1 files changed, 161 insertions, 0 deletions
diff --git a/tvix/eval/src/value/string/context.rs b/tvix/eval/src/value/string/context.rs
new file mode 100644
index 000000000000..e1c04735ddde
--- /dev/null
+++ b/tvix/eval/src/value/string/context.rs
@@ -0,0 +1,161 @@
+use rustc_hash::FxHashSet;
+use serde::Serialize;
+
+use super::NixString;
+
+#[derive(Clone, Debug, Serialize, Hash, PartialEq, Eq)]
+pub enum NixContextElement {
+    /// A plain store path (e.g. source files copied to the store)
+    Plain(String),
+
+    /// Single output of a derivation, represented by its name and its derivation path.
+    Single { name: String, derivation: String },
+
+    /// A reference to a complete derivation
+    /// including its source and its binary closure.
+    /// It is used for the `drvPath` attribute context.
+    /// The referred string is the store path to
+    /// the derivation path.
+    Derivation(String),
+}
+
+/// Nix context strings representation in Tvix. This tracks a set of different kinds of string
+/// dependencies that we can come across during manipulation of our language primitives, mostly
+/// strings. There's some simple algebra of context strings and how they propagate w.r.t. primitive
+/// operations, e.g. concatenation, interpolation and other string operations.
+#[repr(transparent)]
+#[derive(Clone, Debug, Serialize, Default)]
+pub struct NixContext(FxHashSet<NixContextElement>);
+
+impl From<NixContextElement> for NixContext {
+    fn from(value: NixContextElement) -> Self {
+        let mut set = FxHashSet::default();
+        set.insert(value);
+        Self(set)
+    }
+}
+
+impl From<FxHashSet<NixContextElement>> for NixContext {
+    fn from(value: FxHashSet<NixContextElement>) -> Self {
+        Self(value)
+    }
+}
+
+impl<const N: usize> From<[NixContextElement; N]> for NixContext {
+    fn from(value: [NixContextElement; N]) -> Self {
+        let mut set = FxHashSet::default();
+        for elt in value {
+            set.insert(elt);
+        }
+        Self(set)
+    }
+}
+
+impl NixContext {
+    /// Creates an empty context that can be populated
+    /// and passed to form a contextful [NixString], albeit
+    /// if the context is concretly empty, the resulting [NixString]
+    /// will be contextless.
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    /// For internal consumers, we let people observe
+    /// if the [NixContext] is actually empty or not
+    /// to decide whether they want to skip the allocation
+    /// of a full blown [HashSet].
+    pub(crate) fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
+    /// Consumes a new [NixContextElement] and add it if not already
+    /// present in this context.
+    pub fn append(mut self, other: NixContextElement) -> Self {
+        self.0.insert(other);
+        self
+    }
+
+    /// Extends the existing context with more context elements.
+    pub fn extend<T>(&mut self, iter: T)
+    where
+        T: IntoIterator<Item = NixContextElement>,
+    {
+        self.0.extend(iter)
+    }
+
+    /// Copies from another [NixString] its context strings
+    /// in this context.
+    pub fn mimic(&mut self, other: &NixString) {
+        if let Some(context) = other.context() {
+            self.extend(context.iter().cloned());
+        }
+    }
+
+    /// Iterates over "plain" context elements, e.g. sources imported
+    /// in the store without more information, i.e. `toFile` or coerced imported paths.
+    /// It yields paths to the store.
+    pub fn iter_plain(&self) -> impl Iterator<Item = &str> {
+        self.iter().filter_map(|elt| {
+            if let NixContextElement::Plain(s) = elt {
+                Some(s.as_str())
+            } else {
+                None
+            }
+        })
+    }
+
+    /// Iterates over "full derivations" context elements, e.g. something
+    /// referring to their `drvPath`, i.e. their full sources and binary closure.
+    /// It yields derivation paths.
+    pub fn iter_derivation(&self) -> impl Iterator<Item = &str> {
+        self.iter().filter_map(|elt| {
+            if let NixContextElement::Derivation(s) = elt {
+                Some(s.as_str())
+            } else {
+                None
+            }
+        })
+    }
+
+    /// Iterates over "single" context elements, e.g. single derived paths,
+    /// or also known as the single output of a given derivation.
+    /// The first element of the tuple is the output name
+    /// and the second element is the derivation path.
+    pub fn iter_single_outputs(&self) -> impl Iterator<Item = (&str, &str)> {
+        self.iter().filter_map(|elt| {
+            if let NixContextElement::Single { name, derivation } = elt {
+                Some((name.as_str(), derivation.as_str()))
+            } else {
+                None
+            }
+        })
+    }
+
+    /// Iterates over any element of the context.
+    pub fn iter(&self) -> impl Iterator<Item = &NixContextElement> {
+        self.0.iter()
+    }
+
+    /// Produces a list of owned references to this current context,
+    /// no matter its type.
+    pub fn to_owned_references(self) -> Vec<String> {
+        self.0
+            .into_iter()
+            .map(|ctx| match ctx {
+                NixContextElement::Derivation(drv_path) => drv_path,
+                NixContextElement::Plain(store_path) => store_path,
+                NixContextElement::Single { derivation, .. } => derivation,
+            })
+            .collect()
+    }
+}
+
+impl IntoIterator for NixContext {
+    type Item = NixContextElement;
+
+    type IntoIter = std::collections::hash_set::IntoIter<NixContextElement>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.0.into_iter()
+    }
+}