feat: skip over static subtrees (#12849)

* feat: skip over static subtrees

* regenerate

* a few more

* prettier

.changeset/neat-ducks-jam.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+feat: skip over static subtrees

packages/svelte/src/compiler/phases/1-parse/utils/create.js

@@ -7,6 +7,9 @@ export function create_fragment(transparent = false) {
 	return {
 		type: 'Fragment',
 		nodes: [],
-		transparent
+		metadata: {
+			transparent,
+			dynamic: false
+		}
 	};
 }

packages/svelte/src/compiler/phases/2-analyze/index.js

@@ -61,6 +61,7 @@ import { TaggedTemplateExpression } from './visitors/TaggedTemplateExpression.js
 import { Text } from './visitors/Text.js';
 import { TitleElement } from './visitors/TitleElement.js';
 import { UpdateExpression } from './visitors/UpdateExpression.js';
+import { UseDirective } from './visitors/UseDirective.js';
 import { VariableDeclarator } from './visitors/VariableDeclarator.js';
 import is_reference from 'is-reference';
 
@@ -166,6 +167,7 @@ const visitors = {
 	Text,
 	TitleElement,
 	UpdateExpression,
+	UseDirective,
 	VariableDeclarator
 };
 

packages/svelte/src/compiler/phases/2-analyze/visitors/Attribute.js

@@ -1,5 +1,5 @@
 /** @import { ArrowFunctionExpression, Expression, FunctionDeclaration, FunctionExpression } from 'estree' */
-/** @import { Attribute, DelegatedEvent, RegularElement } from '#compiler' */
+/** @import { Attribute, DelegatedEvent, RegularElement, SvelteNode } from '#compiler' */
 /** @import { Context } from '../types' */
 import { is_capture_event, is_delegated } from '../../../../utils.js';
 import {
@@ -7,6 +7,7 @@ import {
 	get_attribute_expression,
 	is_event_attribute
 } from '../../../utils/ast.js';
+import { mark_subtree_dynamic } from './shared/fragment.js';
 
 /**
  * @param {Attribute} node
@@ -15,6 +16,14 @@ import {
 export function Attribute(node, context) {
 	context.next();
 
+	// special case
+	if (node.name === 'value') {
+		const parent = /** @type {SvelteNode} */ (context.path.at(-1));
+		if (parent.type === 'RegularElement' && parent.name === 'option') {
+			mark_subtree_dynamic(context.path);
+		}
+	}
+
 	if (node.value !== true) {
 		for (const chunk of get_attribute_chunks(node.value)) {
 			if (chunk.type !== 'ExpressionTag') continue;

packages/svelte/src/compiler/phases/2-analyze/visitors/AwaitBlock.js

@@ -2,6 +2,7 @@
 /** @import { Context } from '../types' */
 import { validate_block_not_empty, validate_opening_tag } from './shared/utils.js';
 import * as e from '../../../errors.js';
+import { mark_subtree_dynamic } from './shared/fragment.js';
 
 /**
  * @param {AwaitBlock} node
@@ -38,5 +39,7 @@ export function AwaitBlock(node, context) {
 		}
 	}
 
+	mark_subtree_dynamic(context.path);
+
 	context.next();
 }

packages/svelte/src/compiler/phases/2-analyze/visitors/EachBlock.js

@@ -2,6 +2,7 @@
 /** @import { Context } from '../types' */
 /** @import { Scope } from '../../scope' */
 import * as e from '../../../errors.js';
+import { mark_subtree_dynamic } from './shared/fragment.js';
 import { validate_block_not_empty, validate_opening_tag } from './shared/utils.js';
 
 /**
@@ -36,4 +37,6 @@ export function EachBlock(node, context) {
 	context.visit(node.body);
 	if (node.key) context.visit(node.key);
 	if (node.fallback) context.visit(node.fallback);
+
+	mark_subtree_dynamic(context.path);
 }

packages/svelte/src/compiler/phases/2-analyze/visitors/ExpressionTag.js

@@ -2,6 +2,7 @@
 /** @import { Context } from '../types' */
 import { is_tag_valid_with_parent } from '../../../../html-tree-validation.js';
 import * as e from '../../../errors.js';
+import { mark_subtree_dynamic } from './shared/fragment.js';
 
