fix: avoid unnecessary stringify in server attributes (#18232)

### Before submitting the PR, please make sure you do the following

- [x] It's really useful if your PR references an issue where it is
discussed ahead of time. Fixes #10031.
- [x] Prefix your PR title with `feat:`, `fix:`, `chore:`, or `docs:`.
- [x] This message body should clearly illustrate what problems it
solves.
- [x] Ideally, include a test that fails without this PR but passes with
it.
- [x] If this PR changes code within `packages/svelte/src`, add a
changeset (`npx changeset`).

### What this changes

Server attribute template generation currently wraps each dynamic
expression in `$.stringify`, even when the compiler can prove the
expression is a string or a known constant. This reuses the existing
scope evaluation metadata so server output can avoid `$.stringify` for
proven string/constant chunks while keeping it for possibly nullish
unknown values.

The updated snapshot covers a mixed attribute with a known string,
mutable state, `null`, numeric/undefined constants, a known
string-producing `typeof`, and an unknown prop value.

### Tests and linting

- [x] `pnpm test snapshot -t nullish-coallescence-omittance`
- [x] `pnpm test snapshot`
- [x] `pnpm --filter svelte check`
- [x] `pnpm lint`
- [x] `pnpm prettier --check .changeset/slow-bikes-serve.md`

---------

Co-authored-by: Rich Harris <hello@rich-harris.dev>

.changeset/slow-bikes-serve.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: inline primitive constants in attribute values during SSR

packages/svelte/src/compiler/phases/3-transform/server/visitors/shared/utils.js

@@ -229,18 +229,25 @@ export function build_attribute_value(
 				? node.data.replace(regex_whitespaces_strict, ' ')
 				: node.data;
 		} else {
-			expressions.push(
-				b.call(
-					'$.stringify',
-					transform(
-						/** @type {Expression} */ (context.visit(node.expression)),
-						node.metadata.expression
-					)
-				)
-			);
+			const evaluated = context.state.scope.evaluate(node.expression);
 
-			quasi = b.quasi('', i + 1 === value.length);
-			quasis.push(quasi);
+			if (evaluated.is_known) {
+				quasi.value.cooked += (evaluated.value ?? '') + '';
+			} else {
+				const expression = transform(
+					/** @type {Expression} */ (context.visit(node.expression)),
+					node.metadata.expression
+				);
+
+				expressions.push(
+					evaluated.is_string && evaluated.is_defined
+						? expression
+						: b.call('$.stringify', expression)
+				);
+
+				quasi = b.quasi('', i + 1 === value.length);
+				quasis.push(quasi);
+			}
 		}
 	}
 
@@ -248,7 +255,9 @@ export function build_attribute_value(
 		quasi.value.raw = sanitize_template_string(/** @type {string} */ (quasi.value.cooked));
 	}
 
-	return b.template(quasis, expressions);
+	return expressions.length > 0
+		? b.template(quasis, expressions)
+		: b.literal(/** @type {string} */ (quasi.value.cooked));
 }
 
 /**

packages/svelte/tests/snapshot/samples/nullish-coallescence-omittance/_expected/client/index.svelte.js

@@ -1,9 +1,9 @@
 import 'svelte/internal/disclose-version';
 import * as $ from 'svelte/internal/client';
 
-var root = $.from_html(`<h1></h1> <b></b> <button> </button> <h1></h1>`, 1);
+var root = $.from_html(`<h1></h1> <b></b> <button> </button> <h1></h1> <div></div>`, 1);
 
-export default function Nullish_coallescence_omittance($$anchor) {
+export default function Nullish_coallescence_omittance($$anchor, $$props) {
 	let name = 'world';
 	let count = $.state(0);
 	var fragment = root();
@@ -23,7 +23,14 @@ export default function Nullish_coallescence_omittance($$anchor) {
 	var h1_1 = $.sibling(button, 2);
 
 	h1_1.textContent = 'Hello, world';
-	$.template_effect(() => $.set_text(text, `Count is ${$.get(count) ?? ''}`));
+
+	var div = $.sibling(h1_1, 2);
+
+	$.template_effect(() => {
+		$.set_text(text, `Count is ${$.get(count) ?? ''}`);
+		$.set_attribute(div, 'title', `Hello, world ${$.get(count) ?? ''}  1  ${typeof $$props.value} ${$$props.value ?? ''}`);
+	});
+
 	$.delegated('click', button, () => $.update(count));
 	$.append($$anchor, fragment);
 }

packages/svelte/tests/snapshot/samples/nullish-coallescence-omittance/_expected/server/index.svelte.js

@@ -1,8 +1,9 @@
 import * as $ from 'svelte/internal/server';
 
-export default function Nullish_coallescence_omittance($$renderer) {
+export default function Nullish_coallescence_omittance($$renderer, $$props) {
 	let name = 'world';
 	let count = 0;
+	let { value } = $$props;
 
-	$$renderer.push(`<h1>Hello, world!</h1> <b>123</b> <button>Count is ${$.escape(count)}</button> <h1>Hello, world</h1>`);
+	$$renderer.push(`<h1>Hello, world!</h1> <b>123</b> <button>Count is ${$.escape(count)}</button> <h1>Hello, world</h1> <div${$.attr('title', `Hello, world ${$.stringify(count)}  1  ${typeof value} ${$.stringify(value)}`)}></div>`);
 }

packages/svelte/tests/snapshot/samples/nullish-coallescence-omittance/index.svelte

@@ -1,8 +1,10 @@
 <script>
     let name = 'world';
     let count = $state(0);
+    let { value } = $props();
 </script>
 <h1>Hello, {null}{name}!</h1>
 <b>{1 ?? 'stuff'}{2 ?? 'more stuff'}{3 ?? 'even more stuff'}</b>
 <button onclick={()=>count++}>Count is {count}</button>
-<h1>Hello, {name ?? 'earth' ?? null}</h1>
+<h1>Hello, {name ?? 'earth' ?? null}</h1>
+<div title="Hello, {name} {count} {null} {1} {undefined} {typeof value} {value}"></div>