Add -Zallow-features to match rustc's -Z

src/bin/cargo/cli.rs

@@ -35,6 +35,7 @@ pub fn main(config: &mut Config) -> CliResult {
             "
 Available unstable (nightly-only) flags:
 
+    -Z allow-features      -- Allow *only* the listed unstable features
     -Z avoid-dev-deps      -- Avoid installing dev-dependencies if possible
     -Z extra-link-arg      -- Allow `cargo:rustc-link-arg` in build scripts
     -Z minimal-versions    -- Install minimal dependency versions instead of maximum

src/cargo/core/features.rs

@@ -92,6 +92,7 @@
 //!    of that page. Update the rest of the documentation to add the new
 //!    feature.
 
+use std::collections::HashSet;
 use std::env;
 use std::fmt;
 use std::str::FromStr;
@@ -415,13 +416,18 @@ impl Features {
         let mut ret = Features::default();
         ret.nightly_features_allowed = config.nightly_features_allowed;
         for feature in features {
-            ret.add(feature, warnings)?;
+            ret.add(feature, config, warnings)?;
             ret.activated.push(feature.to_string());
         }
         Ok(ret)
     }
 
-    fn add(&mut self, feature_name: &str, warnings: &mut Vec<String>) -> CargoResult<()> {
+    fn add(
+        &mut self,
+        feature_name: &str,
+        config: &Config,
+        warnings: &mut Vec<String>,
+    ) -> CargoResult<()> {
         let nightly_features_allowed = self.nightly_features_allowed;
         let (slot, feature) = match self.status(feature_name) {
             Some(p) => p,
@@ -469,6 +475,24 @@ impl Features {
                 SEE_CHANNELS,
                 see_docs()
             ),
+            Status::Unstable
+                if !config
+                    .cli_unstable()
+                    .allow_features
+                    .as_ref()
+                    .map(|f| f.contains(feature_name))
+                    .unwrap_or(true) =>
+            {
+                bail!(
+                    "the feature `{}` is not in the list of allowed features: {:?}",
+                    feature_name,
+                    config
+                        .cli_unstable()
+                        .allow_features
+                        .as_ref()
+                        .expect("!unwrap_or(true) == false")
+                )
+            }
             Status::Unstable => {}
             Status::Removed => bail!(
                 "the cargo feature `{}` has been removed\n\
@@ -530,6 +554,8 @@ impl Features {
 #[serde(default, rename_all = "kebab-case")]
 pub struct CliUnstable {
     pub print_im_a_teapot: bool,
+    pub allow_features: Option<HashSet<String>>,
+
     pub unstable_options: bool,
     pub no_index_update: bool,
     pub avoid_dev_deps: bool,
@@ -627,8 +653,33 @@ impl CliUnstable {
         }
         let mut warnings = Vec::new();
         for flag in flags {
+            if flag.starts_with("allow-features=") {
                 self.add(flag, &mut warnings)?;
             }
+        }
+        if let Some(allowed) = self.allow_features.take() {
+            for flag in flags {
+                let k = flag
+                    .splitn(2, '=')
+                    .next()
+                    .expect("split always yields >=1 item");
+                if k == "allow-features" {
+                } else if allowed.contains(k) {
+                    self.add(flag, &mut warnings)?;
+                } else {
+                    bail!(
+                        "the feature `{}` is not in the list of allowed features: {:?}",
+                        k,
+                        allowed
+                    );
+                }
+            }
+            self.allow_features = Some(allowed);
+        } else {
+            for flag in flags {
+                self.add(flag, &mut warnings)?;
+            }
+        }
         Ok(warnings)
     }
 
@@ -655,6 +706,7 @@ impl CliUnstable {
         fn parse_features(value: Option<&str>) -> Vec<String> {
             match value {
                 None => Vec::new(),
+                Some("") => Vec::new(),
                 Some(v) => v.split(',').map(|s| s.to_string()).collect(),
             }
         }
@@ -699,6 +751,7 @@ impl CliUnstable {
 
         match k {
             "print-im-a-teapot" => self.print_im_a_teapot = parse_bool(k, v)?,
+            "allow-features" => self.allow_features = Some(parse_features(v).into_iter().collect()),
             "unstable-options" => self.unstable_options = parse_empty(k, v)?,
             "no-index-update" => self.no_index_update = parse_empty(k, v)?,
             "avoid-dev-deps" => self.avoid_dev_deps = parse_empty(k, v)?,

src/doc/src/reference/unstable.md

@@ -58,6 +58,27 @@ Each new feature described below should explain how to use it.
 [nightly channel]: ../../book/appendix-07-nightly-rust.html
 [stabilized]: https://doc.crates.io/contrib/process/unstable.html#stabilization
 
+### allow-features
+
+This permanently-unstable flag makes it so that only a listed set of
+unstable features can be used. Specifically, if you pass
+`-Zallow-features=foo,bar`, you'll continue to be able to pass `-Zfoo`
+and `-Zbar` to `cargo`, but you will be unable to pass `-Zbaz`. You can
+pass an empty string (`-Zallow-features=`) to disallow all unstable
+features.
+
+`-Zallow-features` also restricts which unstable features can be passed
+to the `cargo-features` entry in `Cargo.toml`. If, for example, you want
+to allow
+
+```toml
+cargo-features = ["test-dummy-unstable"]
+```
+
+where `test-dummy-unstable` is unstable, that features would also be
+disallowed by `-Zallow-features=`, and allowed with
+`-Zallow-features=test-dummy-unstable`.
+
 ### extra-link-arg
 * Original Pull Request: [#7811](https://github.com/rust-lang/cargo/pull/7811)
 

tests/testsuite/cargo_features.rs

@@ -113,6 +113,64 @@ release and is no longer necessary to be listed in the manifest
         .run();
 }
 
+#[cargo_test]
+fn allow_features() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                cargo-features = ["test-dummy-unstable"]
+
+                [package]
+                name = "a"
+                version = "0.0.1"
+                authors = []
+                im-a-teapot = true
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("-Zallow-features=test-dummy-unstable build")
+        .masquerade_as_nightly_cargo()
+        .with_stderr(
+            "\
+[COMPILING] a [..]
+[FINISHED] [..]
+",
+        )
+        .run();
+
+    p.cargo("-Zallow-features=test-dummy-unstable,print-im-a-teapot -Zprint-im-a-teapot build")
+        .masquerade_as_nightly_cargo()
+        .with_stdout("im-a-teapot = true")
+        .with_stderr("[FINISHED] [..]")
+        .run();
+
+    p.cargo("-Zallow-features=test-dummy-unstable -Zprint-im-a-teapot build")
+        .masquerade_as_nightly_cargo()
+        .with_status(101)
+        .with_stderr(
+            "\
+error: the feature `print-im-a-teapot` is not in the list of allowed features: {\"test-dummy-unstable\"}
+",
+        )
+        .run();
+
+    p.cargo("-Zallow-features= build")
+        .masquerade_as_nightly_cargo()
+        .with_status(101)
+        .with_stderr(
+            "\
+error: failed to parse manifest at `[..]`
+
+Caused by:
+  the feature `test-dummy-unstable` is not in the list of allowed features: {}
+",
+        )
+        .run();
+}
+
 #[cargo_test]
 fn nightly_feature_requires_nightly() {
     let p = project()