 /**
  * @param {ExpressionTag} node
@@ -14,5 +15,9 @@ export function ExpressionTag(node, context) {
 		}
 	}
 
+	// TODO ideally we wouldn't do this here, we'd just do it on encountering
+	// an `Identifier` within the tag. But we currently need to handle `{42}` etc
+	mark_subtree_dynamic(context.path);
+
 	context.next({ ...context.state, expression: node.metadata.expression });
 }

packages/svelte/src/compiler/phases/2-analyze/visitors/HtmlTag.js

@@ -1,5 +1,6 @@
 /** @import { HtmlTag } from '#compiler' */
 /** @import { Context } from '../types' */
+import { mark_subtree_dynamic } from './shared/fragment.js';
 import { validate_opening_tag } from './shared/utils.js';
 
 /**
@@ -11,5 +12,8 @@ export function HtmlTag(node, context) {
 		validate_opening_tag(node, context.state, '@');
 	}
 
+	// unfortunately this is necessary in order to fix invalid HTML
+	mark_subtree_dynamic(context.path);
+
 	context.next();
 }

packages/svelte/src/compiler/phases/2-analyze/visitors/Identifier.js

@@ -6,6 +6,7 @@ import { should_proxy } from '../../3-transform/client/utils.js';
 import * as e from '../../../errors.js';
 import * as w from '../../../warnings.js';
 import { is_rune } from '../../../../utils.js';
+import { mark_subtree_dynamic } from './shared/fragment.js';
 
 /**
  * @param {Identifier} node
@@ -19,6 +20,8 @@ export function Identifier(node, context) {
 		return;
 	}
 
+	mark_subtree_dynamic(context.path);
+
 	// If we are using arguments outside of a function, then throw an error
 	if (
 		node.name === 'arguments' &&

packages/svelte/src/compiler/phases/2-analyze/visitors/IfBlock.js

@@ -1,5 +1,6 @@
 /** @import { IfBlock } from '#compiler' */
 /** @import { Context } from '../types' */
+import { mark_subtree_dynamic } from './shared/fragment.js';
 import { validate_block_not_empty, validate_opening_tag } from './shared/utils.js';
 
 /**
@@ -22,5 +23,7 @@ export function IfBlock(node, context) {
 		validate_opening_tag(node, context.state, expected);
 	}
 
+	mark_subtree_dynamic(context.path);
+
 	context.next();
 }

packages/svelte/src/compiler/phases/2-analyze/visitors/KeyBlock.js

@@ -1,5 +1,6 @@
 /** @import { KeyBlock } from '#compiler' */
 /** @import { Context } from '../types' */
+import { mark_subtree_dynamic } from './shared/fragment.js';
 import { validate_block_not_empty, validate_opening_tag } from './shared/utils.js';
 
 /**
@@ -13,5 +14,7 @@ export function KeyBlock(node, context) {
 		validate_opening_tag(node, context.state, '#');
 	}
 
+	mark_subtree_dynamic(context.path);
+
 	context.next();
 }

packages/svelte/src/compiler/phases/2-analyze/visitors/OnDirective.js

@@ -1,6 +1,7 @@
 /** @import { OnDirective } from '#compiler' */
 /** @import { Context } from '../types' */
 import * as w from '../../../warnings.js';
+import { mark_subtree_dynamic } from './shared/fragment.js';
 
 /**
  * @param {OnDirective} node
@@ -21,5 +22,7 @@ export function OnDirective(node, context) {
 		context.state.analysis.event_directive_node ??= node;
 	}
 
+	mark_subtree_dynamic(context.path);
+
 	context.next({ ...context.state, expression: node.metadata.expression });
 }

packages/svelte/src/compiler/phases/2-analyze/visitors/RenderTag.js

@@ -3,6 +3,7 @@
 import { unwrap_optional } from '../../../utils/ast.js';
 import * as e from '../../../errors.js';
 import { validate_opening_tag } from './shared/utils.js';
+import { mark_subtree_dynamic } from './shared/fragment.js';
 
