Retrieve and parse --print=check-cfg in target info

src/cargo/core/compiler/build_context/target_info.rs

@@ -14,7 +14,7 @@ use crate::util::context::{GlobalContext, StringList, TargetConfig};
 use crate::util::interning::InternedString;
 use crate::util::{CargoResult, Rustc};
 use anyhow::Context as _;
-use cargo_platform::{Cfg, CfgExpr};
+use cargo_platform::{Cfg, CfgExpr, CheckCfg};
 use cargo_util::{paths, ProcessBuilder};
 use serde::{Deserialize, Serialize};
 use std::cell::RefCell;
@@ -43,6 +43,8 @@ pub struct TargetInfo {
     crate_types: RefCell<HashMap<CrateType, Option<(String, String)>>>,
     /// `cfg` information extracted from `rustc --print=cfg`.
     cfg: Vec<Cfg>,
+    /// `CheckCfg` informations extracted from `rustc --print=check-cfg`.
+    check_cfg: Option<CheckCfg>,
     /// Supported values for `-Csplit-debuginfo=` flag, queried from rustc
     support_split_debuginfo: Vec<String>,
     /// Path to the sysroot.
@@ -204,6 +206,14 @@ impl TargetInfo {
             process.arg("--print=crate-name"); // `___` as a delimiter.
             process.arg("--print=cfg");
 
+            if gctx.cli_unstable().check_target_cfgs {
+                process.arg("--print=crate-name"); // `___` as a delimiter.
+                process.arg("--print=check-cfg");
+
+                process.arg("--check-cfg=cfg()"); // otherwise `--print=check-cfg` won't output
+                process.arg("-Zunstable-options"); // required by `--print=check-cfg`
+            }
+
             let (output, error) = rustc
                 .cached_output(&process, extra_fingerprint)
                 .with_context(|| {
@@ -256,6 +266,11 @@ impl TargetInfo {
             let cfg = {
                 let mut res = Vec::new();
                 for line in &mut lines {
+                    // HACK: abuse `--print=crate-name` to use `___` as a delimiter.
+                    if line == "___" {
+                        break;
+                    }
+
                     let cfg = Cfg::from_str(line).with_context(|| {
                         format!(
                             "failed to parse the cfg from `rustc --print=cfg`, got:\n{}",
@@ -269,6 +284,22 @@ impl TargetInfo {
                 res
             };
 
+            let check_cfg = if gctx.cli_unstable().check_target_cfgs {
+                let mut check_cfg = CheckCfg::default();
+                check_cfg.exhaustive = true;
+
+                for line in lines {
+                    check_cfg
+                        .parse_print_check_cfg_line(line)
+                        .with_context(|| {
+                            format!("unable to parse a line from `--print=check-cfg`")
+                        })?;
+                }
+                Some(check_cfg)
+            } else {
+                None
+            };
+
             // recalculate `rustflags` from above now that we have `cfg`
             // information
             let new_flags = extra_args(
@@ -315,6 +346,7 @@ impl TargetInfo {
                 )?
                 .into(),
                 cfg,
+                check_cfg,
                 support_split_debuginfo,
             });
         }
@@ -337,6 +369,11 @@ impl TargetInfo {
         &self.cfg
     }
 
+    /// The [`CheckCfg`] settings.
+    pub fn check_cfg(&self) -> &Option<CheckCfg> {
+        &self.check_cfg
+    }
+
     /// Returns the list of file types generated by the given crate type.
     ///
     /// Returns `None` if the target does not support the given crate type.