Add --print host-triple

People often parse `-vV` output to get to the host triple, which is
annoying to do. It's easier to just get it directly.

compiler/rustc_driver_impl/src/lib.rs

@@ -731,6 +731,7 @@ fn print_crate_info(
                 targets.sort_unstable();
                 println_info!("{}", targets.join("\n"));
             }
+            HostTuple => println_info!("{}", rustc_session::config::host_tuple()),
             Sysroot => println_info!("{}", sess.sysroot.display()),
             TargetLibdir => println_info!("{}", sess.target_tlib_path.dir.display()),
             TargetSpec => {

compiler/rustc_session/src/config.rs

@@ -813,6 +813,7 @@ pub struct PrintRequest {
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 pub enum PrintKind {
     FileNames,
+    HostTuple,
     Sysroot,
     TargetLibdir,
     CrateName,
@@ -1945,6 +1946,7 @@ fn collect_print_requests(
         ("crate-name", PrintKind::CrateName),
         ("deployment-target", PrintKind::DeploymentTarget),
         ("file-names", PrintKind::FileNames),
+        ("host-tuple", PrintKind::HostTuple),
         ("link-args", PrintKind::LinkArgs),
         ("native-static-libs", PrintKind::NativeStaticLibs),
         ("relocation-models", PrintKind::RelocationModels),

src/doc/rustc/src/command-line-arguments.md

@@ -256,7 +256,8 @@ The valid types of print values are:
 - `crate-name` — The name of the crate.
 - `file-names` — The names of the files created by the `link` emit kind.
 - `sysroot` — Path to the sysroot.
-- `target-libdir` - Path to the target libdir.
+- `target-libdir` — Path to the target libdir.
+- `host-tuple` — The target-tuple string of the host compiler (e.g. `x86_64-unknown-linux-gnu`)
 - `cfg` — List of cfg values. See [conditional compilation] for more
   information about cfg values.
 - `target-list` — List of known targets. The target may be selected with the
@@ -286,7 +287,7 @@ The valid types of print values are:
   exact format of this debugging output is not a stable guarantee, other than
   that it will include the linker executable and the text of each command-line
   argument passed to the linker.
-- `deployment-target` - The currently selected [deployment target] (or minimum OS version)
+- `deployment-target` — The currently selected [deployment target] (or minimum OS version)
   for the selected Apple platform target. This value can be used or passed along to other
   components alongside a Rust build that need this information, such as C compilers.
   This returns rustc's minimum supported deployment target if no `*_DEPLOYMENT_TARGET` variable

tests/ui/invalid-compile-flags/print.stderr

@@ -1,4 +1,4 @@
 error: unknown print request: `yyyy`
   |
-  = help: valid print requests are: `all-target-specs-json`, `calling-conventions`, `cfg`, `check-cfg`, `code-models`, `crate-name`, `deployment-target`, `file-names`, `link-args`, `native-static-libs`, `relocation-models`, `split-debuginfo`, `stack-protector-strategies`, `sysroot`, `target-cpus`, `target-features`, `target-libdir`, `target-list`, `target-spec-json`, `tls-models`
+  = help: valid print requests are: `all-target-specs-json`, `calling-conventions`, `cfg`, `check-cfg`, `code-models`, `crate-name`, `deployment-target`, `file-names`, `host-tuple`, `link-args`, `native-static-libs`, `relocation-models`, `split-debuginfo`, `stack-protector-strategies`, `sysroot`, `target-cpus`, `target-features`, `target-libdir`, `target-list`, `target-spec-json`, `tls-models`
 

tests/ui/print-request/host.rs

@@ -0,0 +1,5 @@
+//@ only-x86_64-unknown-linux-gnu
+//@ compile-flags: --print host-tuple
+//@ check-pass
+
+fn main() {}

tests/ui/print-request/host.stdout

@@ -0,0 +1 @@
+x86_64-unknown-linux-gnu