refactor(tvix/castore): drop {Directory,File,Symlink}Node

Add a `SymlinkTarget` type to represent validated symlink targets.
With this, no invalid states are representable, so we can make `Node` be
just an enum of all three kind of types, and allow access to these
fields directly.

Change-Id: I20bdd480c8d5e64a827649f303c97023b7e390f2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12216
Reviewed-by: benjaminedwardwebb <benjaminedwardwebb@gmail.com>
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: Connor Brewster <cbrewster@hey.com>
Tested-by: BuildkiteCI

tvix/castore/src/nodes/symlink_target.rs

@@ -0,0 +1,82 @@
+// TODO: split out this error
+use crate::ValidateNodeError;
+
+use bstr::ByteSlice;
+use std::fmt::{self, Debug, Display};
+
+/// A wrapper type for symlink targets.
+/// Internally uses a [bytes::Bytes], but disallows empty targets and those
+/// containing null bytes.
+#[repr(transparent)]
+#[derive(Clone, PartialEq, Eq)]
+pub struct SymlinkTarget {
+    inner: bytes::Bytes,
+}
+
+impl AsRef<[u8]> for SymlinkTarget {
+    fn as_ref(&self) -> &[u8] {
+        self.inner.as_ref()
+    }
+}
+
+impl From<SymlinkTarget> for bytes::Bytes {
+    fn from(value: SymlinkTarget) -> Self {
+        value.inner
+    }
+}
+
+impl TryFrom<bytes::Bytes> for SymlinkTarget {
+    type Error = ValidateNodeError;
+
+    fn try_from(value: bytes::Bytes) -> Result<Self, Self::Error> {
+        if value.is_empty() || value.contains(&b'\0') {
+            return Err(ValidateNodeError::InvalidSymlinkTarget(value));
+        }
+
+        Ok(Self { inner: value })
+    }
+}
+
+impl TryFrom<&'static [u8]> for SymlinkTarget {
+    type Error = ValidateNodeError;
+
+    fn try_from(value: &'static [u8]) -> Result<Self, Self::Error> {
+        if value.is_empty() || value.contains(&b'\0') {
+            return Err(ValidateNodeError::InvalidSymlinkTarget(
+                bytes::Bytes::from_static(value),
+            ));
+        }
+
+        Ok(Self {
+            inner: bytes::Bytes::from_static(value),
+        })
+    }
+}
+
+impl TryFrom<&str> for SymlinkTarget {
+    type Error = ValidateNodeError;
+
+    fn try_from(value: &str) -> Result<Self, Self::Error> {
+        if value.is_empty() {
+            return Err(ValidateNodeError::InvalidSymlinkTarget(
+                bytes::Bytes::copy_from_slice(value.as_bytes()),
+            ));
+        }
+
+        Ok(Self {
+            inner: bytes::Bytes::copy_from_slice(value.as_bytes()),
+        })
+    }
+}
+
+impl Debug for SymlinkTarget {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        Debug::fmt(self.inner.as_bstr(), f)
+    }
+}
+
+impl Display for SymlinkTarget {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        Display::fmt(self.inner.as_bstr(), f)
+    }
+}