perf(json5): Remove use of serde(untagged) (#677)

Prep for #472

`serde(untagged)` translates to an `if`/`else if` ladder that ends up
being pretty expensive, including the error generation for each branch
that gets skipped. This switches us to dispatch on type instead,
speeding things up.

This is also prep for speeding up builds by removing the need for
`serde_derive`. Once `serde_core` is published, we can depend on that
and not wait for `syn`, `quote`, etc.

Author: epage

Cargo.lock

@@ -125,7 +125,7 @@ default = ["toml", "json", "yaml", "ini", "ron", "json5", "convert-case", "async
 json = ["serde_json"]
 yaml = ["yaml-rust2"]
 ini = ["rust-ini"]
-json5 = ["json5_rs", "serde/derive"]
+json5 = ["json5_rs", "dep:serde-untagged"]
 convert-case = ["convert_case"]
 preserve_order = ["indexmap", "toml?/preserve_order", "serde_json?/preserve_order", "ron?/indexmap"]
 async = ["async-trait"]
@@ -145,6 +145,7 @@ indexmap = { version = "2.10.0", features = ["serde"], optional = true }
 convert_case = { version = "0.6", optional = true }
 pathdiff = "0.2"
 winnow = "0.7.0"
+serde-untagged = { version = "0.1.8", optional = true }
 
 [dev-dependencies]
 serde_derive = "1.0"

Cargo.toml

@@ -4,8 +4,7 @@ use crate::format;
 use crate::map::Map;
 use crate::value::{Value, ValueKind};
 
-#[derive(serde::Deserialize, Debug)]
-#[serde(untagged)]
+#[derive(Debug)]
 pub(crate) enum Val {
     Null,
     Boolean(bool),
@@ -16,6 +15,23 @@ pub(crate) enum Val {
     Object(Map<String, Self>),
 }
 
+impl<'de> serde::de::Deserialize<'de> for Val {
+    fn deserialize<D>(d: D) -> Result<Self, D::Error>
+    where
+        D: serde::de::Deserializer<'de>,
+    {
+        serde_untagged::UntaggedEnumVisitor::new()
+            .bool(|value| Ok(Self::Boolean(value)))
+            .i64(|value| Ok(Self::Integer(value)))
+            .f64(|value| Ok(Self::Float(value)))
+            .string(|value| Ok(Val::String(value.to_owned())))
+            .none(|| Ok(Self::Null))
+            .seq(|value| value.deserialize().map(Val::Array))
+            .map(|value| value.deserialize().map(Val::Object))
+            .deserialize(d)
+    }
+}
+
 pub(crate) fn parse(
     uri: Option<&String>,
     text: &str,

src/file/format/json5.rs

@@ -676,8 +676,6 @@ impl ser::SerializeStructVariant for Unreachable {
 
 #[cfg(test)]
 mod test {
-    use serde::{Deserialize, Serialize};
-    #[cfg(not(feature = "json5"))]
     use serde_derive::{Deserialize, Serialize};
 
     use super::*;