[feature] Dynamic elements implementation <svelte:element> (#6898)

Closes #2324

Co-authored-by: Alfred Ringstad <alfred.ringstad@hyperlab.se>
Co-authored-by: Simon Holthausen <simon.holthausen@accso.de>
Co-authored-by: tanhauhau <lhtan93@gmail.com>

site/content/docs/02-template-syntax.md

@@ -1627,6 +1627,28 @@ If `this` is falsy, no component is rendered.
 <svelte:component this={currentSelection.component} foo={bar}/>
 ```
 
+### `<svelte:element>`
+
+```sv
+<svelte:element this={expression}/>
+```
+
+---
+
+The `<svelte:element>` element lets you render an element of a dynamically specified type. This is useful for example when rich text content from a CMS. If the tag is changed, the children will be preserved unless there's a transition attached to the element. Any properties and event listeners present will be applied to the element.
+
+The only supported binding is `bind:this`, since the element type specific bindings that Svelte does at build time (e.g. `bind:value` for input elements) does not work with a dynamic tag type.
+
+If `this` has a nullish value, a warning will be logged in development mode.
+
+```sv
+<script>
+	let tag = 'div';
+	export let handler;
+</script>
+
+<svelte:element this={tag} on:click={handler}>Foo</svelte:element>
+```
 
 ### `<svelte:window>`
 

site/content/tutorial/16-special-elements/03-svelte-element/app-a/App.svelte

@@ -0,0 +1,18 @@
+<script>
+	const options = ['h1', 'h3', 'p'];
+	let selected = options[0];
+</script>
+
+<select bind:value={selected}>
+	{#each options as option}
+		<option value={option}>{option}</option>
+	{/each}
+</select>
+
+{#if selected === 'h1'}
+	<h1>I'm a h1 tag</h1>
+{:else if selected === 'h3'}
+	<h3>I'm a h3 tag</h3>
+{:else if selected === 'p'}
+	<p>I'm a p tag</p>
+{/if}

site/content/tutorial/16-special-elements/03-svelte-element/app-b/App.svelte

@@ -0,0 +1,12 @@
+<script>
+	const options = ['h1', 'h3', 'p'];
+	let selected = options[0];
+</script>
+
+<select bind:value={selected}>
+	{#each options as option}
+		<option value={option}>{option}</option>
+	{/each}
+</select>
+
+<svelte:element this={selected}>I'm a {selected} tag</svelte:element>

site/content/tutorial/16-special-elements/03-svelte-element/text.md

@@ -0,0 +1,23 @@
+---
+title: <svelte:element>
+---
+
+Sometimes we don't know in advance what kind of DOM element to render. `<svelte:element>` comes in handy here. Instead of a sequence of `if` blocks...
+
+```html
+{#if selected === 'h1'}
+	<h1>I'm a h1 tag</h1>
+{:else if selected === 'h3'}
+	<h3>I'm a h3 tag</h3>
+{:else if selected === 'p'}
+	<p>I'm a p tag</p>
+{/if}
+```
+
+...we can have a single dynamic component:
+
+```html
+<svelte:element this={selected}>I'm a {selected} tag</svelte:element>
+```
+
+The `this` value can be any string, or a falsy value — if it's falsy, no element is rendered.

src/compiler/compile/compiler_errors.ts

@@ -246,6 +246,10 @@ export default {
 		code: 'invalid-animation',
 		message: 'An element that uses the animate directive must be the sole child of a keyed each block'
 	},
+	invalid_animation_dynamic_element: {
+		code: 'invalid-animation',
+		message: '<svelte:element> cannot have a animate directive'
+	},
 	invalid_directive_value: {
 		code: 'invalid-directive-value',
 		message: 'Can only bind to an identifier (e.g. `foo`) or a member expression (e.g. `foo.bar` or `foo[baz]`)'

src/compiler/compile/nodes/Element.ts

@@ -18,6 +18,9 @@ import Let from './Let';
 import TemplateScope from './shared/TemplateScope';
 import { INode } from './interfaces';
 import Component from '../Component';
+import Expression from './shared/Expression';
+import { string_literal } from '../utils/stringify';
+import { Literal } from 'estree';
 import compiler_warnings from '../compiler_warnings';
 import compiler_errors from '../compiler_errors';
 
@@ -190,11 +193,26 @@ export default class Element extends Node {
 	children: INode[];
 	namespace: string;
 	needs_manual_style_scoping: boolean;
+	tag_expr: Expression;
+
+	get is_dynamic_element() {
+		return this.name === 'svelte:element';
+	}
 
 	constructor(component: Component, parent: Node, scope: TemplateScope, info: any) {
 		super(component, parent, scope, info);
 		this.name = info.name;
 
+		if (info.name === 'svelte:element') {
+			if (typeof info.tag !== 'string') {
+				this.tag_expr = new Expression(component, this, scope, info.tag);
+			} else {
+				this.tag_expr = new Expression(component, this, scope, string_literal(info.tag) as Literal);
+			}
+		} else {
+			this.tag_expr = new Expression(component, this, scope, string_literal(this.name) as Literal);
+		}
+
 		this.namespace = get_namespace(parent as Element, this, component.namespace);
 
 		if (this.namespace !== namespaces.foreign) {

src/compiler/compile/render_dom/Block.ts

@@ -48,6 +48,7 @@ export default class Block {
 		hydrate: Array<Node | Node[]>;
 		mount: Array<Node | Node[]>;
 		measure: Array<Node | Node[]>;
+		restore_measurements: Array<Node | Node[]>;
 		fix: Array<Node | Node[]>;
 		animate: Array<Node | Node[]>;
 		intro: Array<Node | Node[]>;
@@ -96,6 +97,7 @@ export default class Block {
 			hydrate: [],
 			mount: [],
 			measure: [],
+			restore_measurements: [],
 			fix: [],
 			animate: [],
 			intro: [],
@@ -326,6 +328,12 @@ export default class Block {
 				${this.chunks.measure}
 			}`;
 
+			if (this.chunks.restore_measurements.length) {
+				properties.restore_measurements = x`function #restore_measurements(#measurement) {
+					${this.chunks.restore_measurements}
+				}`;
+			}
+
 			properties.fix = x`function #fix() {
 				${this.chunks.fix}
 			}`;
@@ -379,6 +387,7 @@ export default class Block {
 			m: ${properties.mount},
 			p: ${properties.update},
 			r: ${properties.measure},
+			s: ${properties.restore_measurements},
 			f: ${properties.fix},
 			a: ${properties.animate},
 			i: ${properties.intro},