Add -Z check-cfg-features to enable compile-time checking of features

src/cargo/core/compiler/mod.rs

@@ -782,6 +782,28 @@ fn add_allow_features(cx: &Context<'_, '_>, cmd: &mut ProcessBuilder) {
     }
 }
 
+/// Add all features as cfg
+fn add_features(cx: &Context<'_, '_>, cmd: &mut ProcessBuilder, unit: &Unit) {
+    for feat in &unit.features {
+        cmd.arg("--cfg").arg(&format!("feature=\"{}\"", feat));
+    }
+
+    if cx.bcx.config.cli_unstable().check_cfg_features {
+        // This generate something like this:
+        //  - values(feature)
+        //  - values(feature, "foo", "bar")
+        let mut arg = String::from("values(feature");
+        for (&feat, _) in unit.pkg.summary().features() {
+            arg.push_str(", \"");
+            arg.push_str(&feat);
+            arg.push_str("\"");
+        }
+        arg.push(')');
+
+        cmd.arg("-Zunstable-options").arg("--check-cfg").arg(&arg);
+    }
+}
+
 /// Add error-format flags to the command.
 ///
 /// Cargo always uses JSON output. This has several benefits, such as being
@@ -978,9 +1000,7 @@ fn build_base_args(
         cmd.arg("--cfg").arg("test");
     }
 
-    for feat in &unit.features {
-        cmd.arg("--cfg").arg(&format!("feature=\"{}\"", feat));
-    }
+    add_features(cx, cmd, unit);
 
     let meta = cx.files().metadata(unit);
     cmd.arg("-C").arg(&format!("metadata={}", meta));

src/cargo/core/features.rs

@@ -637,6 +637,7 @@ 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_features: bool = ("Enable compile-time checking of features in `cfg`"),
     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"),
@@ -834,6 +835,7 @@ impl CliUnstable {
             "minimal-versions" => self.minimal_versions = parse_empty(k, v)?,
             "advanced-env" => self.advanced_env = parse_empty(k, v)?,
             "config-include" => self.config_include = parse_empty(k, v)?,
+            "check-cfg-features" => self.check_cfg_features = parse_empty(k, v)?,
             "dual-proc-macros" => self.dual_proc_macros = parse_empty(k, v)?,
             // can also be set in .cargo/config or with and ENV
             "mtime-on-use" => self.mtime_on_use = parse_empty(k, v)?,

