# toml-spanner
High-performance, fast compiling, span preserving toml parsing for rust.
Orginally forked from `toml-span` to add TOML 1.1.0 support, `toml-spanner`
has received significant performance improvements and reductions in compile time.
[](https://crates.io/crates/toml-spanner)
[](https://docs.rs/toml-spanner/latest/toml_spanner/)
[](LICENSE)
Like the original `toml-span` temporal values such as timestamps or local times are not supported.
## Example
```rust
use toml_spanner::{Deserialize, Error, Item};
#[derive(Debug)]
struct Things {
name: String,
value: u32,
color: Option<String>,
}
impl<'de> Deserialize<'de> for Things {
fn deserialize(value: &mut Item<'de>) -> Result<Self, Error> {
let table = value.expect_table()?;
Ok(Things {
name: table.required("name")?,
value: table.required("value")?,
color: table.optional("color")?,
})
}
}
struct Config {
things: Vec<Things>,
dev_mode: bool,
}
impl Config {
pub fn parse(content: &str) -> Result<Config, Error> {
let arena = toml_spanner::Arena::new();
let mut table = toml_spanner::parse(content, &arena)?;
let config = Config {
things: table.required("things")?,
dev_mode: table.optional("dev-mode")?.unwrap_or(false),
};
// Report unexpected fields
table.expect_empty()?;
Ok(config)
}
}
fn main() {
let content = r#"
dev-mode = true
[[things]]
name = "hammer"
value = 43
[[things]]
name = "drill"
value = 300
color = "green"
"#;
match Config::parse(content) {
Ok(config) => {
println!("dev_mode: {}", config.dev_mode);
for thing in config.things {
println!("thing: {:?}", thing);
}
}
Err(e) => eprintln!("Error parsing config: {e}"),
}
}
```
## Benchmarks
Measured on AMD Ryzen 9 5950X, 64GB RAM, Linux 6.18, rustc 1.93.0.
Relative parse time across real-world TOML files (lower is better):
```
time(μs) cycles(K) instr(K) branch(K)
zed/Cargo.toml
toml-spanner 29.8 142 501 101
toml 250.1 1196 3074 607
toml-span 381.9 1821 5057 1050
extask.toml
toml-spanner 11.5 55 177 33
toml 81.1 387 1017 196
toml-span 108.4 517 1350 268
devsm.toml
toml-spanner 4.0 19 73 15
toml 34.9 167 439 85
toml-span 59.3 283 732 146
```
### Compile Time
Extra `cargo build --release` time for binaries using respective crates (lower is better):
```
median(ms) added(ms)
null 101
toml-spanner 673 +572
toml-span 1393 +1292
toml 3088 +2987
toml+serde 5214 +5113
```
Checkout `./benchmark` for more details, but numbers should simulate the additional
time added users would experience during source based installs such as via `cargo install`.
## Divergence from `toml-span`
While `toml-spanner` star
ted as a fork of `toml-span`, it has since undergone
extensive changes:
- 10x faster than `toml-span`, and 5-8x faster than `toml` across
real-world workloads.
- Preserved index order: tables retain their insertion order by default,
unlike `toml_span` and the default mode of `toml`.
- Compact `Value` type (on 64bit platforms):
| Crate | Value/Item | TableEntry |
| --------------------- | ---------- | ---------- |
| **toml-spanner** | 24 bytes | 48 bytes |
| toml-span | 48 bytes | 88 bytes |
| toml | 32 bytes | 56 bytes |
| toml (preserve_order) | 80 bytes | 104 bytes |
Note that the `toml` crate `Value` type doesn't contain any span information
and that `toml-span` doesn't support table entry order preservation.
### Trade-offs
`toml-spanner` makes extensive use of `unsafe` code to achieve its performance
and size goals. This is mitigated by fuzzing and running the test suite under
Miri.
### Testing
The `unsafe` in this crate demands thorough testing. The full suite includes
[Miri](https://github.com/rust-lang/miri) for detecting undefined behavior,
fuzzing against the reference `toml` crate, and snapshot-based integration
tests — currently at **97.6%** branch coverage.
```bash
cargo test --workspace # all tests
cargo test -p integ-tests # integration tests only
cargo +nightly miri nextest run # undefined behavior checks
cargo +nightly fuzz run parse_compare_toml # fuzz against the toml crate
cargo +nightly fuzz run parse_value # fuzz the parser directly
```
Integration tests use [insta](https://insta.rs/) for snapshot assertions.
Run `cargo insta test -p integ-tests` and `cargo insta review` to review
changes.
Code coverage:
```bash
cargo +nightly llvm-cov --branch --show-missing-lines -- -q
```
## Differences from `toml`
First off I just want to be up front and clear about the differences/limitations of this crate versus `toml`
1. No `serde` support for deserialization, there is a `serde` feature, but that only enables serialization of the `Value` and `Spanned` types.
1. No toml serialization. This crate is only intended to be a span preserving deserializer, there is no intention to provide serialization to toml, especially the advanced format preserving kind provided by `toml-edit`.
1. No datetime deserialization. It would be trivial to add support for this (behind an optional feature), I just have no use for it at the moment. PRs welcome.
### License
This contribution is dual licensed under EITHER OF
- Apache License, Version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or <http://www.apache.org/licenses/LICENSE-2.0>)
- MIT license ([LICENSE-MIT](LICENSE-MIT) or <http://opensource.org/licenses/MIT>)
at your option.