[fix] emit deprecation warning only once

Fixes #8009

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Svelte changelog
 
+## Unreleased
+
+* Emit deprecation warning only once
+
 ## 3.53.0
 
 * Check whether `parentNode` exists before removing child ([#6037](https://github.com/sveltejs/svelte/issues/6037))

src/compiler/compile/index.ts

@@ -46,6 +46,8 @@ const valid_css_values = [
 const regex_valid_identifier = /^[a-zA-Z_$][a-zA-Z_$0-9]*$/;
 const regex_starts_with_lowercase_character = /^[a-z]/;
 
+let has_shown_deprecation = false;
+
 function validate_options(options: CompileOptions, warnings: Warning[]) {
 	const { name, filename, loopGuardTimeout, dev, namespace, css } = options;
 
@@ -89,6 +91,8 @@ function validate_options(options: CompileOptions, warnings: Warning[]) {
 
 	if (css === true || css === false) {
 		options.css = css === true ? 'injected' : 'external';
+		if (!has_shown_deprecation) {
+			has_shown_deprecation=true;
 			const message = `options.css as a boolean is deprecated. Use '${options.css}' instead of ${css}.`;
 			warnings.push({
 				code: 'options-css-boolean-deprecated',
@@ -97,6 +101,7 @@ function validate_options(options: CompileOptions, warnings: Warning[]) {
 				toString: () => message
 			});
 		}
+	}
 
 	if (namespace && valid_namespaces.indexOf(namespace) === -1) {
 		const match = fuzzymatch(namespace, valid_namespaces);