Create declaration emit diagnostic selectors on demand

[Gavin Kline (gwkline)]

Fixes #64720

Analysis

Every declaration the declaration transformer visits installs a diagnostic context. setupDiagnosticContext calls createGetSymbolAccessibilityDiagnosticForNode(input), which allocates a closure that selects the error message in case a symbol used by the declaration is inaccessible; the wrapped selectors allocate two. It then returns a second closure that restores the previous context. The selector is only invoked from handleSymbolAccessibilityError, that is, when an accessibility error is actually reported, which almost never happens.

This runs for every emitted declaration file. Under noEmit with declaration enabled, it runs for every file's declaration diagnostics. On the generator from the issue the closures are about 40% of all allocations. On a private 8.6k-file monorepo package checked with --noEmit they were 11%.

Fix

• SymbolTrackerSharedState holds a symbolAccessibilityDiagnosticContext{node, fn} instead of a function. The common path stores the declaration node, and get creates the selector only when an error is reported. The few sites that need a custom selector store a function as before: default export assignments, class extends expressions, dynamic names, and the initial throwDiagnostic sentinel.
• setupDiagnosticContext returns the saved state as a struct, and restoreDiagnosticContext reinstates it. The deferred restore therefore no longer allocates.

The selector is created from the same node with the same inputs, only later, so behaviour is unchanged.

New test declarationEmitInaccessibleNameContexts gives an unnameable inferred type to every kind of declaration the selectors distinguish. It covers variables, destructuring, function return types and parameters, class properties and static properties, constructor parameter properties, methods and static methods, getters and accessor pairs, namespace members, object literal members and default exports, and reports 13 distinct diagnostics. Its baselines were generated with the old code and are unchanged. No existing baseline changes.

Results on an M3 Pro:

Program	Allocations	Check time
generator from the issue (1,557 files, noEmit + declaration)	14.5M → 8.7M (−40%)	0.300 s → 0.276 s
private monorepo package, --noEmit (8.6k files)	46.1M → 41.1M (−10.9%)	4.19 s → 4.09 s
same package, declaration build	35.4M → 32.5M (−8.0%)	emitted output identical

The profiling, the patch and this description were produced with Claude Code (Claude Fable 5.1 and Claude Opus 5.5). I have read and understand the change and will handle review myself.

Copilot Checklist

I successfully ran the applicable command at the end of my session, and it completed without error:

• npx hereby validate
• npx hereby validate --api (for TypeScript API changes; no API changes here)

🤖 Generated with Claude Code

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