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configure: Use CARGO_CFG_*_{F16,F128} rather than invoking rustc

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Currently we run the `rustc` from the `RUSTC` environment variable to
figure out whether or not to enable `f16` and `f128`, based on the
`target_has_reliable_{f16,f128}` config. However, this does not know
about the codegen backend used, and the backend isn't trivial to check
in a build script (usually it gets set via `RUSTFLAGS`).

It turns out we don't actually need to run `rustc` here: Cargo
unconditionally emits all config from the relevant compiler as
`CARGO_CFG_*` variables, regardless of whether or not they are known
options. Switch to checking these for setting config rather than
invoking `rustc`.

As an added advantage, this will work with target.json files without any
special handling.

Fixes: ed17b95715dd ("Use the compiler to determine whether or not to enable `f16` and `f128`")
This commit is contained in:
Trevor Gross 2025-08-05 20:56:27 +00:00
parent ecf6d3c6ce
commit fbc700f92b
2 changed files with 10 additions and 47 deletions
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27
library/compiler-builtins/compiler-builtins/configure.rs
View file

@ -1,6 +1,5 @@
// Configuration that is shared between `compiler_builtins` and `builtins_test`.
use std::process::{Command, Stdio};
use std::{env, str};
#[derive(Debug)]
@ -35,26 +34,6 @@ impl Target {
.map(|s| s.to_lowercase().replace("_", "-"))
.collect();
// Query rustc for options that Cargo does not provide env for. The bootstrap hack is used
// to get consistent output regardless of channel (`f16`/`f128` config options are hidden
// on stable otherwise).
let mut cmd = Command::new(env::var("RUSTC").unwrap());
cmd.args(["--print=cfg", "--target", &triple])
.env("RUSTC_BOOTSTRAP", "1")
.stderr(Stdio::inherit());
let out = cmd
.output()
.unwrap_or_else(|e| panic!("failed to run `{cmd:?}`: {e}"));
let rustc_cfg = str::from_utf8(&out.stdout).unwrap();
// If we couldn't query `rustc` (e.g. a custom JSON target was used), make the safe
// choice and leave `f16` and `f128` disabled.
let rustc_output_ok = out.status.success();
let reliable_f128 =
rustc_output_ok && rustc_cfg.lines().any(|l| l == "target_has_reliable_f128");
let reliable_f16 =
rustc_output_ok && rustc_cfg.lines().any(|l| l == "target_has_reliable_f16");
Self {
triple,
triple_split,
@ -74,8 +53,10 @@ impl Target {
.split(",")
.map(ToOwned::to_owned)
.collect(),
reliable_f128,
reliable_f16,
// Note that these are unstable options, so only show up with the nightly compiler or
// with `RUSTC_BOOTSTRAP=1` (which is required to use the types anyway).
reliable_f128: env::var_os("CARGO_CFG_TARGET_HAS_RELIABLE_F128").is_some(),
reliable_f16: env::var_os("CARGO_CFG_TARGET_HAS_RELIABLE_F16").is_some(),
}
}

30
library/compiler-builtins/libm/configure.rs
View file

@ -1,9 +1,9 @@
// Configuration shared with both libm and libm-test
use std::env;
use std::path::PathBuf;
use std::process::{Command, Stdio};
use std::{env, str};
#[derive(Debug)]
#[allow(dead_code)]
pub struct Config {
pub manifest_dir: PathBuf,
@ -33,26 +33,6 @@ impl Config {
.map(|s| s.to_lowercase().replace("_", "-"))
.collect();
// Query rustc for options that Cargo does not provide env for. The bootstrap hack is used
// to get consistent output regardless of channel (`f16`/`f128` config options are hidden
// on stable otherwise).
let mut cmd = Command::new(env::var("RUSTC").unwrap());
cmd.args(["--print=cfg", "--target", &target_triple])
.env("RUSTC_BOOTSTRAP", "1")
.stderr(Stdio::inherit());
let out = cmd
.output()
.unwrap_or_else(|e| panic!("failed to run `{cmd:?}`: {e}"));
let rustc_cfg = str::from_utf8(&out.stdout).unwrap();
// If we couldn't query `rustc` (e.g. a custom JSON target was used), make the safe
// choice and leave `f16` and `f128` disabled.
let rustc_output_ok = out.status.success();
let reliable_f128 =
rustc_output_ok && rustc_cfg.lines().any(|l| l == "target_has_reliable_f128");
let reliable_f16 =
rustc_output_ok && rustc_cfg.lines().any(|l| l == "target_has_reliable_f16");
Self {
target_triple,
manifest_dir: PathBuf::from(env::var("CARGO_MANIFEST_DIR").unwrap()),
@ -66,8 +46,10 @@ impl Config {
target_string: env::var("TARGET").unwrap(),
target_vendor: env::var("CARGO_CFG_TARGET_VENDOR").unwrap(),
target_features,
reliable_f128,
reliable_f16,
// Note that these are unstable options, so only show up with the nightly compiler or
// with `RUSTC_BOOTSTRAP=1` (which is required to use the types anyway).
reliable_f128: env::var_os("CARGO_CFG_TARGET_HAS_RELIABLE_F128").is_some(),
reliable_f16: env::var_os("CARGO_CFG_TARGET_HAS_RELIABLE_F16").is_some(),
}
}
}