Add unstable rustc-check-cfg build script output

src/cargo/core/compiler/context/mod.rs

@@ -235,6 +235,14 @@ impl<'a, 'cfg> Context<'a, 'cfg> {
                             args.push(cfg.into());
                         }
 
+                        if !output.check_cfgs.is_empty() {
+                            args.push("-Zunstable-options".into());
+                            for check_cfg in &output.check_cfgs {
+                                args.push("--check-cfg".into());
+                                args.push(check_cfg.into());
+                            }
+                        }
+
                         for (lt, arg) in &output.linker_args {
                             if lt.applies_to(&unit.target) {
                                 args.push("-C".into());

src/cargo/core/compiler/custom_build.rs

@@ -29,6 +29,8 @@ pub struct BuildOutput {
     pub linker_args: Vec<(LinkType, String)>,
     /// Various `--cfg` flags to pass to the compiler.
     pub cfgs: Vec<String>,
+    /// Various `--check-cfg` flags to pass to the compiler.
+    pub check_cfgs: Vec<String>,
     /// Additional environment variables to run the compiler with.
     pub env: Vec<(String, String)>,
     /// Metadata to pass to the immediate dependencies.
@@ -322,6 +324,10 @@ fn build_work(cx: &mut Context<'_, '_>, unit: &Unit) -> CargoResult<Job> {
     paths::create_dir_all(&script_out_dir)?;
 
     let nightly_features_allowed = cx.bcx.config.nightly_features_allowed;
+    let extra_check_cfg = match cx.bcx.config.cli_unstable().check_cfg {
+        Some((_, _, _, output)) => output,
+        None => false,
+    };
     let targets: Vec<Target> = unit.pkg.targets().to_vec();
     // Need a separate copy for the fresh closure.
     let targets_fresh = targets.clone();
@@ -432,6 +438,7 @@ fn build_work(cx: &mut Context<'_, '_>, unit: &Unit) -> CargoResult<Job> {
             &pkg_descr,
             &script_out_dir,
             &script_out_dir,
+            extra_check_cfg,
             nightly_features_allowed,
             &targets,
         )?;
@@ -459,6 +466,7 @@ fn build_work(cx: &mut Context<'_, '_>, unit: &Unit) -> CargoResult<Job> {
                 &pkg_descr,
                 &prev_script_out_dir,
                 &script_out_dir,
+                extra_check_cfg,
                 nightly_features_allowed,
                 &targets_fresh,
             )?,
@@ -511,6 +519,7 @@ impl BuildOutput {
         pkg_descr: &str,
         script_out_dir_when_generated: &Path,
         script_out_dir: &Path,
+        extra_check_cfg: bool,
         nightly_features_allowed: bool,
         targets: &[Target],
     ) -> CargoResult<BuildOutput> {
@@ -521,6 +530,7 @@ impl BuildOutput {
             pkg_descr,
             script_out_dir_when_generated,
             script_out_dir,
+            extra_check_cfg,
             nightly_features_allowed,
             targets,
         )
@@ -536,13 +546,15 @@ impl BuildOutput {
         pkg_descr: &str,
         script_out_dir_when_generated: &Path,
         script_out_dir: &Path,
+        extra_check_cfg: bool,
         nightly_features_allowed: bool,
         targets: &[Target],
     ) -> CargoResult<BuildOutput> {
         let mut library_paths = Vec::new();
         let mut library_links = Vec::new();
         let mut linker_args = Vec::new();
         let mut cfgs = Vec::new();
+        let mut check_cfgs = Vec::new();
         let mut env = Vec::new();
         let mut metadata = Vec::new();
         let mut rerun_if_changed = Vec::new();
@@ -669,6 +681,13 @@ impl BuildOutput {
                     linker_args.push((LinkType::All, value));
                 }
                 "rustc-cfg" => cfgs.push(value.to_string()),
+                "rustc-check-cfg" => {
+                    if extra_check_cfg {
+                        check_cfgs.push(value.to_string());
+                    } else {
+                        warnings.push(format!("cargo:{} requires -Zcheck-cfg=output flag", key));
+                    }
+                }
                 "rustc-env" => {
                     let (key, val) = BuildOutput::parse_rustc_env(&value, &whence)?;
                     // Build scripts aren't allowed to set RUSTC_BOOTSTRAP.
@@ -728,6 +747,7 @@ impl BuildOutput {
             library_links,
             linker_args,
             cfgs,
+            check_cfgs,
             env,
             metadata,
             rerun_if_changed,
@@ -968,6 +988,10 @@ fn prev_build_output(cx: &mut Context<'_, '_>, unit: &Unit) -> (Option<BuildOutp
             &unit.pkg.to_string(),
             &prev_script_out_dir,
             &script_out_dir,
+            match cx.bcx.config.cli_unstable().check_cfg {
+                Some((_, _, _, output)) => output,
+                None => false,
+            },
             cx.bcx.config.nightly_features_allowed,
             unit.pkg.targets(),
         )

