chore(bundle-size-tools): drop unused/flawed legacy behaviors

[ChumpChief]

Description

Follow-up to #27224 (analyzer.json swap) and #27239 (dead-code cleanup). Part of AB#56981 (re-enable PR bundle size comparison).

Three pre-existing behaviors are removed because they don't fit the bundleSizeDiff design that #27158 began (a producer that emits a structured artifact, consumed by a future GitHub Actions workflow). Each is independent; commits are split for review.

1. sizeRegressionDetected field on bundleSizeDiff output (commit 1)

Background. flub generate bundleSizeDiff runs a producer-side regression check: any non-total metric growing by more than a hardcoded 5KB threshold sets a sizeRegressionDetected: boolean field on result.json's "changes" variant.

Why remove. That threshold is a policy decision, not raw data. The broader design has the producer emit unopinionated per-bundle deltas and the consumer (the future GH Actions workflow that will post PR comments / manage labels) apply its own threshold. Producer-side hardcoding pre-commits us to one definition of "regression" that the consumer can't override.

Removed.

• detectSizeRegression function
• sizeRegressionThresholdBytes constant
• sizeRegressionDetected field from BundleSizeDiffResult's "changes" variant
• BundleMetric type import (only used by the removed function)

Compatibility. No consumer reads result.json yet — the GH workflow that will is a future PR — so this is a no-op at the seam and lets that future consumer pick its own threshold.

2. tagWaiting mechanism in ADOSizeComparator (commit 2)

Background. When the comparator runs against a baseline commit whose CI build hasn't completed yet, it tags the PR's own build with a marker (via tagBuildAsWaiting) so that a downstream process could re-run the comparison once the baseline finished.

Why remove. The downstream half (consuming the tag and retriggering the PR build) was never written, so the tagging is effectively no-op overhead today. The new bundleSizeDiff design — and PR 3's planned scheduled re-run — replaces this need entirely with workflow-side polling.

Removed.

• bundle-size-tools/src/ADO/getBuildTagForCommit.ts (whole file)
• ADOSizeComparator.tagBuildAsWaiting private method
• ADOSizeComparator constructor param adoBuildId
• getSizeComparison's tagWaiting parameter
• IADOConstants fields prBuildDefinitionId and projectRepoGuid (only used by tagWaiting and the now-gone PR-comments machinery)
• Corresponding barrel exports

3. Naive fallback in ADOSizeComparator (commit 2)

Background. When no CI build is found for the baseline commit (or the build has no usable artifact), the comparator walks prior commits via getPriorCommit looking for one with a usable build, and compares the PR against that.

Why remove. The resulting comparison is incorrect-by-design: the PR is being compared against a commit on main that's N commits older than the merge-base, so main-side deltas between merge-base and merge-base−N get falsely attributed to the PR.

Removed.

• ADOSizeComparator.naiveFallbackCommitGenerator static method
• ADOSizeComparator constructor param getFallbackCommit
• utilities/gitCommands.ts's getPriorCommit (only used by the fallback)
• Corresponding barrel exports

Cleanup side-effect. With both tagWaiting and the fallback gone, getSizeComparison flattens from a while-loop with continue/fallback branches into a linear sequence of early-return validations, and error messages become more specific — "No CI build found for baseline commit X" / "Baseline build did not publish bundle artifacts" instead of the generic "Could not find a usable baseline build".

4. BannedModulesPlugin (commit 3)

Background. A custom Webpack plugin shipped by bundle-size-tools and used by bundle-size-tests to fail the build if any chunk imports a banned module. The only banned module configured today is Node-core assert, whose polyfill is large in browser builds.

Why remove. Webpack 5 supports the same effect natively via resolve.fallback: { assert: false }, which blocks the polyfill from being bundled at all. The plugin existed because earlier Webpack versions didn't have this option. The native config is a few lines; the plugin is a ~70-line file plus the burden of being a public API export of bundle-size-tools.

Removed.

