refactor(core): parse @value with postcss-value-parser (#412)

Replace the regex-based parser with postcss-value-parser, which provides
node positions directly. This removes the manual line/column/offset
arithmetic and aligns the implementation with composes-parser.

Behavior for syntax supported by css-loader is unchanged. For syntax not
supported by css-loader, the leading word is now accepted as the token
name instead of emitting a diagnostic.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: mizdra

.changeset/at-value-parser-with-value-parser.md

@@ -0,0 +1,12 @@
+---
+'@css-modules-kit/core': minor
+---
+
+refactor(core): parse `@value` with postcss-value-parser
+
+The `@value` parser is reimplemented on top of postcss-value-parser. Behavior for syntax supported by css-loader is unchanged.
+
+For syntax that css-loader does not support (where css-modules-kit does not guarantee a specific behavior), the result changed:
+
+- `@value \\c: #000;` and `@value \'d: #000;` are now parsed as a token declaration instead of reporting an error.
+- `@value \31 e: #000;` is now read as the token name `\31` instead of `e`.

packages/core/src/parser/at-value-parser.test.ts

@@ -4,7 +4,7 @@ import { fakeAtValues, fakeRoot } from '../test/ast.js';
 import { parseAtValue } from './at-value-parser.js';
 
 describe('parseAtValue', () => {
-  test('valid', () => {
+  test('parses syntax supported by css-loader', () => {
     const atValues = fakeAtValues(
       fakeRoot(dedent`
         @value basic: #000;
@@ -411,10 +411,14 @@ describe('parseAtValue', () => {
       ]
     `);
   });
-  test('invalid', () => {
-    // NOTE: These test cases are invalid as the syntax of @value. The behavior when using
-    // these syntaxes is undefined. A diagnostic like "`@value` is a invalid syntax." may
-    // or may not be reported.
+  test('parses syntax unsupported by css-loader', () => {
+    // NOTE: These syntaxes are not supported by css-loader, so css-modules-kit does not
+    // guarantee any specific behavior for them. The snapshots below document how the current
+    // implementation parses them rather than a behavior we commit to.
+    //
+    // The `@value \31 e` case is tokenized as `\31` and `e` instead of a single ident `1e`,
+    // because postcss-value-parser does not interpret CSS escape sequences in identifiers.
+    // This is a known bug: https://github.com/postcss/postcss-value-parser/issues/64
     const atValues = fakeAtValues(
       fakeRoot(dedent`
         @value;
@@ -494,38 +498,74 @@ describe('parseAtValue', () => {
               "category": "error",
               "length": 0,
               "start": {
-                "column": 8,
+                "column": 10,
                 "line": 2,
               },
               "text": "\`\` is invalid syntax.",
             },
           ],
         },
         {
-          "diagnostics": [
-            {
-              "category": "error",
-              "length": 17,
+          "atValue": {
+            "declarationLoc": {
+              "end": {
+                "column": 17,
+                "line": 3,
+                "offset": 53,
+              },
               "start": {
                 "column": 1,
                 "line": 3,
+                "offset": 37,
               },
-              "text": "\`@value \\\\c: #000\` is a invalid syntax.",
             },
-          ],
+            "loc": {
+              "end": {
+                "column": 11,
+                "line": 3,
+                "offset": 47,
+              },
+              "start": {
+                "column": 8,
+                "line": 3,
+                "offset": 44,
+              },
+            },
+            "name": "\\\\c",
+            "type": "valueDeclaration",
+          },
+          "diagnostics": [],
         },
         {
-          "diagnostics": [
-            {
-              "category": "error",
-              "length": 17,
+          "atValue": {
+            "declarationLoc": {
+              "end": {
+                "column": 17,
+                "line": 4,
+                "offset": 71,
+              },
               "start": {
                 "column": 1,
                 "line": 4,
+                "offset": 55,
               },
-              "text": "\`@value \\'d: #000\` is a invalid syntax.",
             },
-          ],
+            "loc": {
+              "end": {
+                "column": 11,
+                "line": 4,
+                "offset": 65,
+              },
+              "start": {
+                "column": 8,
+                "line": 4,
+                "offset": 62,
+              },
+            },
+            "name": "\\'d",
+            "type": "valueDeclaration",
+          },
+          "diagnostics": [],
         },
         {
           "atValue": {
@@ -543,17 +583,17 @@ describe('parseAtValue', () => {
             },
             "loc": {
               "end": {
-                "column": 13,
+                "column": 11,
                 "line": 5,
-                "offset": 85,
+                "offset": 83,
               },
               "start": {
-                "column": 12,
+                "column": 8,
                 "line": 5,
-                "offset": 84,
+                "offset": 80,
               },
             },
-            "name": "e",
+            "name": "\\31",
             "type": "valueDeclaration",
           },
           "diagnostics": [],