Add experimental [cfgs] section to decribe the expected cfgs

src/cargo/core/compiler/custom_build.rs

@@ -325,7 +325,7 @@ fn build_work(cx: &mut Context<'_, '_>, unit: &Unit) -> CargoResult<Job> {
 
     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,
+        Some((_, _, _, output, _)) => output,
         None => false,
     };
     let targets: Vec<Target> = unit.pkg.targets().to_vec();
@@ -989,7 +989,7 @@ fn prev_build_output(cx: &mut Context<'_, '_>, unit: &Unit) -> (Option<BuildOutp
             &prev_script_out_dir,
             &script_out_dir,
             match cx.bcx.config.cli_unstable().check_cfg {
-                Some((_, _, _, output)) => output,
+                Some((_, _, _, output, _)) => output,
                 None => false,
             },
             cx.bcx.config.nightly_features_allowed,

src/cargo/core/compiler/mod.rs

@@ -1131,7 +1131,7 @@ 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, _output)) =
+    if let Some((features, well_known_names, well_known_values, _output, _cfgs)) =
         cx.bcx.config.cli_unstable().check_cfg
     {
         let mut args = Vec::with_capacity(unit.pkg.summary().features().len() * 2 + 4);
@@ -1153,6 +1153,46 @@ fn check_cfg_args(cx: &Context<'_, '_>, unit: &Unit) -> Vec<OsString> {
             args.push(arg);
         }
 
+        if let Some(cfgs) = unit.pkg.cfgs() {
+            let mut names = OsString::from("names(");
+            let mut has_names = false;
+
+            for name in cfgs
+                .iter()
+                .filter_map(|(name, values)| values.as_ref().is_none().then_some(name))
+            {
+                if has_names {
+                    names.push(",");
+                }
+                names.push(name);
+                has_names = true;
+            }
+
+            if has_names {
+                names.push(")");
+                args.push(OsString::from("--check-cfg"));
+                args.push(names);
+            }
+
+            for (name, values) in cfgs
+                .iter()
+                .filter_map(|(name, values)| values.as_ref().map(|values| (name, values)))
+            {
+                let mut arg = OsString::from("values(");
+
+                arg.push(name);
+                for val in values {
+                    arg.push(", \"");
+                    arg.push(&val);
+                    arg.push("\"");
+                }
+                arg.push(")");
+
+                args.push(OsString::from("--check-cfg"));
+                args.push(arg);
+            }
+        }
+
         if well_known_names {
             args.push(OsString::from("--check-cfg"));
             args.push(OsString::from("names()"));

src/cargo/core/features.rs

@@ -671,7 +671,7 @@ unstable_cli_options!(
     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"),
     #[serde(deserialize_with = "deserialize_check_cfg")]
-    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"),
+    check_cfg: Option<(/*features:*/ bool, /*well_known_names:*/ bool, /*well_known_values:*/ bool, /*output:*/ bool, /*cfgs:*/ 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"),
@@ -774,7 +774,7 @@ where
 
 fn deserialize_check_cfg<'de, D>(
     deserializer: D,
-) -> Result<Option<(bool, bool, bool, bool)>, D::Error>
+) -> Result<Option<(bool, bool, bool, bool, bool)>, D::Error>
 where
     D: serde::Deserializer<'de>,
 {
@@ -789,27 +789,36 @@ where
 
 fn parse_check_cfg(
     it: impl Iterator<Item = impl AsRef<str>>,
-) -> CargoResult<Option<(bool, bool, bool, bool)>> {
+) -> CargoResult<Option<(bool, bool, bool, bool, bool)>> {
     let mut features = false;
     let mut well_known_names = false;
     let mut well_known_values = false;
     let mut output = false;
+    let mut cfgs = false;
 
+    let mut had_options = false;
     for e in it {
         match e.as_ref() {
             "features" => features = true,
             "names" => well_known_names = true,
             "values" => well_known_values = true,
             "output" => output = true,
-            _ => bail!("unstable check-cfg only takes `features`, `names`, `values` or `output` as valid inputs"),
+            "cfgs" => cfgs = true,
+            _ => bail!("unstable check-cfg only takes `features`, `names`, `values`, `output` or `cfgs` as valid inputs"),
         }
+        had_options = true;
+    }
+
+    if !had_options {
+        bail!("unstable check-cfg must take one or more of these options: `features`, `names`, `values`, `output` or `cfgs`");
     }
 
