xmlschema 0.0.8

XML Schema (XSD) validation for Rust, with zero unsafe code
Documentation

[!NOTE] The rewrite has landed. Schema parsing, structural validation, simple types with restriction facets, and xs:pattern all work; see Status for exactly what is and is not supported.

0.0.1 on crates.io is the old crate, which exposed no public API at all. Do not use it.

Contents

Getting started

The oxml ecosystem

Reference

Practical

Status

State
Schema parsing ✅ elements, model groups, cardinality, attributes
Simple types ✅ all 44 built-ins, 12 restriction facets
xs:list and xs:union ✅
xs:all, xs:group, xs:attributeGroup ✅
xs:any and xs:anyAttribute ✅ namespace and processContents
Complex-type derivation ✅ extension, restriction, and Particle Valid (Restriction)
Schema validity ✅ XSD's own structural rules
xs:pattern ✅ own engine, XSD dialect including class subtraction
Diagnostics ✅ every violation, each with a path
Conformance ✅ 95.0% of the W3C suite's decided tests, ratcheted
Tests ✅ 236, plus the conformance suite
Identity constraints (key, keyref, unique) ✗
xs:import / include ✗
Substitution groups ✗

An unsupported construct is skipped rather than rejected: the surrounding rules still apply, so a schema using one validates everything else correctly instead of failing wholesale.

What was skipped is reported. support::unsupported audits a schema against what this crate enforces and names everything it does not, so "this document is valid" and "this document was checked" are never confused for one another.

Install

[dependencies]
xmlschema = { git = "https://github.com/sebastienrousseau/xmlschema" }
oxml = { git = "https://github.com/sebastienrousseau/oxml" }

Published releases follow once the suite cuts its first version together.

Quick Start

use xmlschema::{parse_schema, validate};

let xsd = r#"
  <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
    <xs:element name="book">
      <xs:complexType>
        <xs:sequence>
          <xs:element name="title" type="xs:string"/>
        </xs:sequence>
        <xs:attribute name="lang" type="xs:string" use="required"/>
      </xs:complexType>
    </xs:element>
  </xs:schema>
"#;

let schema = parse_schema(xsd)?;
let doc = oxml::parse("<book lang='en'><title>Dune</title></book>")?;

assert!(validate(&doc, &schema).is_valid());
# Ok::<(), Box<dyn std::error::Error>>(())

Every violation is reported, each with a path:

/invoice/issued — `22/08/2026` is not a valid date (YYYY-MM-DD)
/invoice/line[1]/@currency — `pounds` does not match the pattern `[A-Z]{3}`
/invoice/line[1]/amount — -5 must be greater than 0
/invoice/line[2] — missing required attribute `currency`
/invoice/line[2]/amount — `not a number` is not a valid decimal

Why this crate exists

Rust has no pure-Rust XSD validator. The options today are:

  • libxml — bindings to libxml2. Complete and battle-tested, but it is C: it needs a build toolchain, contains unsafe, does not work in WebAssembly, and inherits libxml2's CVE stream.
  • Nothing else. There is no maintained pure-Rust implementation.

For a project already committed to safe Rust — no C toolchain, WASM targets, an auditable dependency tree — that is not a choice so much as an absence.

xmlschema exists to close it, with the same constraints as the rest of the suite: #![forbid(unsafe_code)], no FFI, no C.

The oxml ecosystem

Every member ships the same version number, so there is never a compatibility table to consult.

Crate What it is Status
oxml Core — parser, tree, XPath 1.0 Available
oxml-cli Command-line querying and formatting Available
oxml-lsp XML analysis and linting; the LSP transport is not yet implemented Available
oxml-mcp Model Context Protocol server Available
oxml-wasm WebAssembly bindings Available
xmlschema XSD validation Being rewritten

This crate keeps its published name rather than being folded into oxml. The name means XSD validation specifically, and repurposing it into a general toolkit would have handed existing users something entirely different under a name they already depend on.

