perf: optimize imports that are not mutated or reassigned (#8948)

this means such imports are seen as static and subsequently Svelte needs to generate way less code

Author: benmccann

.changeset/shaggy-pans-repair.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+perf: optimize imports that are not mutated or reassigned

packages/svelte/src/compiler/compile/Component.js

@@ -770,14 +770,12 @@ export default class Component {
 			if (name[0] === '$') {
 				return this.error(/** @type {any} */ (node), compiler_errors.illegal_declaration);
 			}
-			const writable =
-				node.type === 'VariableDeclaration' && (node.kind === 'var' || node.kind === 'let');
-			const imported = node.type.startsWith('Import');
+			const { type } = node;
 			this.add_var(node, {
 				name,
 				initialised: instance_scope.initialised_declarations.has(name),
-				writable,
-				imported
+				imported: type.startsWith('Import'),
+				writable: type === 'VariableDeclaration' && (node.kind === 'var' || node.kind === 'let')
 			});
 			this.node_for_declaration.set(name, node);
 		});

packages/svelte/src/compiler/compile/internal_exports.js

@@ -54,7 +54,8 @@ export default class Expression {
 
 	/** @type {Array<import('estree').Node | import('estree').Node[]>} */
 	declarations = [];
-	/** */
+
+	/** @type {boolean} */
 	uses_context = false;
 
 	/** @type {import('estree').Node} */
@@ -129,7 +130,10 @@ export default class Expression {
 						}
 					} else {
 						if (!lazy) {
-							dependencies.add(name);
+							const variable = component.var_lookup.get(name);
+							if (!variable || !variable.imported || variable.mutated || variable.reassigned) {
+								dependencies.add(name);
+							}
 						}
 						component.add_reference(node, name);
 						component.warn_if_undefined(name, nodes[0], template_scope, owner);
@@ -231,6 +235,8 @@ export default class Expression {
 		if (this.manipulated) return this.manipulated;
 		const { component, declarations, scope_map: map, template_scope, owner } = this;
 		let scope = this.scope;
+
+		/** @type {import('estree').FunctionExpression | import('estree').ArrowFunctionExpression | null} */
 		let function_expression;
 
 		/** @type {Set<string>} */