[feat] implement constants in markup (#6413)

Author: tanhauhau

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

@@ -453,6 +453,29 @@ The `{@debug ...}` tag offers an alternative to `console.log(...)`. It logs the
 The `{@debug}` tag without any arguments will insert a `debugger` statement that gets triggered when *any* state changes, as opposed to the specified variables.
 
 
+### {@const ...}
+
+```sv
+{@const assignment}
+```
+
+---
+
+The `{@const ...}` tag defines a local constant.
+
+```sv
+<script>
+  export let boxes;
+</script>
+
+{#each boxes as box}
+  {@const area = box.width * box.height}
+	{box.width} * {box.height} = {area}
+{/each}
+```
+
+`{@const}` is only allowed as direct child of `{#each}`, `{:then}`, `{:catch}`, `<Component />` or `<svelte:fragment />`.
+
 
 ### Element directives
 

src/compiler/compile/compiler_errors.ts

@@ -40,6 +40,10 @@ export default {
 		code: 'invalid-binding',
 		message: 'Cannot bind to a variable declared with {#await ... then} or {:catch} blocks'
 	},
+	invalid_binding_const: {
+		code: 'invalid-binding',
+		message: 'Cannot bind to a variable declared with {@const ...}'
+	},
 	invalid_binding_writibale: {
 		code: 'invalid-binding',
 		message: 'Cannot bind to a variable which is not writable'
@@ -208,7 +212,7 @@ export default {
 	},
 	invalid_attribute_value: (name: string) => ({
 		code: `invalid-${name}-value`,
-		message: `${name} attribute must be true or false`	
+		message: `${name} attribute must be true or false`
 	}),
 	invalid_options_attribute_unknown: {
 		code: 'invalid-options-attribute',
@@ -241,5 +245,21 @@ export default {
 	invalid_directive_value: {
 		code: 'invalid-directive-value',
 		message: 'Can only bind to an identifier (e.g. `foo`) or a member expression (e.g. `foo.bar` or `foo[baz]`)'
-	}
+	},
+	invalid_const_placement: {
+		code: 'invalid-const-placement',
+		message: '{@const} must be the immediate child of {#each}, {:then}, {:catch}, <svelte:fragment> or <Component>'
+	},
+	invalid_const_declaration: (name: string) => ({
+		code: 'invalid-const-declaration',
+		message: `'${name}' has already been declared`
+	}),
+	invalid_const_update: (name: string) => ({
+		code: 'invalid-const-update',
+		message: `'${name}' is declared using {@const ...} and is read-only`
+	}),
+	cyclical_const_tags: (cycle: string[]) => ({
+		code: 'cyclical-const-tags',
+		message: `Cyclical dependency detected: ${cycle.join(' → ')}`
+	})
 };

src/compiler/compile/nodes/Binding.ts

@@ -57,6 +57,9 @@ export default class Binding extends Node {
 				component.error(this, compiler_errors.invalid_binding_await);
 				return;
 			}
+			if (scope.is_const(name)) {
+				component.error(this, compiler_errors.invalid_binding_const);
+			}
 
 			scope.dependencies_for_name.get(name).forEach(name => {
 				const variable = component.var_lookup.get(name);

src/compiler/compile/nodes/CatchBlock.ts

@@ -1,13 +1,15 @@
-import map_children from './shared/map_children';
 import TemplateScope from './shared/TemplateScope';
 import AbstractBlock from './shared/AbstractBlock';
 import AwaitBlock from './AwaitBlock';
 import Component from '../Component';
 import { TemplateNode } from '../../interfaces';
+import get_const_tags from './shared/get_const_tags';
+import ConstTag from './ConstTag';
 
 export default class CatchBlock extends AbstractBlock {
 	type: 'CatchBlock';
 	scope: TemplateScope;
+	const_tags: ConstTag[];
 
 	constructor(component: Component, parent: AwaitBlock, scope: TemplateScope, info: TemplateNode) {
 		super(component, parent, scope, info);
@@ -18,7 +20,8 @@ export default class CatchBlock extends AbstractBlock {
 				this.scope.add(context.key.name, parent.expression.dependencies, this);
 			});
 		}
-		this.children = map_children(component, parent, this.scope, info.children);
+
+		([this.const_tags, this.children] = get_const_tags(info.children, component, this, parent));
 
 		if (!info.skip) {
 			this.warn_if_empty_block();