src/doc/src/reference/unstable.md

@@ -1078,6 +1078,20 @@ For instance:
 cargo doc -Z unstable-options -Z rustdoc-scrape-examples=examples
 ```
 
+### check-cfg-features
+
+* RFC: [#3013](https://github.com/rust-lang/rfcs/pull/3013)
+
+The `-Z check-cfg-features` argument tells Cargo to pass all possible features of a package to
+`rustc` unstable `--check-cfg` command line as `--check-cfg=values(feature, ...)`. This enables
+compile time checking of feature values in `#[cfg]`, `cfg!` and `#[cfg_attr]`. Note than this
+command line options will probably become the default when stabilizing.
+For instance:
+
+```
+cargo check -Z unstable-options -Z check-cfg-features
+```
+
 ## Stabilized and removed features
 
 ### Compile progress

tests/testsuite/build.rs

@@ -5948,3 +5948,122 @@ fn primary_package_env_var() {
 
     foo.cargo("test").run();
 }
+
+#[cargo_test]
+fn check_cfg_features() {
+    if !is_nightly() {
+        // --check-cfg is a nightly only rustc command line
+        return;
+    }
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [project]
+                name = "foo"
+                version = "0.1.0"
+
+                [features]
+                f_a = []
+                f_b = []
+            "#,
+        )
+        .file("src/main.rs", "fn main() {}")
+        .build();
+
+    p.cargo("build -v -Z check-cfg-features")
+        .masquerade_as_nightly_cargo()
+        .with_stderr(
+            "\
+[COMPILING] foo v0.1.0 [..]
+[RUNNING] `rustc [..] --check-cfg 'values(feature, \"f_a\", \"f_b\")' [..]
+[FINISHED] dev [unoptimized + debuginfo] target(s) in [..]
+",
+        )
+        .run();
+}
+
+#[cargo_test]
+fn check_cfg_features_with_deps() {
+    if !is_nightly() {
+        // --check-cfg is a nightly only rustc command line
+        return;
+    }
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [project]
+                name = "foo"
+                version = "0.1.0"
+
+                [dependencies]
+                bar = { path = "bar/" }
+
+                [features]
+                f_a = []
+                f_b = []
+            "#,
+        )
+        .file("src/main.rs", "fn main() {}")
+        .file("bar/Cargo.toml", &basic_manifest("bar", "0.1.0"))
+        .file("bar/src/lib.rs", "#[allow(dead_code)] fn bar() {}")
+        .build();
+
+    p.cargo("build -v -Z check-cfg-features")
+        .masquerade_as_nightly_cargo()
+        .with_stderr(
+            "\
+[COMPILING] bar v0.1.0 [..]
+[RUNNING] `rustc [..] --check-cfg 'values(feature)' [..]
+[COMPILING] foo v0.1.0 [..]
+[RUNNING] `rustc --crate-name foo [..] --check-cfg 'values(feature, \"f_a\", \"f_b\")' [..]
+[FINISHED] dev [unoptimized + debuginfo] target(s) in [..]
+",
+        )
+        .run();
+}
+#[cargo_test]
+fn check_cfg_features_with_opt_deps() {
+    if !is_nightly() {
+        // --check-cfg is a nightly only rustc command line
+        return;
+    }
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [project]
+                name = "foo"
+                version = "0.1.0"
+
+                [dependencies]
+                bar = { path = "bar/", optional = true }
+
+                [features]
+                default = ["bar"]
+                f_a = []
+                f_b = []
+            "#,
+        )
+        .file("src/main.rs", "fn main() {}")
+        .file("bar/Cargo.toml", &basic_manifest("bar", "0.1.0"))
+        .file("bar/src/lib.rs", "#[allow(dead_code)] fn bar() {}")
+        .build();
+
+    p.cargo("build -v -Z check-cfg-features")
+        .masquerade_as_nightly_cargo()
+        .with_stderr(
+            "\
+[COMPILING] bar v0.1.0 [..]
+[RUNNING] `rustc [..] --check-cfg 'values(feature)' [..]
+[COMPILING] foo v0.1.0 [..]
+[RUNNING] `rustc --crate-name foo [..] --check-cfg 'values(feature, \"bar\", \"default\", \"f_a\", \"f_b\")' [..]
+[FINISHED] dev [unoptimized + debuginfo] target(s) in [..]
+",
+        )
+        .run();
+}

tests/testsuite/check.rs

@@ -3,6 +3,7 @@
 use std::fmt::{self, Write};
 
 use cargo_test_support::install::exe;
+use cargo_test_support::is_nightly;
 use cargo_test_support::paths::CargoPathExt;
 use cargo_test_support::registry::Package;
 use cargo_test_support::tools;
@@ -997,3 +998,38 @@ fn rustc_workspace_wrapper_excludes_published_deps() {
         .with_stdout_does_not_contain("WRAPPER CALLED: rustc --crate-name baz [..]")
         .run();
 }
+
+#[cargo_test]
+fn check_cfg_features() {
+    if !is_nightly() {
+        // --check-cfg is a nightly only rustc command line
+        return;
+    }
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [project]
+                name = "foo"
+                version = "0.1.0"
+
+                [features]
+                f_a = []
+                f_b = []
+            "#,
+        )
+        .file("src/main.rs", "fn main() {}")
+        .build();
+
+    p.cargo("check -v -Z check-cfg-features")
+        .masquerade_as_nightly_cargo()
+        .with_stderr(
+            "\
+[CHECKING] foo v0.1.0 [..]
+[RUNNING] `rustc [..] --check-cfg 'values(feature, \"f_a\", \"f_b\")' [..]
+[FINISHED] dev [unoptimized + debuginfo] target(s) in [..]
+",
+        )
+        .run();
+}

tests/testsuite/test.rs

@@ -4499,3 +4499,39 @@ fn test_workspaces_cwd() {
         .with_stdout_contains("test test_integration_deep_cwd ... ok")
         .run();
 }
+
+#[cargo_test]
+fn check_cfg_features() {
+    if !is_nightly() {
+        // --check-cfg is a nightly only rustc command line
+        return;
+    }
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [project]
+                name = "foo"
+                version = "0.1.0"
+
+                [features]
+                f_a = []
+                f_b = []
+            "#,
+        )
+        .file("src/main.rs", "fn main() {}")
+        .build();
+
+    p.cargo("test -v -Z check-cfg-features")
+        .masquerade_as_nightly_cargo()
+        .with_stderr(
+            "\
+[COMPILING] foo v0.1.0 [..]
+[RUNNING] `rustc [..] --check-cfg 'values(feature, \"f_a\", \"f_b\")' [..]
+[FINISHED] test [unoptimized + debuginfo] target(s) in [..]
+[RUNNING] [..]
+",
+        )
+        .run();
+}