fix(fast-build): :prop bindings forward typed state to child without rendering as HTML attribute

[janechu]

Pull Request

📖 Description

Attributes with a : prefix (FAST property bindings) are now forwarded to the child element's SSR rendering state but are stripped from the rendered HTML output. Previously, :prop bindings were skipped entirely — not added to child state and not rendered in HTML.

This change enables structured data (arrays, objects) to be passed from the parent state to a custom element's SSR template — for use with f-repeat and similar directives — without the value appearing as a visible string attribute (e.g. [Object]) in the rendered HTML.

Attribute form	In rendered HTML	In child state
:items="{{items}}"	❌ stripped	✅ {"items": <array from parent state>}
items="{{items}}"	✅ rendered	✅ {"items": <array from parent state>}
@click="{handler()}"	❌ stripped	❌ skipped
f-ref, f-slotted, f-children	❌ stripped	❌ skipped

Example

<!-- entry.html -->
<item-list :items="{{items}}"></item-list>

<!-- templates.html -->
<f-template name="item-list">
  <template>
    <f-repeat value="{{item in items}}"><li>{{item}}</li></f-repeat>
  </template>
</f-template>

state.json supplies {"items": ["a", "b", "c"]}. The :items binding resolves the typed array from the parent state and places it in the child state without rendering it as an HTML attribute.

📑 Test Plan

• test_custom_element_colon_property_in_state_not_html — verifies :prop resolves into child state and is absent from rendered HTML
• test_custom_element_repeat_from_colon_binding — verifies f-repeat iterates an array passed via :prop
• test_custom_element_repeat_empty_array_from_colon_binding — empty array case
• test_custom_element_object_from_colon_binding — object passed via :prop resolves in child template

All existing tests pass (cargo test in crates/microsoft-fast-build).

✅ 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 @microsoft/fast-build SSR custom element rendering so :prop (FAST property bindings) are forwarded into the child element’s SSR rendering state while being stripped from the emitted HTML, enabling typed values (arrays/objects) to flow into child templates (e.g., for f-repeat) without polluting attributes.

Changes:

• Forward :prop bindings into custom element child state (lowercased key, : removed) while continuing to omit them from rendered HTML.
• Add/adjust tests validating :prop typed forwarding (string/object/array) and verifying the attribute is absent from output HTML.
• Update documentation (README + DESIGN) to reflect the new attribute-to-state behavior; add a patch change file.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
crates/microsoft-fast-build/src/directive.rs	Implements forwarding of :prop bindings into child SSR state while keeping @event skipped.
crates/microsoft-fast-build/tests/custom_elements.rs	Adds coverage for :prop state forwarding (including arrays/objects for f-repeat) and stripping from HTML.
crates/microsoft-fast-build/README.md	Updates attribute→state mapping and guidance for using :prop to pass structured data without rendering attributes.
crates/microsoft-fast-build/DESIGN.md	Updates the SSR design notes to match the new :prop handling.
change/@microsoft-fast-build-fix-json-attrs-72c1b67a-84b2-484c-90bd-9e9f9a065e6c.json	Adds a patch change entry for the behavior update.