm-ir|obj|link|dep-info]"),
         opt::multi_s("", "print", "Comma separated list of compiler information to \
-                               print on stdout",
-                 "[crate-name|file-names|sysroot|cfg|target-list|target-cpus|\
-                   target-features|relocation-models|code-models]"),
+                               print on stdout", &print_opts.join("|")),
         opt::flagmulti_s("g",  "",  "Equivalent to -C debuginfo=2"),
         opt::flagmulti_s("O", "", "Equivalent to -C opt-level=2"),
         opt::opt_s("o", "", "Write output to <filename>", "FILENAME"),
@@ -1469,6 +1475,8 @@ pub fn build_session_options_and_crate_config(matches: &getopts::Matches)
             "target-features" => PrintRequest::TargetFeatures,
             "relocation-models" => PrintRequest::RelocationModels,
             "code-models" => PrintRequest::CodeModels,
+            "target-spec-json" if nightly_options::is_unstable_enabled(matches)
+                => PrintRequest::TargetSpec,
             req => {
                 early_error(error_format, &format!("unknown print request `{}`", req))
             }

src/librustc_driver/lib.rs

@@ -80,6 +80,8 @@ use rustc_metadata::locator;
 use rustc_metadata::cstore::CStore;
 use rustc::util::common::time;
 
+use serialize::json::ToJson;
+
 use std::cmp::max;
 use std::cmp::Ordering::Equal;
 use std::default::Default;
@@ -584,6 +586,7 @@ impl RustcDefaultCalls {
                     println!("{}", targets.join("\n"));
                 },
                 PrintRequest::Sysroot => println!("{}", sess.sysroot().display()),
+                PrintRequest::TargetSpec => println!("{}", sess.target.target.to_json().pretty()),
                 PrintRequest::FileNames |
                 PrintRequest::CrateName => {
                     let input = match input {

src/test/run-make/target-specs/Makefile

@@ -6,3 +6,4 @@ all:
 	$(RUSTC) foo.rs --target=my-incomplete-platform.json 2>&1 | grep 'Field llvm-target'
 	RUST_TARGET_PATH=. $(RUSTC) foo.rs --target=my-awesome-platform --crate-type=lib --emit=asm
 	RUST_TARGET_PATH=. $(RUSTC) foo.rs --target=x86_64-unknown-linux-gnu --crate-type=lib --emit=asm
+	$(RUSTC) -Z unstable-options --target=my-awesome-platform.json --print target-spec-json > $(TMPDIR)/test-platform.json && $(RUSTC) -Z unstable-options --target=$(TMPDIR)/test-platform.json --print target-spec-json | diff -q $(TMPDIR)/test-platform.json -