Ecosystem comparison

Crate XSD validation Pure Rust WASM Status
xmlschema ✅ 95.0% of the W3C suite's decided tests ✅ ✅ active
libxml ✅ ✗ (C-FFI) ✗ active
quick-xml ✗ ✅ ✅ active
roxmltree ✗ ✅ ✅ active
xot ✗ ✅ ✅ active

libxml remains the more complete implementation. The difference is what it costs: a C toolchain, unsafe, no WebAssembly target, and libxml2's CVE stream. This crate trades completeness for those.

What is not implemented

Everything in the list this section used to hold has shipped — schema parsing, structural validation, simple types, complex types and diagnostics are all ✅ in Status above. What remains:

  1. Identity constraints — xs:key, xs:keyref, xs:unique.
  2. Import mechanisms — xs:import, xs:include, xs:redefine. These come after the core is correct, because they multiply the surface without adding validation power.
  3. Substitution groups, and the undecidable corners of derivation validity.

An unsupported construct is skipped rather than rejected, and the conformance harness counts such a test as unsupported whatever answer it produced — so the published rate never flatters itself with accidental agreement.

Benchmarks

cargo bench --bench schema     # parsing an .xsd into the model
cargo bench --bench validate   # validating documents against it
cargo bench --bench pattern    # the `xs:pattern` engine

Three benchmarks, split because they answer different questions. Parsing a schema is the expensive half and happens once; validating is the cheap half and repeats. The pattern engine is separate because it is a regex implementation of its own, in XSD's dialect rather than PCRE's.

No absolute figures are published here. The same benchmarks on this machine returned confidence intervals spanning 566–906 µs for a single case — a spread wider than most changes worth measuring — because the machine was busy. A figure without its conditions is not a measurement. See oxml's BENCHMARKS.md for the method and what a published number has to carry.

Reading a report

validate returns a Report, not a Result. A document can be wrong in several independent ways, and stopping at the first means fixing them one build at a time.

/order: missing required attribute `id`
/order: expected `customer` exactly once, found 0
/order/line[1]: expected `sku` exactly once, found 0
/order/line[1]/qty: `many` is not a valid integer
/order/line[1]/sku: unexpected element `sku`; this content model allows sku, qty in that order

Each Violation carries a path and a message. The path is positional — line[1] is the first line child — so it identifies one element rather than a set.

Migration

From xmllint --schema

xmllint xmlschema
xmllint --schema s.xsd --noout f.xml validate(&parse(xml)?, &parse_schema(xsd)?)
exit status report.is_valid()
stderr text report.violations, each with a path
--schema with xs:import not supported yet

The useful difference is that violations are data rather than a stream of text to grep.

From libxml's XmlSchemaValidationContext

libxml xmlschema
SchemaParserContext::from_buffer parse_schema
SchemaValidationContext::validate_document validate
error callbacks report.violations
a libxml2 C dependency none

libxml2 implements XSD 1.0 completely and this crate does not — see Status. If you need xs:import, identity constraints or complex-type derivation today, stay.

Examples

examples/ is compiled and run in CI.

Example What it shows
validate Parsing a schema once, validating many, and reading a Report 
cargo run --example validate

When not to use xmlschema

  • You need complete XSD 1.0. This is early; check Status against your schemas first.
  • You need XSD 1.1 — assertions, conditional type assignment. Xerces has it.
  • Your schemas use xs:import or xs:include. Not supported; those constructs are skipped, so validation is incomplete rather than wrong.
  • You need identity constraints — xs:key, xs:keyref, xs:unique.
  • You need to validate while streaming. The document is parsed in full first.

FAQ

Why does an unsupported construct get skipped rather than rejected?

Because a schema using one construct this crate lacks would otherwise be unusable in full. Skipping means the surrounding rules still apply, so a schema with an xs:all block validates everything else correctly.

The cost is that a document can be reported valid when a construct that was skipped would have rejected it. Validation is incomplete, not wrong — and the distinction matters, so check Status before relying on a pass.

