feat(nix-compat/{aterm,derivation}): init parser

This provides a nom-based parser for Nix derivations in ATerm format,
which can be reached via `Derivation::from_aterm_bytes`.

Some of the lower-level ATerm primitives are moved into a (new) aterm
module, and some more higher-level ones that construct derivation-
specific types.

Also, move the escape_bytes function into there, this is a generic ATerm
thing.

Change-Id: I2b03b8a1461c7ea2fcb8640c2fc3d1fa3ea719fb
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9730
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

tvix/nix-compat/src/derivation/parse_error.rs

@@ -0,0 +1,73 @@
+//! This contains error and result types that can happen while parsing
+//! Derivations from ATerm.
+use nom::IResult;
+
+use crate::nixhash;
+
+pub type NomResult<I, O> = IResult<I, O, NomError<I>>;
+
+#[derive(Debug, PartialEq)]
+pub enum ErrorKind {
+    // duplicate key in map
+    DuplicateMapKey(String),
+
+    // Digest parsing error
+    NixHashError(nixhash::Error),
+
+    // error kind wrapped from native nom errors
+    Nom(nom::error::ErrorKind),
+}
+
+/// Our own error type to pass along parser-related errors.
+#[derive(Debug, PartialEq)]
+pub struct NomError<I> {
+    /// position of the error in the input data
+    pub input: I,
+    /// error code
+    pub code: ErrorKind,
+}
+
+impl<I, E> nom::error::FromExternalError<I, E> for NomError<I> {
+    fn from_external_error(input: I, kind: nom::error::ErrorKind, _e: E) -> Self {
+        Self {
+            input,
+            code: ErrorKind::Nom(kind),
+        }
+    }
+}
+
+impl<I> nom::error::ParseError<I> for NomError<I> {
+    fn from_error_kind(input: I, kind: nom::error::ErrorKind) -> Self {
+        Self {
+            input,
+            code: ErrorKind::Nom(kind),
+        }
+    }
+
+    // FUTUREWORK: implement, so we have support for backtracking through the
+    // parse tree?
+    fn append(_input: I, _kind: nom::error::ErrorKind, other: Self) -> Self {
+        other
+    }
+}
+
+/// This wraps a [nom::error::Error] into our error.
+impl<I> From<nom::error::Error<I>> for NomError<I> {
+    fn from(value: nom::error::Error<I>) -> Self {
+        Self {
+            input: value.input,
+            code: ErrorKind::Nom(value.code),
+        }
+    }
+}
+
+/// This essentially implements
+/// From<nom::Err<nom::error::Error<I>>> for nom::Err<NomError<I>>,
+/// which we can't because nom::Err<_> is a foreign type.
+pub(crate) fn into_nomerror<I>(e: nom::Err<nom::error::Error<I>>) -> nom::Err<NomError<I>> {
+    match e {
+        nom::Err::Incomplete(n) => nom::Err::Incomplete(n),
+        nom::Err::Error(e) => nom::Err::Error(e.into()),
+        nom::Err::Failure(e) => nom::Err::Failure(e.into()),
+    }
+}