 /**
  * @param {RenderTag} node
@@ -33,5 +34,7 @@ export function RenderTag(node, context) {
 		e.render_tag_invalid_call_expression(node);
 	}
 
+	mark_subtree_dynamic(context.path);
+
 	context.next({ ...context.state, render_tag: node });
 }

packages/svelte/src/compiler/phases/2-analyze/visitors/SlotElement.js

@@ -3,6 +3,7 @@
 import { is_text_attribute } from '../../../utils/ast.js';
 import * as e from '../../../errors.js';
 import * as w from '../../../warnings.js';
+import { mark_subtree_dynamic } from './shared/fragment.js';
 
 /**
  * @param {SlotElement} node
@@ -13,6 +14,8 @@ export function SlotElement(node, context) {
 		w.slot_element_deprecated(node);
 	}
 
+	mark_subtree_dynamic(context.path);
+
 	/** @type {string} */
 	let name = 'default';
 

packages/svelte/src/compiler/phases/2-analyze/visitors/SpreadAttribute.js

@@ -1,10 +1,13 @@
 /** @import { SpreadAttribute } from '#compiler' */
 /** @import { Context } from '../types' */
 
+import { mark_subtree_dynamic } from './shared/fragment.js';
+
 /**
  * @param {SpreadAttribute} node
  * @param {Context} context
  */
 export function SpreadAttribute(node, context) {
+	mark_subtree_dynamic(context.path);
 	context.next({ ...context.state, expression: node.metadata.expression });
 }

packages/svelte/src/compiler/phases/2-analyze/visitors/StyleDirective.js

@@ -2,6 +2,7 @@
 /** @import { Context } from '../types' */
 import * as e from '../../../errors.js';
 import { get_attribute_chunks } from '../../../utils/ast.js';
+import { mark_subtree_dynamic } from './shared/fragment.js';
 
 /**
  * @param {StyleDirective} node
@@ -21,6 +22,8 @@ export function StyleDirective(node, context) {
 				node.metadata.expression.has_state = true;
 			}
 		}
+
+		mark_subtree_dynamic(context.path);
 	} else {
 		context.next();
 

packages/svelte/src/compiler/phases/2-analyze/visitors/SvelteElement.js

@@ -4,6 +4,7 @@ import { NAMESPACE_MATHML, NAMESPACE_SVG } from '../../../../constants.js';
 import { is_text_attribute } from '../../../utils/ast.js';
 import { check_element } from './shared/a11y.js';
 import { validate_element } from './shared/element.js';
+import { mark_subtree_dynamic } from './shared/fragment.js';
 
 /**
  * @param {SvelteElement} node
@@ -58,5 +59,7 @@ export function SvelteElement(node, context) {
 		}
 	}
 
+	mark_subtree_dynamic(context.path);
+
 	context.next({ ...context.state, parent_element: null });
 }

packages/svelte/src/compiler/phases/2-analyze/visitors/SvelteHead.js

@@ -1,6 +1,7 @@
 /** @import { SvelteHead } from '#compiler' */
 /** @import { Context } from '../types' */
 import * as e from '../../../errors.js';
+import { mark_subtree_dynamic } from './shared/fragment.js';
 
 /**
  * @param {SvelteHead} node
@@ -11,5 +12,7 @@ export function SvelteHead(node, context) {
 		e.svelte_head_illegal_attribute(attribute);
 	}
 
+	mark_subtree_dynamic(context.path);
+
 	context.next();
 }

packages/svelte/src/compiler/phases/2-analyze/visitors/UseDirective.js

@@ -0,0 +1,12 @@
+/** @import { UseDirective } from '#compiler' */
+/** @import { Context } from '../types' */
+import { mark_subtree_dynamic } from './shared/fragment.js';
+
+/**
+ * @param {UseDirective} node
+ * @param {Context} context
+ */
+export function UseDirective(node, context) {
+	mark_subtree_dynamic(context.path);
+	context.next();
+}

packages/svelte/src/compiler/phases/2-analyze/visitors/shared/component.js

@@ -8,12 +8,15 @@ import {
 	validate_attribute_name,
 	validate_slot_attribute
 } from './attribute.js';
+import { mark_subtree_dynamic } from './fragment.js';
 