src/compiler/compile/nodes/ConstTag.ts

@@ -0,0 +1,72 @@
+import Node from './shared/Node';
+import Expression from './shared/Expression';
+import Component from '../Component';
+import TemplateScope from './shared/TemplateScope';
+import { Context, unpack_destructuring } from './shared/Context';
+import { ConstTag as ConstTagType } from '../../interfaces';
+import { INodeAllowConstTag } from './interfaces';
+import { walk } from 'estree-walker';
+import { extract_identifiers } from 'periscopic';
+import is_reference, { NodeWithPropertyDefinition } from 'is-reference';
+import get_object from '../utils/get_object';
+import compiler_errors from '../compiler_errors';
+
+const allowed_parents = new Set(['EachBlock', 'CatchBlock', 'ThenBlock', 'InlineComponent', 'SlotTemplate']);
+
+export default class ConstTag extends Node {
+	type: 'ConstTag';
+	expression: Expression;
+	contexts: Context[] = [];
+	node: ConstTagType;
+	scope: TemplateScope;
+
+	assignees: Set<string> = new Set();
+  dependencies: Set<string> = new Set();
+
+	constructor(component: Component, parent: INodeAllowConstTag, scope: TemplateScope, info: ConstTagType) {
+		super(component, parent, scope, info);
+
+		if (!allowed_parents.has(parent.type)) {
+			component.error(info, compiler_errors.invalid_const_placement);
+		}
+		this.node = info;
+		this.scope = scope;
+
+		const { assignees, dependencies } = this;
+
+		extract_identifiers(info.expression.left).forEach(({ name }) => {
+      assignees.add(name);
+			const owner = this.scope.get_owner(name);
+			if (owner === parent) {
+				component.error(info, compiler_errors.invalid_const_declaration(name));
+			}
+    });
+
+    walk(info.expression.right, {
+      enter(node, parent) {
+        if (is_reference(node as NodeWithPropertyDefinition, parent as NodeWithPropertyDefinition)) {
+          const identifier = get_object(node as any);
+          const { name } = identifier;
+          dependencies.add(name);
+        }
+      }
+    });
+	}
+
+	parse_expression() {
+		unpack_destructuring({
+			contexts: this.contexts,
+			node: this.node.expression.left,
+			scope: this.scope,
+			component: this.component
+		});
+		this.expression = new Expression(this.component, this, this.scope, this.node.expression.right);
+		this.contexts.forEach(context => {
+			const owner = this.scope.get_owner(context.key.name);
+			if (owner && owner.type === 'ConstTag' && owner.parent === this.parent) {
+				this.component.error(this.node, compiler_errors.invalid_const_declaration(context.key.name));
+			}
+			this.scope.add(context.key.name, this.expression.dependencies, this);
+		});
+	}
+}

src/compiler/compile/nodes/DefaultSlotTemplate.ts

@@ -1,14 +1,15 @@
 import ElseBlock from './ElseBlock';
 import Expression from './shared/Expression';
-import map_children from './shared/map_children';
 import TemplateScope from './shared/TemplateScope';
 import AbstractBlock from './shared/AbstractBlock';
 import Element from './Element';
+import ConstTag from './ConstTag';
 import { Context, unpack_destructuring } from './shared/Context';
 import { Node } from 'estree';
 import Component from '../Component';
 import { TemplateNode } from '../../interfaces';
 import compiler_errors from '../compiler_errors';
+import get_const_tags from './shared/get_const_tags';
 
 export default class EachBlock extends AbstractBlock {
 	type: 'EachBlock';
@@ -22,6 +23,7 @@ export default class EachBlock extends AbstractBlock {
 	key: Expression;
 	scope: TemplateScope;
 	contexts: Context[];
+	const_tags: ConstTag[];
 	has_animation: boolean;
 	has_binding = false;
 	has_index_binding = false;
@@ -57,7 +59,7 @@ export default class EachBlock extends AbstractBlock {
 
 		this.has_animation = false;
 
-		this.children = map_children(component, this, this.scope, info.children);
+		([this.const_tags, this.children] = get_const_tags(info.children, component, this, this));
 
 		if (this.has_animation) {
 			if (this.children.length !== 1) {

src/compiler/compile/nodes/EachBlock.ts

@@ -126,7 +126,15 @@ export default class InlineComponent extends Node {
 						slot_template.attributes.push(attribute);
 					}
 				}
-		
+				// transfer const
+				for (let i = child.children.length - 1; i >= 0; i--) {
+					const child_child = child.children[i];
+					if (child_child.type === 'ConstTag') {
+						slot_template.children.push(child_child);
+						child.children.splice(i, 1);
+					}
+				}
+
 				children.push(slot_template);
 				info.children.splice(i, 1);
 			}

src/compiler/compile/nodes/InlineComponent.ts

@@ -1,17 +1,19 @@
-import map_children from './shared/map_children';
 import Component from '../Component';
 import TemplateScope from './shared/TemplateScope';
 import Node from './shared/Node';
 import Let from './Let';
 import Attribute from './Attribute';
 import { INode } from './interfaces';
 import compiler_errors from '../compiler_errors';
+import get_const_tags from './shared/get_const_tags';
+import ConstTag from './ConstTag';
 
 export default class SlotTemplate extends Node {
 	type: 'SlotTemplate';
 	scope: TemplateScope;
 	children: INode[];
 	lets: Let[] = [];
+	const_tags: ConstTag[];
 	slot_attribute: Attribute;
 	slot_template_name: string = 'default';
 
@@ -63,7 +65,7 @@ export default class SlotTemplate extends Node {
 		});
 
 		this.scope = scope;
-		this.children = map_children(component, this, this.scope, info.children);
+		([this.const_tags, this.children] = get_const_tags(info.children, component, this, this));
 	}
 
 	validate_slot_template_placement() {

src/compiler/compile/nodes/SlotTemplate.ts

@@ -1,13 +1,15 @@
-import map_children from './shared/map_children';
 import TemplateScope from './shared/TemplateScope';
 import AbstractBlock from './shared/AbstractBlock';
 import AwaitBlock from './AwaitBlock';
 import Component from '../Component';
 import { TemplateNode } from '../../interfaces';
+import get_const_tags from './shared/get_const_tags';
+import ConstTag from './ConstTag';
 
 export default class ThenBlock extends AbstractBlock {
 	type: 'ThenBlock';
 	scope: TemplateScope;
+	const_tags: ConstTag[];
 
 	constructor(component: Component, parent: AwaitBlock, scope: TemplateScope, info: TemplateNode) {
 		super(component, parent, scope, info);
@@ -18,7 +20,8 @@ export default class ThenBlock extends AbstractBlock {
 				this.scope.add(context.key.name, parent.expression.dependencies, this);
 			});
 		}
-		this.children = map_children(component, parent, this.scope, info.children);
+
+		([this.const_tags, this.children] = get_const_tags(info.children, component, this, parent));
 
 		if (!info.skip) {
 			this.warn_if_empty_block();