feat: propagate unbound state to all descendant custom elements

[janechu]

Pull Request

📖 Description

This change updates the microsoft-fast-build Rust crate so that all custom elements — whether at the entry level or deeply nested — receive the current root state as a base for their child state. Per-element HTML attributes are overlaid on top, with attribute-derived values taking precedence over any matching state keys.

Previously, only entry-level (root) custom elements received the full root state; nested elements built their state entirely from HTML attributes. This meant that if a child element template referenced a state key like {{text}}, the parent element had to explicitly pass it via an attribute binding (e.g. <my-child text="{{text}}">). With this change, unbound state keys propagate through the entire element tree automatically.

Example

<!-- state.json: {"text": "Hello world"} -->

<!-- my-el template -->
<f-template name="my-el">
    <template>
        {{text}}
        <my-child-el></my-child-el>
    </template>
</f-template>

<!-- my-child-el template — {{text}} resolves automatically -->
<f-template name="my-child-el">
    <template>
        {{text}}
    </template>
</f-template>

Both elements now render "Hello world" without explicit attribute bindings.

👩‍💻 Reviewer Notes

• The core change is in crates/microsoft-fast-build/src/directive.rs — the render_custom_element function now always uses the current root state as a base for child state (removing the is_entry branching for state building).
• The is_entry flag still controls opening-tag attribute handling (root elements resolve primitive bindings, strip non-primitives; nested elements render resolved values and inject data-fe-c markers).
• The test test_nested_custom_element_still_uses_attr_state was updated to test_nested_custom_element_inherits_parent_state to reflect the new expected behavior.
• DESIGN.md and README.md for the crate have been updated.

📑 Test Plan

• Updated existing test (test_nested_custom_element_inherits_parent_state) to verify state propagation works.
• Added 4 new tests:
  • test_unbound_state_propagates_to_child_elements — exact scenario from the spec
  • test_state_propagates_through_multiple_levels — three levels of nesting
  • test_child_attr_overrides_propagated_state — attribute values take precedence
  • test_non_entry_render_also_propagates_state — non-entry mode also propagates
• All 250 Rust crate tests pass.
• Full npm build and 10,917 npm tests pass.

✅ Checklist

General

• I have included a change request file using $ npm run change
• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.

[Copilot]

Pull request overview

Updates the microsoft-fast-build Rust renderer so nested custom elements inherit their parent/root state by default, allowing unbound state keys (e.g. {{text}}) to resolve throughout the custom element tree without requiring explicit attribute bindings.

Changes:

• Adjusts render_custom_element state construction to always base child state on the current root/parent state, then overlay per-element attributes.
• Updates and adds Rust tests to validate state propagation through nested custom elements and override precedence.
• Updates crate docs (README + DESIGN) to reflect the new state propagation model and clarify is_entry behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
crates/microsoft-fast-build/src/directive.rs	Changes child state building for custom elements to always inherit current root/parent state before overlaying attrs.
crates/microsoft-fast-build/tests/custom_elements.rs	Renames/updates an existing nested-element test and adds new tests for propagation, multi-level nesting, overrides, and non-entry rendering.
crates/microsoft-fast-build/README.md	Documents state propagation to nested elements and clarifies entry/root element attribute handling behavior.
crates/microsoft-fast-build/DESIGN.md	Updates internal design docs to reflect that is_entry now only affects opening-tag attribute handling, not child state building.