Support FORCE_COLOR

[jakebailey]

Same as microsoft/typescript-go#2596

[Copilot]

Pull request overview

Adds support for the FORCE_COLOR environment variable to control tsc’s “pretty” (colored) output, while ensuring NO_COLOR continues to disable color even when FORCE_COLOR is present.

Changes:

• Update pretty-output defaulting logic to honor FORCE_COLOR (and keep NO_COLOR as an override).
• Add command-line unit tests for FORCE_COLOR and NO_COLOR + FORCE_COLOR precedence.
• Add new reference baselines for the new test scenarios.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/compiler/executeCommandLine.ts	Updates defaultIsPretty to consider NO_COLOR and FORCE_COLOR.
src/testRunner/unittests/tsc/commandLine.ts	Adds verifyTsc cases covering FORCE_COLOR and precedence with NO_COLOR.
tests/baselines/reference/tsc/commandLine/adds-color-when-FORCE_COLOR-is-set.js	Baseline output for the new FORCE_COLOR scenario.
tests/baselines/reference/tsc/commandLine/does-not-add-color-when-NO_COLOR-is-set-even-if-FORCE_COLOR-is-set.js	Baseline output ensuring NO_COLOR disables color even when FORCE_COLOR is set.

[Copilot]

The new "adds color when FORCE_COLOR is set" test uses TestServerHost.createWatchedSystem, whose writeOutputIsTTY() always returns true. That means the output would already be colored even without FORCE_COLOR, so this test doesn't actually validate the new FORCE_COLOR behavior (forcing color in a non-TTY scenario). Consider overriding writeOutputIsTTY to return false for this test (or enhancing the host to support a non-TTY mode) so the baseline would differ without FORCE_COLOR and the test would catch regressions.

[Copilot]

FORCE_COLOR is treated as enabled for any non-empty value, but in common FORCE_COLOR semantics a value of "0" explicitly disables color. With the current check, FORCE_COLOR=0 will incorrectly force pretty output on. Consider parsing the value and treating "0" (and possibly "false") as disabling, while still allowing "1"/"2"/"3" to force-enable.

Suggested change

	if (sys.getEnvironmentVariable("FORCE_COLOR")) {
	const forceColor = sys.getEnvironmentVariable("FORCE_COLOR");
	if (forceColor !== undefined) {
	const normalized = forceColor.trim().toLowerCase();
	if (normalized === "0" || normalized === "false") {
	return false;
	}