     Ok(Some((
         features,
         well_known_names,
         well_known_values,
         output,
+        cfgs,
     )))
 }
 

src/cargo/core/manifest.rs

@@ -64,6 +64,7 @@ pub struct Manifest {
     default_run: Option<String>,
     metabuild: Option<Vec<String>>,
     resolve_behavior: Option<ResolveBehavior>,
+    cfgs: Option<BTreeMap<String, Option<Vec<String>>>>,
 }
 
 /// When parsing `Cargo.toml`, some warnings should silenced
@@ -405,6 +406,7 @@ impl Manifest {
         original: Rc<TomlManifest>,
         metabuild: Option<Vec<String>>,
         resolve_behavior: Option<ResolveBehavior>,
+        cfgs: Option<BTreeMap<String, Option<Vec<String>>>>,
     ) -> Manifest {
         Manifest {
             summary,
@@ -430,6 +432,7 @@ impl Manifest {
             default_run,
             metabuild,
             resolve_behavior,
+            cfgs,
         }
     }
 
@@ -497,6 +500,9 @@ impl Manifest {
     pub fn links(&self) -> Option<&str> {
         self.links.as_deref()
     }
+    pub fn cfgs(&self) -> &Option<BTreeMap<String, Option<Vec<String>>>> {
+        &self.cfgs
+    }
 
     pub fn workspace_config(&self) -> &WorkspaceConfig {
         &self.workspace

src/cargo/core/package.rs

@@ -87,6 +87,7 @@ pub struct SerializedPackage {
     dependencies: Vec<Dependency>,
     targets: Vec<Target>,
     features: BTreeMap<InternedString, Vec<InternedString>>,
+    // cfgs: BTreeMap<String, Option<Vec<String>>>,
     manifest_path: PathBuf,
     metadata: Option<toml::Value>,
     publish: Option<Vec<String>>,
@@ -164,6 +165,10 @@ impl Package {
     pub fn authors(&self) -> &Vec<String> {
         &self.manifest().metadata().authors
     }
+    /// Gets the cfgs.
+    pub fn cfgs(&self) -> &Option<BTreeMap<String, Option<Vec<String>>>> {
+        self.manifest().cfgs()
+    }
 
     /// Returns `None` if the package is set to publish.
     /// Returns `Some(allowed_registries)` if publishing is limited to specified

src/cargo/util/config/target.rs

@@ -138,7 +138,7 @@ fn parse_links_overrides(
 ) -> CargoResult<BTreeMap<String, BuildOutput>> {
     let mut links_overrides = BTreeMap::new();
     let extra_check_cfg = match config.cli_unstable().check_cfg {
-        Some((_, _, _, output)) => output,
+        Some((_, _, _, output, _)) => output,
         None => false,
     };
 

src/cargo/util/toml/mod.rs

@@ -360,13 +360,31 @@ pub struct TomlManifest {
     #[serde(rename = "build_dependencies")]
     build_dependencies2: Option<BTreeMap<String, MaybeWorkspaceDependency>>,
     features: Option<BTreeMap<InternedString, Vec<InternedString>>>,
+    cfgs: Option<BTreeMap<String, TomlCfg>>,
     target: Option<BTreeMap<String, TomlPlatform>>,
     replace: Option<BTreeMap<String, TomlDependency>>,
     patch: Option<BTreeMap<String, BTreeMap<String, TomlDependency>>>,
     workspace: Option<TomlWorkspace>,
     badges: Option<MaybeWorkspaceField<BTreeMap<String, BTreeMap<String, String>>>>,
 }
 
+#[derive(Clone, Debug, Serialize, Deserialize)]
+#[serde(untagged)]
+pub enum TomlCfg {
+    List(Vec<String>),
+    Values { values: bool },
+}
+
+impl TomlCfg {
+    pub(crate) fn into_option_vec(self) -> Option<Vec<String>> {
+        match self {
+            TomlCfg::List(l) => Some(l),
+            TomlCfg::Values { values: true } => Some(Vec::new()),
+            TomlCfg::Values { values: false } => None,
+        }
+    }
+}
+
 #[derive(Deserialize, Serialize, Clone, Debug, Default)]
 pub struct TomlProfiles(BTreeMap<InternedString, TomlProfile>);
 
