Add makeContext: render-tree-scoped provide/consume (RFC #1154)

[NullVoxPopuli-ai-agent]

Summary

Implements makeContext as the user-facing context API discussed in RFC emberjs#1154, on top of an internal render-tree scope tracker.

import { makeContext } from '@ember/renderer';

class Theme {
  color = 'dark';
}

const theme = makeContext(Theme);

<template>
  <theme.Provide>
    {{#let (theme.consume) as |t|}}
      {{t.color}}  {{! "dark" }}
    {{/let}}
  </theme.Provide>

  {{ (theme.consume) }}  {{! throws — no provider in the hierarchy }}
</template>

Behavior (matching the RFC clarifications)

• consume() throws when no <Provide> is in the hierarchy. A missing provider is a bug, not a "fall back to undefined" state.
• consume() throws when called outside of rendering.
• The value the factory returns is not itself tracked. @tracked state on it is — mutating those fields re-renders consumers, which is the reactivity story called for in the RFC.
• Two factory forms supported (NVP's class form + rtablada's factory overload):

  makeContext(SomeClass)       // each <Provide> calls `new SomeClass()`
  makeContext(() => someValue) // each <Provide> calls the factory

How it works

Two layers:

1. Render-tree scope tracker (@glimmer/runtime/lib/render-scope.ts, internal). Always-on, mirrors DebugRenderTree's lifecycle. Each component pushes its own scope node at VM_CREATE_COMPONENT_OP (before the user constructor runs) and pops it at VM_DID_RENDER_LAYOUT_OP. An updating opcode re-pushes/pops on re-render so descendant lookups stay correct. scope.entries iterates the current node's own additions newest-first, then walks up — exactly the search shape consume() needs.
2. makeContext (packages/@ember/-internals/glimmer/lib/make-context.ts, public). Builds Provide on the same internal-component infrastructure as Input/Textarea/LinkTo (so ember-source doesn't take a dep on @glimmer/component). Each <Provide> constructor instantiates the factory and pushes [key, value] onto the scope; consume walks scope.entries for the closure-captured key. The shared {{yield}} template is precompiled once.

Scope

• Components only for now (where Provide lives). The underlying scope tracker is bucket-agnostic; extending to helpers / modifiers / plain curlies later is mechanical.
• No @value override on <Provide> yet — the factory is the value source. If we want explicit value passing later (à la rtablada's PermissionContext example) it's an additive change.
• getScope/addToScope are kept private to @glimmer/runtime (renamed getCurrentRenderScope/addToCurrentRenderScope internally). If a future RFC wants those public, exposing them is a one-line re-export.

Tests

packages/@ember/-internals/glimmer/tests/integration/render-tree-scope-test.ts covers what consumers actually care about:

• consume() throws outside a render
• consume() throws with no provider in the tree
• Nearest-provider lookup with nested <Provide> shadowing
• Factory form (makeContext(() => value))
• (context.consume) works as a template helper
• @tracked fields on the provided value cause consumer re-render

Validation

• pnpm exec tsc --noEmit clean.
• pnpm build:js succeeds for both dist/dev and dist/prod.
• pnpm run lint:docs / lint:format clean.
• pnpm test:node passes.
• Browser testem suite needs your local environment to validate end-to-end.

Test plan

• pnpm install && pnpm test runs green
• pnpm exec tsc --noEmit clean
• Spot-check the new integration tests render and assert as described
• Confirm nothing in the existing render-tree / debug-render-tree suite regresses (the scope tracker is parallel-but-independent of DebugRenderTree)

References

• RFC Resolved issue with cursor jumping in Ember.TextArea in IE/Opera emberjs/ember.js#1154: Public API for render-tree-based scope exploration emberjs/rfcs#1154
• NVP's makeContext sketch: Public API for render-tree-based scope exploration emberjs/rfcs#1154 (comment)
• rtablada's factory overload: Public API for render-tree-based scope exploration emberjs/rfcs#1154 (comment)
• Original Context RFC Ember.Select optionValuePath doesn't work as expected emberjs/ember.js#975: Add context API emberjs/rfcs#975

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