 /**
  * @param {Component | SvelteComponent | SvelteSelf} node
  * @param {Context} context
  */
 export function visit_component(node, context) {
+	mark_subtree_dynamic(context.path);
+
 	for (const attribute of node.attributes) {
 		if (
 			attribute.type !== 'Attribute' &&

packages/svelte/src/compiler/phases/2-analyze/visitors/shared/fragment.js

@@ -0,0 +1,15 @@
+/** @import { SvelteNode } from '#compiler' */
+
+/**
+ * @param {SvelteNode[]} path
+ */
+export function mark_subtree_dynamic(path) {
+	let i = path.length;
+	while (i--) {
+		const node = path[i];
+		if (node.type === 'Fragment') {
+			if (node.metadata.dynamic) return;
+			node.metadata.dynamic = true;
+		}
+	}
+}

packages/svelte/src/compiler/phases/3-transform/client/visitors/RegularElement.js

@@ -313,9 +313,8 @@ export function RegularElement(node, context) {
 	/** Whether or not we need to wrap the children in `{...}` to avoid declaration conflicts */
 	const has_declaration = node.fragment.nodes.some((node) => node.type === 'SnippetBlock');
 
-	const child_state = has_declaration
+	/** @type {typeof state} */
-		? { ...state, init: [], update: [], after_update: [] }
+	const child_state = { ...state, init: [], update: [], after_update: [] };
-		: state;
 
 	for (const node of hoisted) {
 		context.visit(node, child_state);
@@ -353,6 +352,10 @@ export function RegularElement(node, context) {
 				...child_state.after_update
 			])
 		);
+	} else if (node.fragment.metadata.dynamic) {
+		context.state.init.push(...child_state.init);
+		context.state.update.push(...child_state.update);
+		context.state.after_update.push(...child_state.after_update);
 	}
 
 	if (has_direction_attribute) {

packages/svelte/src/compiler/phases/scope.js

@@ -636,7 +636,7 @@ export function create_scopes(ast, root, allow_reactive_declarations, parent) {
 		},
 
 		Fragment: (node, context) => {
-			const scope = context.state.scope.child(node.transparent);
+			const scope = context.state.scope.child(node.metadata.transparent);
 			scopes.set(node, scope);
 			context.next({ scope });
 		},

packages/svelte/src/compiler/types/template.d.ts

@@ -29,12 +29,18 @@ export interface BaseNode {
 export interface Fragment {
 	type: 'Fragment';
 	nodes: Array<Text | Tag | ElementLike | Block | Comment>;
-	/**
+	metadata: {
-	 * Fragments declare their own scopes. A transparent fragment is one whose scope
+		/**
-	 * is not represented by a scope in the resulting JavaScript (e.g. an element scope),
+		 * Fragments declare their own scopes. A transparent fragment is one whose scope
-	 * and should therefore delegate to parent scopes when generating unique identifiers
+		 * is not represented by a scope in the resulting JavaScript (e.g. an element scope),
-	 */
+		 * and should therefore delegate to parent scopes when generating unique identifiers
-	transparent: boolean;
+		 */
+		transparent: boolean;
+		/**
+		 * Whether or not we need to traverse into the fragment during mount/hydrate
+		 */
+		dynamic: boolean;
+	};
 }
 
 /**

packages/svelte/tests/parser-modern/samples/comment-before-script/output.json

@@ -20,8 +20,7 @@
 				"raw": "\n",
 				"data": "\n"
 			}
-		],
+		]
-		"transparent": false
 	},
 	"options": null,
 	"instance": {

packages/svelte/tests/parser-modern/samples/css-nth-syntax/output.json

@@ -1108,12 +1108,10 @@
 							"raw": "Foo",
 							"data": "Foo"
 						}
-					],
+					]
-					"transparent": true
 				}
 			}
-		],
+		]
-		"transparent": false
 	},
 	"options": null
 }

packages/svelte/tests/parser-modern/samples/css-pseudo-classes/output.json

@@ -403,8 +403,7 @@
 	"type": "Root",
 	"fragment": {
 		"type": "Fragment",
-		"nodes": [],
+		"nodes": []
-		"transparent": false
 	},
 	"options": null
 }

packages/svelte/tests/parser-modern/samples/each-block-object-pattern/output.json