Why is xs:pattern a hand-written engine?

Because XSD's regular expression dialect is not PCRE and not Rust's regex. It has different anchoring semantics — the whole value must match — its own character-class escapes, and Unicode block and category escapes that neither crate spells the same way.

Using a general-purpose engine would mean translating one dialect into another and being subtly wrong at the edges. The engine is a few hundred lines and does exactly what the specification says.

Is a schema reusable across documents?

Yes, and that is the intended shape. parse_schema is the expensive half; validate is the half you repeat. A Schema is immutable after parsing.

Does it fetch schemas over the network?

No. parse_schema takes the schema's text. There is no code that opens a file or a socket, which is also why xs:import and xs:include are not supported — they name a location to fetch.

When they arrive, the shape will be a caller-supplied map from location to content, never a fetch.

What does a path like /order/line[1]/qty mean?

The qty child of the first line child of order. It is positional so that it identifies one element and not a set — which is what you need when the message is "this one is wrong".

Does it validate the schema itself?

Partly. It rejects a schema that is not well-formed XML, reports what it cannot understand, and enforces the structural rules XSD imposes on schemas themselves — where xs:annotation may appear, which children are mutually exclusive, that two element declarations of one name must agree on their type, that a facet's value must belong to the type it narrows, and that a restriction's content model must be a valid restriction of its base.

It does not validate a schema against the full XSD schema-for-schemas, which would be a second validator.

How is this tested?

236 tests over schema parsing, every built-in type, every facet, the pattern engine, the validator and the derivation relation. The XML underneath carries the W3C XML conformance suite — 2,557 of 2,557 decided tests, zero panics.

And the W3C XML Schema Test Suite, xsts-2007-06-20, pinned by SHA-256: 39,420 tests, of which 95.0% of the decided ones pass, with zero panics. This was the main gap in the crate's verification until 0.0.6, and closing it is most of what 0.0.6 is.

A pass rate over that suite is only worth reporting because of how it is counted. This crate implements a subset of XSD and skips what it does not understand — and a schema whose constraints were all skipped accepts every document, so agreeing with a test proves nothing. A test counts as a pass only when the schema is enforced in full. On the first run, 20,682 tests would otherwise have counted as passes with nothing checked; that figure is published alongside the rate, in doc/CONFORMANCE.md.

Development

./scripts/gate.sh

That runs everything CI runs, in the order that fails fastest: format, clippy, tests, rustdoc, the #![forbid(unsafe_code)] check, the example, the W3C XSD conformance suite, the 95% coverage floor and an MSRV build. It pins the toolchain rather than trusting rust-toolchain.toml, because a RUSTUP_TOOLCHAIN in the environment silently overrides that file and a lint that exists in one release and not another then makes a green local run and a red CI one.

The conformance step is skipped loudly when the suite has not been downloaded, and counts as a failure rather than vanishing. A skipped conformance test is a passing one as far as cargo test is concerned, and this crate's headline figure rests on that suite.

The individual steps, if you want them one at a time:

cargo test --all-features
cargo clippy --workspace --all-targets --all-features -- -D warnings
cargo fmt --all --check
cargo bench --bench schema
cargo run --example validate
cargo run --release -p xmlschema-conformance --bin download
cargo test --release -p xmlschema-conformance

CI runs the same set on Linux, macOS and Windows.

Security

XSD validation is normally applied to untrusted documents, which makes the parser's threat model part of this crate's threat model. It inherits oxml's posture:

  • No entity expansion. Only the five predefined entities and numeric character references are resolved, so XXE and billion-laughs are foreclosed by construction rather than by a flag.
  • No unsafe. #![forbid(unsafe_code)], enforced at compile time.

Report vulnerabilities privately — see SECURITY.md.

Documentation

Acknowledgements

  • libxml2 — the reference implementation, and the yardstick for behaviour.
  • W3C — for the XML Schema specification.
  • python-xmlschema — proof that a readable, standalone XSD implementation is achievable.

License

Licensed under either of

at your option.