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

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Merged
merged 1 commit into from
Aug 12, 2025
Merged

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

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48 changes: 38 additions & 10 deletions Cargo.lock
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3 changes: 2 additions & 1 deletion Cargo.toml
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Original file line number Diff line number Diff line change
Expand Up @@ -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"]
Expand All @@ -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"
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20 changes: 18 additions & 2 deletions src/file/format/json5.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -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),
Expand All @@ -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))
Comment on lines +29 to +30
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Since this PR is partially motivated by performance, it is worth knowing about dtolnay/erased-serde#61 (which is used by serde-untagged). For very deeply nested config you would see O(n²) performance in the new implementation compared to O(n) in the previous one. Ultimately the best performance would be from a more verbose impl that resembles https://github.com/serde-rs/json/blob/v1.0.142/src/value/de.rs#L22, which completely eliminates the quadratic factor and also reduces linear factor from heap allocations inside erased-serde and constant factor from heap allocations inside UntaggedEnumVisitor.

.deserialize(d)
}
}

pub(crate) fn parse(
uri: Option<&String>,
text: &str,
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2 changes: 0 additions & 2 deletions src/ser.rs
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Expand Up @@ -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::*;
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