How can I find the current Rust compiler's default LLVM target triple?

Question

I want to override a build script, which means adding a configuration section that looks like this:

[target.x86_64-unknown-linux-gnu.foo]
rustc-link-search = ["/path/to/foo"]
rustc-link-lib = ["foo"]
root = "/path/to/foo"
key = "value"

But I'm using a Mac, so x86_64-unknown-linux-gnu isn't the correct target triple.

How do I discover which target triple rustc or cargo are currently using?

rustc --print cfg prints a list of values that don't seem to correspond to the triple (there's no unknown in there, in particular).

rustc --print target-list displays all available targets; I just want the default.

Answer 1

cargo uses rustc -vV to detect the default target triple (source). We can do the same thing:

use std::process::Command;

use anyhow::{format_err, Context, Result};
use std::str;

fn get_target() -> Result<String> {
    let output = Command::new("rustc")
        .arg("-vV")
        .output()
        .context("Failed to run rustc to get the host target")?;
    let output = str::from_utf8(&output.stdout).context("`rustc -vV` didn't return utf8 output")?;

    let field = "host: ";
    let host = output
        .lines()
        .find(|l| l.starts_with(field))
        .map(|l| &l[field.len()..])
        .ok_or_else(|| {
            format_err!(
                "`rustc -vV` didn't have a line for `{}`, got:\n{}",
                field.trim(),
                output
            )
        })?
        .to_string();
    Ok(host)
}

fn main() -> Result<()> {
    let host = get_target()?;
    println!("target triple: {}", host);
    Ok(())
}

Answer 2

rustc --print cfg will output something like the following:

$ rustc --print cfg
debug_assertions
target_arch="x86_64"
target_endian="little"
target_env="gnu"
target_family="unix"
target_feature="fxsr"
target_feature="sse"
target_feature="sse2"
target_os="linux"
target_pointer_width="64"
target_vendor="unknown"
unix

The target is <arch>-<vendor>-<os>-<env>.