How can I query rustc for the host triple?

Question

When working with gcc, I can get what gcc thinks is my host's triplet by running gcc -dumpmachine. On my current system, this gives me x86_64-linux-gnu.

How can I get stable rustc to print my host triple? (x86_64-unknown-linux-gnu in this case)

rustc's documentation doesn't seem to contain anything relevant besides the --print and --version. Neither seem to produce the host target triple.

Clarification: with two answers being given about nightly so far, I want to stress that this question is specifically about stable rustc compilers.

Answer 1

With Rust nightly, you can print the "target spec JSON":

$ rustc +nightly -Z unstable-options --print target-spec-json
{
  "arch": "x86_64",
  "cpu": "x86-64",
  "data-layout": "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128",
  "dynamic-linking": true,
  "env": "gnu",
  "executables": true,
  "has-elf-tls": true,
  "has-rpath": true,
  "is-builtin": true,
  "linker-flavor": "gcc",
  "linker-is-gnu": true,
  "llvm-target": "x86_64-unknown-linux-gnu",
  "max-atomic-width": 64,
  "os": "linux",
  "position-independent-executables": true,
  "pre-link-args": {
    "gcc": [
      "-Wl,--as-needed",
      "-Wl,-z,noexecstack",
      "-m64"
    ]
  },
  "relro-level": "full",
  "stack-probes": true,
  "target-c-int-width": "32",
  "target-endian": "little",
  "target-family": "unix",
  "target-pointer-width": "64",
  "vendor": "unknown"
}

To parse the target triple from this on the command line, you could use a tool like jq:

$ rustc +nightly -Z unstable-options --print target-spec-json | jq -r '."llvm-target"'
x86_64-unknown-linux-gnu

This isn't stable yet (and therefore requires the -Z unstable-options compiler option), but it probably will be in the future. The feature was added in #38061.

Answer 2

rustc --version --verbose

Will give you some output like:

rustc 1.35.0-nightly (474e7a648 2019-04-07)
binary: rustc
commit-hash: 474e7a6486758ea6fc761893b1a49cd9076fb0ab
commit-date: 2019-04-07
host: x86_64-unknown-linux-gnu
release: 1.35.0-nightly
LLVM version: 8.0

Where host is your target triple.