@@ -144,8 +144,7 @@
 											}
 										}
 									}
-								],
+								]
-								"transparent": true
 							}
 						},
 						{
@@ -155,8 +154,7 @@
 							"raw": "\n",
 							"data": "\n"
 						}
-					],
+					]
-					"transparent": false
 				},
 				"context": {
 					"type": "ObjectPattern",
@@ -310,8 +308,7 @@
 					]
 				}
 			}
-		],
+		]
-		"transparent": false
 	},
 	"options": null
 }

packages/svelte/tests/parser-modern/samples/if-block-else/output.json

@@ -54,8 +54,7 @@
 										"raw": "foo",
 										"data": "foo"
 									}
-								],
+								]
-								"transparent": true
 							}
 						},
 						{
@@ -65,8 +64,7 @@
 							"raw": "\n",
 							"data": "\n"
 						}
-					],
+					]
-					"transparent": false
 				},
 				"alternate": {
 					"type": "Fragment",
@@ -94,8 +92,7 @@
 										"raw": "not foo",
 										"data": "not foo"
 									}
-								],
+								]
-								"transparent": true
 							}
 						},
 						{
@@ -105,12 +102,10 @@
 							"raw": "\n",
 							"data": "\n"
 						}
-					],
+					]
-					"transparent": false
 				}
 			}
-		],
+		]
-		"transparent": false
 	},
 	"options": null
 }

packages/svelte/tests/parser-modern/samples/if-block-elseif/output.json

@@ -87,8 +87,7 @@
 										"raw": "x is greater than 10",
 										"data": "x is greater than 10"
 									}
-								],
+								]
-								"transparent": true
 							}
 						},
 						{
@@ -98,8 +97,7 @@
 							"raw": "\n",
 							"data": "\n"
 						}
-					],
+					]
-					"transparent": false
 				},
 				"alternate": {
 					"type": "Fragment",
@@ -184,8 +182,7 @@
 													"raw": "x is less than 5",
 													"data": "x is less than 5"
 												}
-											],
+											]
-											"transparent": true
 										}
 									},
 									{
@@ -195,17 +192,14 @@
 										"raw": "\n",
 										"data": "\n"
 									}
-								],
+								]
-								"transparent": false
 							},
 							"alternate": null
 						}
-					],
+					]
-					"transparent": false
 				}
 			}
-		],
+		]
-		"transparent": false
 	},
 	"options": null
 }

packages/svelte/tests/parser-modern/samples/if-block/output.json

@@ -38,13 +38,11 @@
 							"raw": "bar",
 							"data": "bar"
 						}
-					],
+					]
-					"transparent": false
 				},
 				"alternate": null
 			}
-		],
+		]
-		"transparent": false
 	},
 	"options": null
 }

packages/svelte/tests/parser-modern/samples/options/output.json

@@ -21,8 +21,7 @@
 				"raw": "\n\n",
 				"data": "\n\n"
 			}
-		],
+		]
-		"transparent": false
 	},
 	"options": {
 		"start": 0,

packages/svelte/tests/parser-modern/samples/semicolon-inside-quotes/output.json

@@ -98,8 +98,7 @@
 							"raw": "\n\tSemicolon inside quotes\n",
 							"data": "\n\tSemicolon inside quotes\n"
 						}
-					],
+					]
-					"transparent": true
 				}
 			},
 			{
@@ -109,8 +108,7 @@
 				"raw": "\n",
 				"data": "\n"
 			}
-		],
+		]
-		"transparent": false
 	},
 	"options": null
 }

