[designspace] Deserialize lib fields

This was about as painful as expected, and requires us to manually glue
together the xml data model with the plist model, using serde.

To briefly summarize the problem: XML data, by definition, does not have
any defined semantics. Semantics are added on top of XML on a per-case
basis, by indicating the document type inline
(<https://en.wikipedia.org/wiki/Document_type_declaration>).

The plist crate does implement derive for plist types, but they are based on the
assumption that type information is being provided by some deserializer
(e.g. for a specific file format). This is the right thing for the plist
crate to do. Unfortunately quick_xml cannot provide this type
information, since quick_xml does not know that we are deserializing a
plist.

To make this all work, I have added a bunch of manual deserialize code.
This code will only work when deserializing from XML; if in the future
we wish to serialize to other formats, we will need to revise it.

Some notes:

- This only implements deserialization, not serialization. (This is
  fine, since we do not currently serialize anyway, but we'll have to
  figure that out eventually)
- this does not support the UID type, which is not accepted in XML
  plists

Cargo.toml

@@ -28,6 +28,7 @@ rayon = { version = "1.3.0", optional = true }
 kurbo = { version = "0.9.0", optional = true }
 thiserror = "1.0"
 indexmap = {version = "2.0.0", features = ["serde"] }
+base64 = "0.21.2"
 
 [dependencies.druid]
 default-features = false

src/designspace.rs

@@ -4,7 +4,10 @@
 
 use std::{fs::File, io::BufReader, path::Path};
 
+use plist::Dictionary;
+
 use crate::error::DesignSpaceLoadError;
+use crate::serde_xml_plist as serde_plist;
 
 /// A [designspace].
 ///
@@ -24,6 +27,9 @@ pub struct DesignSpaceDocument {
     /// One or more instances.
     #[serde(default, deserialize_with = "serde_impls::deserialize_instances")]
     pub instances: Vec<Instance>,
+    /// Additional arbitrary user data
+    #[serde(default, deserialize_with = "serde_plist::deserialize_dict")]
+    pub lib: Dictionary,
 }
 
 /// An [axis].
@@ -131,6 +137,9 @@ pub struct Instance {
     /// Location in designspace.
     #[serde(deserialize_with = "serde_impls::deserialize_location")]
     pub location: Vec<Dimension>,
+    /// Arbitrary data about this instance
+    #[serde(default, deserialize_with = "serde_plist::deserialize_dict")]
+    pub lib: Dictionary,
 }
 
 /// A [design space dimension].
@@ -238,6 +247,7 @@ mod serde_impls {
 mod tests {
     use std::path::Path;
 
+    use plist::Value;
     use pretty_assertions::assert_eq;
 
     use crate::designspace::{AxisMapping, Dimension};
@@ -305,4 +315,25 @@ mod tests {
         assert!(ds.instances[1].name.starts_with("unnamed_instance_"));
         assert_ne!(ds.instances[0].name, ds.instances[1].name);
     }
+
+    #[test]
+    fn load_lib() {
+        let loaded = DesignSpaceDocument::load("testdata/wght.designspace").unwrap();
+        assert_eq!(
+            loaded.lib.get("org.linebender.hasLoadedLibCorrectly"),
+            Some(&Value::String("Absolutely!".into()))
+        );
+
+        let params = loaded.instances[0]
+            .lib
+            .get("com.schriftgestaltung.customParameters")
+            .and_then(Value::as_array)
+            .unwrap();
+        assert_eq!(params[0].as_array().unwrap()[0].as_string(), Some("xHeight"));
+        assert_eq!(params[0].as_array().unwrap()[1].as_string(), Some("536"));
+        assert_eq!(
+            params[1].as_array().unwrap()[1].as_array().unwrap()[0].as_unsigned_integer(),
+            Some(2)
+        );
+    }
 }

src/lib.rs

@@ -84,6 +84,7 @@ mod kerning;
 mod layer;
 mod name;
 mod names;
+mod serde_xml_plist;
 mod shared_types;
 mod upconversion;
 pub(crate) mod util;