• bundle-size-tools/src/bannedModulesPlugin/bannedModulesPlugin.ts (whole file)
• BannedModule, BannedModulesPlugin, BannedModulesPluginOptions exports

Dependency drops (consequences of the swap):

• webpack removed from bundle-size-tools/package.json (was only used by the plugin)
• @fluidframework/bundle-size-tools removed from bundle-size-tests/package.json devDependencies (was only used here for the plugin)

Reviewer Guidance

The review process is outlined on this wiki page.

Three independent removals, each in its own commit — easy to review commit-by-commit.

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🔭 PR Review Fleet Report

Note

This report is generated by an experimental AI review fleet and is provided as a beta feature. Findings are a starting point for discussion, not a gate. Use your own judgement.

Verdict: ❌ Request Changes

4 Spicy, 3 Pungent, 0 Smelly

Findings

Sev	#	Area	File	What	Fix
🌶️ Spicy	C1	API Compatibility	build-tools/packages/bundle-size-tools/src/bannedModulesPlugin/bannedModulesPlugin.ts:1	BannedModulesPlugin, BannedModule, and BannedModulesPluginOptions were all @public exports and have been deleted outright with no deprecation period. Consumers who imported any of these (e.g. examples/utils/bundle-size-tests/webpack.config.cjs did exactly that) will get a hard compile/runtime error after upgrading.	Before removing, add @deprecated annotations with migration guidance (e.g. 'Use webpack resolve.fallback or a custom plugin instead') and ship at least one release with the deprecated symbols intact, then remove in a follow-up.
🌶️ Spicy	C2	API Compatibility	build-tools/packages/bundle-size-tools/src/ADO/getBuildTagForCommit.ts:1	getBuildTagForCommit was a @public export and has been deleted without deprecation. Any consumer who imported it from @fluidframework/bundle-size-tools will get a compile error after upgrading.	Deprecate the function first with a @deprecated JSDoc tag and a migration note (or note that there is no replacement), then remove it in a later release.
🌶️ Spicy	C3	API Compatibility	build-tools/packages/bundle-size-tools/src/utilities/gitCommands.ts:1	getPriorCommit was a @public export and has been deleted without deprecation. Any consumer importing it directly from @fluidframework/bundle-size-tools will get a compile error after upgrading.	Deprecate first, then remove in a follow-up release.
🌶️ Spicy	C4	API Compatibility	build-tools/packages/bundle-size-tools/src/ADO/AdoSizeComparator.ts:240	ADOSizeComparator.naiveFallbackCommitGenerator was a @public static method and has been deleted without deprecation. Consumers who referenced this static directly will break at compile time.	Add @deprecated to the static method and keep it for at least one release before removing.
🧄 Pungent	H1	API Compatibility	build-tools/packages/bundle-size-tools/src/ADO/AdoSizeComparator.ts:62	The ADOSizeComparator constructor had two @public parameters (adoBuildId: number | undefined and getFallbackCommit?: (startingCommit: string) => Generator<string>) that have been removed. Any call site passing more than three positional arguments will fail at runtime or compile time after upgrading. The API report confirms this is a public signature.	This is a breaking change. If the package must ship this way, bump the major version or document it clearly as a breaking change in the changelog. Alternatively, consider keeping the parameters with @deprecated annotations before removing them.
🧄 Pungent	H2	API Compatibility	build-tools/packages/bundle-size-tools/src/ADO/AdoSizeComparator.ts:259	getSizeComparison(tagWaiting: boolean) changed signature to getSizeComparison() — the required tagWaiting boolean parameter was removed. Callers who pass false or true will still compile (extra args are silently dropped in JS), but any callers who relied on tagWaiting: true to trigger the tagging behavior will now silently lose that behavior with no warning.	Document the behavioral removal explicitly in the changelog. If any callers pass true, they need to be migrated to an alternative mechanism.
🧄 Pungent	H3	API Compatibility	build-tools/packages/bundle-size-tools/src/ADO/Constants.ts:13	IADOConstants.prBuildDefinitionId and IADOConstants.projectRepoGuid were @public optional properties on a public interface and have been removed. Any consumer who implemented or extended this interface with those fields will get a TS error (excess-property checks on object literals, or structural mismatch). Any code that reads these properties will also fail to compile.	Mark the properties @deprecated first and remove in a follow-up release, or accept this as a documented breaking change requiring a major/minor version bump with clear changelog entry.

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