@@ -1558,6 +1576,7 @@ impl TomlManifest {
             )?,
             build_dependencies2: None,
             features: self.features.clone(),
+            cfgs: self.cfgs.clone(),
             target: match self.target.as_ref().map(|target_map| {
                 target_map
                     .iter()
@@ -1793,6 +1812,15 @@ impl TomlManifest {
             )));
         }
 
+        if me.cfgs.is_some()
+            && matches!(
+                &config.cli_unstable().check_cfg,
+                None | Some((_, _, _, _, false))
+            )
+        {
+            bail!("`[cfgs]` section requires `-Zcheck-cfg=cfgs`");
+        }
+
         let rust_version = if let Some(rust_version) = &package.rust_version {
             let rust_version = rust_version
                 .clone()
@@ -2220,6 +2248,12 @@ impl TomlManifest {
             }
         }
 
+        let cfgs = me.cfgs.as_ref().map(|cfgs| {
+            cfgs.iter()
+                .map(|(name, toml_cfg)| (name.clone(), toml_cfg.clone().into_option_vec()))
+                .collect()
+        });
+
         let default_kind = package
             .default_target
             .as_ref()
@@ -2249,6 +2283,7 @@ impl TomlManifest {
             build_dependencies: build_deps,
             build_dependencies2: None,
             features: me.features.clone(),
+            cfgs: me.cfgs.clone(),
             target,
             replace: me.replace.clone(),
             patch: me.patch.clone(),
@@ -2281,6 +2316,7 @@ impl TomlManifest {
             Rc::new(resolved_toml),
             package.metabuild.clone().map(|sov| sov.0),
             resolve_behavior,
+            cfgs,
         );
         if package.license_file.is_some() && package.license.is_some() {
             manifest.warnings_mut().add_warning(
@@ -2344,6 +2380,9 @@ impl TomlManifest {
         if me.features.is_some() {
             bail!("this virtual manifest specifies a [features] section, which is not allowed");
         }
+        if me.cfgs.is_some() {
+            bail!("this virtual manifest specifies a [cfgs] section, which is not allowed");
+        }
         if me.target.is_some() {
             bail!("this virtual manifest specifies a [target] section, which is not allowed");
         }

src/doc/src/reference/unstable.md

@@ -1197,17 +1197,26 @@ It's values are:
  - `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.
+ - `cfgs`: enable the use of the `#[cfgs]` in the `Cargo.toml` manifest.
 
 For instance:
 
 ```
 cargo check -Z unstable-options -Z check-cfg=features
 cargo check -Z unstable-options -Z check-cfg=names
 cargo check -Z unstable-options -Z check-cfg=values
-cargo check -Z unstable-options -Z check-cfg=features,names,values
+cargo check -Z unstable-options -Z check-cfg=cfgs
+cargo check -Z unstable-options -Z check-cfg=features,names,values,cfgs
 ```
 
-Or for `output`:
+Those values can also be set in the `[unstable]` section of `.cargo/config.toml`:
+
+```toml
+[unstable]
+check-cfg = ["cfgs", "features"]
+```
+
+#### Usage of `output`
 
 ```rust,no_run
 // build.rs
@@ -1218,6 +1227,23 @@ println!("cargo:rustc-check-cfg=names(foo, bar)");
 cargo check -Z unstable-options -Z check-cfg=output
 ```
 
+#### Usage of `cfgs`
+
+In `Cargo.toml`:
+
+```toml
+[cfgs]
+"use_libc" = []                # means `--check-cfg=values('use_libc')`
+"loom" = ["yes", "no", "auto"] # means `--check-cfg=values('feature', "yes", "no", "auto")`
+
+[cfgs."unknow_values"] # means `--check-cfg=names('unknow_values')`
+values = false         # aka values are unknown (so everything is accepted)
+```
+
+```
+cargo check -Z unstable-options -Z check-cfg=cfgs
+```
+
 ### `cargo:rustc-check-cfg=CHECK_CFG`
 
 The `rustc-check-cfg` instruction tells Cargo to pass the given value to the