[libgraphql-core-v1] Task 16.6b: Implement type extension merging

[jeffmo]

Summary

Task 16.6b (second of the six schema-hardening PRs): implement type-extension and schema-extension support. Previously SchemaBuilder silently dropped every extension (TypeExtension(_) | SchemaExtension(_) => {}) — schema data loss relative to v0, with ExtensionOfUndefinedType and InvalidExtensionTypeKind sitting as dead error kinds.

Design

• All 6 type-extension kinds merge in place onto the registered type: Object/Interface append fields + implements entries; Enum appends values; Union appends members; InputObject appends input fields; Scalar appends directives; every kind appends extension directives. Duplicate contributions reuse the existing duplicate error kinds with "first defined here" + spec notes. Per-item error accumulation: a duplicate is skipped, the extension's remaining valid contributions still merge (v1's error-accumulation style; v0 aborted on first duplicate).
• Extension-before-definition defers via owned PendingTypeExtension entries (new schema/pending_type_extension.rs; data extracted eagerly from the borrowed AST via the existing ast_helpers/builder conversions), applied in arrival order when the target is absorbed — v0-parity behavior, resolving the plan's Unresolved Question 3. Extensions still pending at build() each produce ExtensionOfUndefinedType. Kind mismatch produces InvalidExtensionTypeKind, which now carries actual-vs-extension GraphQLTypeKind for an informative message.
• extend schema merges root operations via load_root_operations(), factored out of load_schema_definition so both paths share duplicate handling. Works with implicitly-defined schemas (no explicit schema {} block), matching graphql-js. Schema-level directive annotations aren't stored on Schema at all yet (true for schema {} definitions too) — recorded honestly in the plan notes.
• @oneOf × extensions, both directions (surfaced by the pre-PR Opus review, which caught my initial implementation violating Input Object Extensions rule 5): extend input X @oneOf is rejected with the new OneOfDirectiveProvidedByInputObjectExtension error and the directive is not merged — even an all-nullable input must not become oneOf via extension. Conversely, a non-nullable field contributed by an extension to an already-@oneOf input is caught by the 16.6a constraints, because validation runs over the fully-merged type (merge-before-validate ordering, now locked in by test).
• Extension-contributed fields get the same __-prefix rejection as the definition path.

Ordering-correctness notes (review-verified)

Pending extensions are consumed on the first successful type registration, and the duplicate-type check returns before consumption — so a duplicate second definition cannot double-apply extensions. Ext-vs-ext collisions are caught (the second extension sees the first's merged contribution). A mismatched-kind extension errors without affecting sibling extensions.

Tests

39 new tests (v1 crate: 217 → 256): per-kind merge / extension-before-definition / undefined target / kind mismatch / duplicate contribution; implements merge + duplicate; dunder rejections; extend schema merge + duplicate root-op + implicit-schema case; spec-note assertions; oneOf rule-5 rejection (all-nullable smoking-gun + not-merged-no-constraint-errors) and rule-6 merged-field constraint.

Tracking

Non-repeatable-directive uniqueness across extensions ("must not already apply to the previous type", present in every extension section) is owned by Task 16.6f's directive-application validator, which the plan now requires to run over merged types.

Verification

• cargo test --workspace — 1361 tests green
• cargo clippy --workspace --tests -- -Dwarnings — clean
• Opus sub-agent review pre-PR: 1 spec blocker found & fixed (rule 5 above), 2 coverage gaps filled (rule-6 test, implicit-schema test), ordering/dunder/convention checks passed
• Plan doc updated with [x] boxes + Completion Notes per the Execution Protocol

🤖 Generated with Claude Code

https://claude.ai/code/session_01TipQtpvuLuHjHmRsVEgwJw

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Generated by Claude Code

[Copilot]

Pull request overview

Implements GraphQL SDL schema/type extension support in libgraphql-core-v1’s SchemaBuilder, fixing prior behavior where all extensions were silently dropped and enabling v0-parity merging semantics (including extension-before-definition deferral) with error accumulation.

Changes:

• Add owned pending-extension infrastructure and merge logic for all type extension kinds (object/interface/enum/union/input/scalar), including duplicate detection and kind-mismatch/undefined-target errors.
• Implement extend schema { ... } root operation merging by factoring shared root-op loading/duplicate handling.
• Add a comprehensive new test suite covering merge behavior, ordering, error cases, extend schema, __-name rejection, and @oneOf × extensions rule handling.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
libgraphql-core-v1-plan.md	Marks Task 16.6b complete and documents the final design/semantics (pending extensions, merge-before-validate, schema extension behavior, oneOf rule 5).
crates/libgraphql-core-v1/src/schema/tests/schema_builder_extension_tests.rs	Adds extensive tests for all extension kinds, ordering, duplicates, undefined targets, kind mismatch, schema extensions, and oneOf-related rules.
crates/libgraphql-core-v1/src/schema/tests/mod.rs	Registers the new extension test module.
crates/libgraphql-core-v1/src/schema/schema_builder.rs	Implements schema/type extension loading, pending extension application, build-time undefined-target errors, and merge helpers.
crates/libgraphql-core-v1/src/schema/schema_build_error.rs	Extends InvalidExtensionTypeKind diagnostics and adds OneOfDirectiveProvidedByInputObjectExtension.
crates/libgraphql-core-v1/src/schema/pending_type_extension.rs	Introduces owned PendingTypeExtension representations extracted from AST for deferral and in-place merging.
crates/libgraphql-core-v1/src/schema/mod.rs	Wires in the new pending-extension module.

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