feat(tvix/serde): initial Nix->serde::Deserialize impl

This will make it possible fairly easily use Nix to represent
arbitrary data structures, e.g. for using Nix as a config language.

Only pure Nix (i.e. no `import` etc.) is supported for now.

Not all types, specifically no struct traversal, are implemented in
this commit.

Change-Id: I9ac91a229a0d12bf818e6e3249f3e5a691599a2c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7712
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/serde/src/error.rs

@@ -0,0 +1,92 @@
+//! When serialising Nix goes wrong ...
+
+use std::error;
+use std::fmt::Display;
+
+#[derive(Clone, Debug)]
+pub enum Error {
+    /// Attempted to deserialise an unsupported Nix value (such as a
+    /// function) that can not be represented by the
+    /// [`serde::Deserialize`] trait.
+    Unserializable { value_type: &'static str },
+
+    /// Expected to deserialize a value that is unsupported by Nix.
+    Unsupported { wanted: &'static str },
+
+    /// Expected a specific type, but got something else on the Nix side.
+    UnexpectedType {
+        expected: &'static str,
+        got: &'static str,
+    },
+
+    /// Deserialisation error returned from `serde::de`.
+    Deserialization(String),
+
+    /// Deserialized integer did not fit.
+    IntegerConversion { got: i64, need: &'static str },
+
+    /// Evaluation of the supplied Nix code failed while computing the
+    /// value for deserialisation.
+    NixErrors {
+        errors: Vec<tvix_eval::Error>,
+        source: tvix_eval::SourceCode,
+    },
+}
+
+impl Display for Error {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Error::Unserializable { value_type } => write!(
+                f,
+                "can not deserialise a Nix '{}' into a Rust type",
+                value_type
+            ),
+
+            Error::Unsupported { wanted } => {
+                write!(f, "can not deserialize a '{}' from a Nix value", wanted)
+            }
+
+            Error::UnexpectedType { expected, got } => {
+                write!(f, "expected type {}, but got Nix type {}", expected, got)
+            }
+
+            Error::NixErrors { errors, source } => {
+                writeln!(
+                    f,
+                    "{} occured during Nix evaluation: ",
+                    if errors.len() == 1 { "error" } else { "errors" }
+                )?;
+
+                for err in errors {
+                    writeln!(f, "{}", err.fancy_format_str(&source))?;
+                }
+
+                Ok(())
+            }
+
+            Error::Deserialization(err) => write!(f, "deserialisation error occured: {}", err),
+
+            Error::IntegerConversion { got, need } => {
+                write!(f, "i64({}) does not fit in a {}", got, need)
+            }
+        }
+    }
+}
+
+impl error::Error for Error {
+    fn source(&self) -> Option<&(dyn error::Error + 'static)> {
+        match self {
+            Self::NixErrors { errors, .. } => errors.first().map(|e| e as &dyn error::Error),
+            _ => None,
+        }
+    }
+}
+
+impl serde::de::Error for Error {
+    fn custom<T>(err: T) -> Self
+    where
+        T: Display,
+    {
+        Self::Deserialization(err.to_string())
+    }
+}