Add support for custom deserializers on model fields

[jhendrixMSFT]

This uses the @clientOption decorator to specify a custom deserializer function. Note that this will override any deserialize_with that would have been created by default.

[Copilot]

Pull request overview

Adds support in the TypeSpec Rust emitter for specifying a custom serde deserialize_with function on individual model fields via the @clientOption decorator.

Changes:

• Introduces a model-field customization pipeline (customizations) and wires it through the code model.
• Updates the tcgc adapter to read @clientOption on model properties and record deserialize_with.
• Updates model codegen to emit a custom #[serde(deserialize_with = "...")] and adds a new end-to-end test fixture/workspace crate.

Reviewed changes

Copilot reviewed 12 out of 17 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
packages/typespec-rust/src/tcgcadapter/adapter.ts	Reads @clientOption from property decorators and stores a deserializeWith customization.
packages/typespec-rust/src/codemodel/customizations.ts	Adds a customization type/class (DeserializeWith) for model fields.
packages/typespec-rust/src/codemodel/types.ts	Adds customizations to ModelField and initializes it.
packages/typespec-rust/src/codemodel/index.ts	Re-exports customization types.
packages/typespec-rust/src/codegen/models.ts	Emits serde deserialize_with based on stored customization.
packages/typespec-rust/test/tsp/ClientOption/main.tsp	New TypeSpec test input applying @@clientOption(..., "deserialize_with", ..., "rust").
packages/typespec-rust/test/other/client_option/src/lib.rs	Test crate providing the referenced deserializer function.
packages/typespec-rust/test/other/client_option/src/generated/models/models.rs	Expected generated model with #[serde(deserialize_with = "...")].
packages/typespec-rust/test/other/client_option/src/generated/models/mod.rs	Generated models module plumbing for the test crate.
packages/typespec-rust/test/other/client_option/src/generated/mod.rs	Generated top-level module plumbing for the test crate.
packages/typespec-rust/test/other/client_option/Cargo.toml	New test crate manifest.
packages/typespec-rust/test/Cargo.toml	Adds the new test crate to the workspace members.
packages/typespec-rust/test/Cargo.lock	Adds lock entry for the new test crate.
packages/typespec-rust/.scripts/tspcompile.js	Adds compilation/generation step for the new ClientOption test.
packages/typespec-rust/package.json	Bumps @azure-tools/typespec-client-generator-core dependency and peer range.
packages/typespec-rust/pnpm-lock.yaml	Lockfile update for the dependency bump.
packages/typespec-rust/CHANGELOG.md	Documents the new @clientOption support.

Files not reviewed (1)

• packages/typespec-rust/pnpm-lock.yaml: Language not supported

[Copilot]

The new override skips addSerDeHelper entirely when a custom deserialize_with is present. addSerDeHelper also emits required non-deserialization serde params (e.g. serialize_with for base64/literal helpers, with = "azure_core::time::..." for date-times). Skipping it can break compilation or serialization; instead, keep the helper generation and only override the deserialize portion.

Suggested change

	if (deserializeWith) {
	serdeParams.add(`deserialize_with = "${deserializeWith.name}"`);
	} else if (unwrappedType.kind === 'encodedBytes' || unwrappedType.kind === 'enumValue' || unwrappedType.kind === 'literal' || unwrappedType.kind === 'offsetDateTime' || encodeAsString(unwrappedType)) {
	addSerDeHelper(field, serdeParams, bodyFormat, use);
	// always generate serde helpers for supported types; custom deserialize_with will override only the
	// deserialize_with portion while preserving other helper-generated serde params
	if (unwrappedType.kind === 'encodedBytes' || unwrappedType.kind === 'enumValue' || unwrappedType.kind === 'literal' || unwrappedType.kind === 'offsetDateTime' || encodeAsString(unwrappedType)) {
	addSerDeHelper(field, serdeParams, bodyFormat, use);
	}
	if (deserializeWith) {
	// remove any existing deserialize_with param emitted by addSerDeHelper so the custom one takes precedence
	for (const param of Array.from(serdeParams)) {
	if (param.startsWith('deserialize_with')) {
	serdeParams.delete(param);
	}
	}
	serdeParams.add(`deserialize_with = "${deserializeWith.name}"`);

[jhendrixMSFT]

Hmm I might need to retool this a bit.