src/cargo/core/compiler/mod.rs

@@ -292,7 +292,7 @@ fn rustc(cx: &mut Context<'_, '_>, unit: &Unit, exec: &Arc<dyn Executor>) -> Car
                 )?;
                 add_plugin_deps(&mut rustc, &script_outputs, &build_scripts, &root_output)?;
             }
-            add_custom_env(&mut rustc, &script_outputs, script_metadata);
+            add_custom_flags(&mut rustc, &script_outputs, script_metadata)?;
         }
 
         for output in outputs.iter() {
@@ -408,9 +408,6 @@ fn rustc(cx: &mut Context<'_, '_>, unit: &Unit, exec: &Arc<dyn Executor>) -> Car
             }
 
             if key.0 == current_id {
-                for cfg in &output.cfgs {
-                    rustc.arg("--cfg").arg(cfg);
-                }
                 if pass_l_flag {
                     for name in output.library_links.iter() {
                         rustc.arg("-l").arg(name);
@@ -431,22 +428,6 @@ fn rustc(cx: &mut Context<'_, '_>, unit: &Unit, exec: &Arc<dyn Executor>) -> Car
         }
         Ok(())
     }
-
-    // Add all custom environment variables present in `state` (after they've
-    // been put there by one of the `build_scripts`) to the command provided.
-    fn add_custom_env(
-        rustc: &mut ProcessBuilder,
-        build_script_outputs: &BuildScriptOutputs,
-        metadata: Option<Metadata>,
-    ) {
-        if let Some(metadata) = metadata {
-            if let Some(output) = build_script_outputs.get(metadata) {
-                for &(ref name, ref value) in output.env.iter() {
-                    rustc.env(name, value);
-                }
-            }
-        }
-    }
 }
 
 /// Link the compiled target (often of form `foo-{metadata_hash}`) to the
@@ -713,16 +694,11 @@ fn rustdoc(cx: &mut Context<'_, '_>, unit: &Unit) -> CargoResult<Work> {
     let mut output_options = OutputOptions::new(cx, unit);
     let script_metadata = cx.find_build_script_metadata(unit);
     Ok(Work::new(move |state| {
-        if let Some(script_metadata) = script_metadata {
-            if let Some(output) = build_script_outputs.lock().unwrap().get(script_metadata) {
-                for cfg in output.cfgs.iter() {
-                    rustdoc.arg("--cfg").arg(cfg);
-                }
-                for &(ref name, ref value) in output.env.iter() {
-                    rustdoc.env(name, value);
-                }
-            }
-        }
+        add_custom_flags(
+            &mut rustdoc,
+            &build_script_outputs.lock().unwrap(),
+            script_metadata,
+        )?;
         let crate_dir = doc_dir.join(&crate_name);
         if crate_dir.exists() {
             // Remove output from a previous build. This ensures that stale
@@ -1055,7 +1031,7 @@ fn features_args(unit: &Unit) -> Vec<OsString> {
 
 /// Generate the --check-cfg arguments for the unit
 fn check_cfg_args(cx: &Context<'_, '_>, unit: &Unit) -> Vec<OsString> {
-    if let Some((features, well_known_names, well_known_values)) =
+    if let Some((features, well_known_names, well_known_values, _output)) =
         cx.bcx.config.cli_unstable().check_cfg
     {
         let mut args = Vec::with_capacity(unit.pkg.summary().features().len() * 2 + 4);
@@ -1184,6 +1160,32 @@ fn build_deps_args(
     Ok(())
 }
 
+/// Add custom flags from the output a of build-script to a `ProcessBuilder`
+fn add_custom_flags(
+    cmd: &mut ProcessBuilder,
+    build_script_outputs: &BuildScriptOutputs,
+    metadata: Option<Metadata>,
+) -> CargoResult<()> {
+    if let Some(metadata) = metadata {
+        if let Some(output) = build_script_outputs.get(metadata) {
+            for cfg in output.cfgs.iter() {
+                cmd.arg("--cfg").arg(cfg);
+            }
+            if !output.check_cfgs.is_empty() {
+                cmd.arg("-Zunstable-options");
+                for check_cfg in &output.check_cfgs {
+                    cmd.arg("--check-cfg").arg(check_cfg);
+                }
+            }
+            for &(ref name, ref value) in output.env.iter() {
+                cmd.env(name, value);
+            }
+        }
+    }
+
+    Ok(())
+}
+
 /// Generates a list of `--extern` arguments.
 pub fn extern_args(
     cx: &Context<'_, '_>,

src/cargo/core/features.rs

@@ -641,7 +641,7 @@ unstable_cli_options!(
     build_std_features: Option<Vec<String>>  = ("Configure features enabled for the standard library itself when building the standard library"),
     config_include: bool = ("Enable the `include` key in config files"),
     credential_process: bool = ("Add a config setting to fetch registry authentication tokens by calling an external process"),
-    check_cfg: Option<(/*features:*/ bool, /*well_known_names:*/ bool, /*well_known_values:*/ bool)> = ("Specify scope of compile-time checking of `cfg` names/values"),
+    check_cfg: Option<(/*features:*/ bool, /*well_known_names:*/ bool, /*well_known_values:*/ bool, /*output:*/ bool)> = ("Specify scope of compile-time checking of `cfg` names/values"),
     doctest_in_workspace: bool = ("Compile doctests with paths relative to the workspace root"),
     doctest_xcompile: bool = ("Compile and run doctests for non-host target using runner config"),
     dual_proc_macros: bool = ("Build proc-macros for both the host and the target"),
@@ -783,22 +783,29 @@ impl CliUnstable {
             }
         }
 
