Basic feature flag mechanism (#5350)

crates/typst-cli/src/args.rs

@@ -274,8 +274,17 @@ pub struct SharedArgs {
     /// defaults to number of CPUs. Setting it to 1 disables parallelism.
     #[clap(long, short)]
     pub jobs: Option<usize>,
+
+    /// Enables in-development features that may be changed or removed at any
+    /// time.
+    #[arg(long = "feature", value_delimiter = ',')]
+    pub feature: Vec<Feature>,
 }
 
+/// An in-development feature that may be changed or removed at any time.
+#[derive(Debug, Copy, Clone, Eq, PartialEq, ValueEnum)]
+pub enum Feature {}
+
 /// Arguments related to where packages are stored in the system.
 #[derive(Debug, Clone, Args)]
 pub struct PackageStorageArgs {

crates/typst-cli/src/world.rs

@@ -112,7 +112,10 @@ impl SystemWorld {
                 .map(|(k, v)| (k.as_str().into(), v.as_str().into_value()))
                 .collect();
 
-            Library::builder().with_inputs(inputs).build()
+            let features =
+                command.feature.iter().map(|&feature| match feature {}).collect();
+
+            Library::builder().with_inputs(inputs).with_features(features).build()
         };
 
         let fonts = Fonts::searcher()

crates/typst-library/src/lib.rs

@@ -30,7 +30,7 @@ use std::ops::{Deref, Range};
 use ecow::EcoString;
 use typst_syntax::package::PackageSpec;
 use typst_syntax::{FileId, Source, Span};
-use typst_utils::LazyHash;
+use typst_utils::{LazyHash, SmallBitSet};
 
 use crate::diag::FileResult;
 use crate::foundations::{Array, Bytes, Datetime, Dict, Module, Scope, Styles, Value};
@@ -161,9 +161,10 @@ pub struct Library {
     /// The default style properties (for page size, font selection, and
     /// everything else configurable via set and show rules).
     pub styles: Styles,
-    /// The standard library as a value.
-    /// Used to provide the `std` variable.
+    /// The standard library as a value. Used to provide the `std` variable.
     pub std: Value,
+    /// In-development features that were enabled.
+    pub features: Features,
 }
 
 impl Library {
@@ -186,6 +187,7 @@ impl Default for Library {
 #[derive(Debug, Clone, Default)]
 pub struct LibraryBuilder {
     inputs: Option<Dict>,
+    features: Features,
 }
 
 impl LibraryBuilder {
@@ -195,16 +197,58 @@ impl LibraryBuilder {
         self
     }
 
+    /// Configure in-development features that should be enabled.
+    ///
+    /// No guarantees whatsover!
+    pub fn with_features(mut self, features: Features) -> Self {
+        self.features = features;
+        self
+    }
+
     /// Consumes the builder and returns a `Library`.
     pub fn build(self) -> Library {
         let math = math::module();
         let inputs = self.inputs.unwrap_or_default();
         let global = global(math.clone(), inputs);
         let std = Value::Module(global.clone());
-        Library { global, math, styles: Styles::new(), std }
+        Library {
+            global,
+            math,
+            styles: Styles::new(),
+            std,
+            features: self.features,
+        }
     }
 }
 
+/// A selection of in-development features that should be enabled.
+///
+/// Can be collected from an iterator of [`Feature`]s.
+#[derive(Debug, Default, Clone, Hash)]
+pub struct Features(SmallBitSet);
+
+impl Features {
+    /// Check whether the given feature is enabled.
+    pub fn is_enabled(&self, feature: Feature) -> bool {
+        self.0.contains(feature as usize)
+    }
+}
+
+impl FromIterator<Feature> for Features {
+    fn from_iter<T: IntoIterator<Item = Feature>>(iter: T) -> Self {
+        let mut set = SmallBitSet::default();
+        for feature in iter {
+            set.insert(feature as usize);
+        }
+        Self(set)
+    }
+}
+
+/// An in-development feature that should be enabled.
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
+#[non_exhaustive]
+pub enum Feature {}
+
 /// Construct the module with global definitions.
 fn global(math: Module, inputs: Dict) -> Module {
     let mut global = Scope::deduplicating();