packages/svelte/tests/parser-modern/samples/snippets/output.json

@@ -112,8 +112,7 @@
 											"name": "msg"
 										}
 									}
-								],
+								]
-								"transparent": true
 							}
 						},
 						{
@@ -123,8 +122,7 @@
 							"raw": "\n",
 							"data": "\n"
 						}
-					],
+					]
-					"transparent": false
 				}
 			},
 			{
@@ -189,8 +187,7 @@
 					"optional": false
 				}
 			}
-		],
+		]
-		"transparent": false
 	},
 	"options": null,
 	"instance": {

packages/svelte/tests/parser-modern/samples/template-shadowroot/output.json

@@ -56,12 +56,10 @@
 										"attributes": [],
 										"fragment": {
 											"type": "Fragment",
-											"nodes": [],
+											"nodes": []
-											"transparent": true
 										}
 									}
-								],
+								]
-								"transparent": true
 							}
 						},
 						{
@@ -71,8 +69,7 @@
 							"raw": "\n",
 							"data": "\n"
 						}
-					],
+					]
-					"transparent": true
 				}
 			},
 			{
@@ -115,12 +112,10 @@
 										"attributes": [],
 										"fragment": {
 											"type": "Fragment",
-											"nodes": [],
+											"nodes": []
-											"transparent": true
 										}
 									}
-								],
+								]
-								"transparent": true
 							}
 						},
 						{
@@ -130,8 +125,7 @@
 							"raw": "\n",
 							"data": "\n"
 						}
-					],
+					]
-					"transparent": true
 				}
 			},
 			{
@@ -149,12 +143,10 @@
 				"attributes": [],
 				"fragment": {
 					"type": "Fragment",
-					"nodes": [],
+					"nodes": []
-					"transparent": true
 				}
 			}
-		],
+		]
-		"transparent": false
 	},
 	"options": null
 }

packages/svelte/tests/parser-modern/samples/typescript-in-event-handler/output.json

@@ -350,12 +350,10 @@
 								"name": "count"
 							}
 						}
-					],
+					]
-					"transparent": true
 				}
 			}
-		],
+		]
-		"transparent": false
 	},
 	"options": null,
 	"instance": {

packages/svelte/tests/snapshot/samples/skip-static-subtree/_expected/client/index.svelte.js

@@ -0,0 +1,10 @@
+import "svelte/internal/disclose-version";
+import * as $ from "svelte/internal/client";
+
+var root = $.template(`<header><nav><a href="/">Home</a> <a href="/away">Away</a></nav></header>`);
+
+export default function Skip_static_subtree($$anchor) {
+	var header = root();
+
+	$.append($$anchor, header);
+}

packages/svelte/tests/snapshot/samples/skip-static-subtree/_expected/server/index.svelte.js

@@ -0,0 +1,5 @@
+import * as $ from "svelte/internal/server";
+
+export default function Skip_static_subtree($$payload) {
+	$$payload.out += `<header><nav><a href="/">Home</a> <a href="/away">Away</a></nav></header>`;
+}

packages/svelte/tests/snapshot/samples/skip-static-subtree/index.svelte

@@ -0,0 +1,6 @@
+<header>
+	<nav>
+		<a href="/">Home</a>
+		<a href="/away">Away</a>
+	</nav>
+</header>

packages/svelte/types/index.d.ts

@@ -1486,12 +1486,18 @@ declare module 'svelte/compiler' {
 	interface Fragment {
 		type: 'Fragment';
 		nodes: Array<Text | Tag | ElementLike | Block | Comment>;
-		/**
+		metadata: {
-		 * Fragments declare their own scopes. A transparent fragment is one whose scope
+			/**
-		 * is not represented by a scope in the resulting JavaScript (e.g. an element scope),
+			 * Fragments declare their own scopes. A transparent fragment is one whose scope
-		 * and should therefore delegate to parent scopes when generating unique identifiers
+			 * is not represented by a scope in the resulting JavaScript (e.g. an element scope),
-		 */
+			 * and should therefore delegate to parent scopes when generating unique identifiers
-		transparent: boolean;
+			 */
+			transparent: boolean;
+			/**
+			 * Whether or not we need to traverse into the fragment during mount/hydrate
+			 */
+			dynamic: boolean;
+		};
 	}
 
 	/**