-        fn parse_check_cfg(value: Option<&str>) -> CargoResult<Option<(bool, bool, bool)>> {
+        fn parse_check_cfg(value: Option<&str>) -> CargoResult<Option<(bool, bool, bool, bool)>> {
             if let Some(value) = value {
                 let mut features = false;
                 let mut well_known_names = false;
                 let mut well_known_values = false;
+                let mut output = false;
 
                 for e in value.split(',') {
                     match e {
                         "features" => features = true,
                         "names" => well_known_names = true,
                         "values" => well_known_values = true,
-                        _ => bail!("flag -Zcheck-cfg only takes `features`, `names` or `values` as valid inputs"),
+                        "output" => output = true,
+                        _ => bail!("flag -Zcheck-cfg only takes `features`, `names`, `values` or `output` as valid inputs"),
                     }
                 }
 
-                Ok(Some((features, well_known_names, well_known_values)))
+                Ok(Some((
+                    features,
+                    well_known_names,
+                    well_known_values,
+                    output,
+                )))
             } else {
                 Ok(None)
             }

src/cargo/util/config/target.rs

@@ -120,7 +120,7 @@ fn load_config_table(config: &Config, prefix: &str) -> CargoResult<TargetConfig>
     // Links do not support environment variables.
     let target_key = ConfigKey::from_str(prefix);
     let links_overrides = match config.get_table(&target_key)? {
-        Some(links) => parse_links_overrides(&target_key, links.val)?,
+        Some(links) => parse_links_overrides(&target_key, links.val, config)?,
         None => BTreeMap::new(),
     };
     Ok(TargetConfig {
@@ -134,8 +134,14 @@ fn load_config_table(config: &Config, prefix: &str) -> CargoResult<TargetConfig>
 fn parse_links_overrides(
     target_key: &ConfigKey,
     links: HashMap<String, CV>,
+    config: &Config,
 ) -> CargoResult<BTreeMap<String, BuildOutput>> {
     let mut links_overrides = BTreeMap::new();
+    let extra_check_cfg = match config.cli_unstable().check_cfg {
+        Some((_, _, _, output)) => output,
+        None => false,
+    };
+
     for (lib_name, value) in links {
         // Skip these keys, it shares the namespace with `TargetConfig`.
         match lib_name.as_str() {
@@ -200,6 +206,17 @@ fn parse_links_overrides(
                     let list = value.list(key)?;
                     output.cfgs.extend(list.iter().map(|v| v.0.clone()));
                 }
+                "rustc-check-cfg" => {
+                    if extra_check_cfg {
+                        let list = value.list(key)?;
+                        output.check_cfgs.extend(list.iter().map(|v| v.0.clone()));
+                    } else {
+                        config.shell().warn(format!(
+                            "target config `{}.{}` requires -Zcheck-cfg=output flag",
+                            target_key, key
+                        ))?;
+                    }
+                }
                 "rustc-env" => {
                     for (name, val) in value.table(key)?.0 {
                         let val = val.string(name)?.0;

src/doc/src/reference/unstable.md

@@ -1201,6 +1201,7 @@ It's values are:
     Note than this command line options will probably become the default when stabilizing.
  - `names`: enables well known names checking via `--check-cfg=names()`.
  - `values`: enables well known values checking via `--check-cfg=values()`.
+ - `output`: enable the use of `rustc-check-cfg` in build script.
 
 For instance:
 
@@ -1211,6 +1212,29 @@ cargo check -Z unstable-options -Z check-cfg=values
 cargo check -Z unstable-options -Z check-cfg=features,names,values
 ```
 
+Or for `output`:
+
+```rust,no_run
+// build.rs
+println!("cargo:rustc-check-cfg=names(foo, bar)");
+```
+
+```
+cargo check -Z unstable-options -Z check-cfg=output
+```
+
+### `cargo:rustc-check-cfg=CHECK_CFG`
+
+The `rustc-check-cfg` instruction tells Cargo to pass the given value to the
+`--check-cfg` flag to the compiler. This may be used for compile-time
+detection of unexpected conditional compilation name and/or values.
+
+This can only be used in combination with `-Zcheck-cfg=output` otherwise it is ignored
+with a warning.
+
+If you want to integrate with Cargo features, use `-Zcheck-cfg=features` instead of
+trying to do it manually with this option.
+
 ### workspace-inheritance
 
 * RFC: [#2906](https://github.com/rust-lang/rfcs/blob/master/text/2906-cargo-workspace-deduplicate.md)