Report better errors for values parsed from a PostCSS stylesheet

Closes shellscape#98

Author: nex3

lib/SubInput.js

@@ -0,0 +1,55 @@
+/*
+  Copyright © 2018 Andrew Powell
+
+  This Source Code Form is subject to the terms of the Mozilla Public
+  License, v. 2.0. If a copy of the MPL was not distributed with this
+  file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+  The above copyright notice and this permission notice shall be
+  included in all copies or substantial portions of this Source Code Form.
+*/
+
+// A PostCSS Input that exposes a substring of a larger Input as though it were
+// the entire text to be parsed.
+module.exports = class SubInput {
+  constructor(css, context, lineInContext, columnInContext) {
+    this.css = css;
+    this.context = context;
+    this.lineInContext = lineInContext;
+    this.columnInContext = columnInContext;
+  }
+
+  error(message, line, column, opts = {}) {
+    let lineInContext;
+    let columnInContext;
+    if (line === 1) {
+      lineInContext = this.lineInContext;
+      columnInContext = column + this.columnInContext - 1;
+    } else {
+      lineInContext = this.lineInContext + line - 1;
+    }
+
+    return this.context.error(message, lineInContext, columnInContext, opts);
+  }
+
+  origin(line, column) {
+    let lineInContext;
+    let columnInContext;
+    if (line === 1) {
+      lineInContext = this.lineInContext;
+      columnInContext = column + this.columnInContext - 1;
+    } else {
+      lineInContext = this.lineInContext + line - 1;
+    }
+
+    return this.context.origin(lineInContext, columnInContext);
+  }
+
+  mapResolve(file) {
+    return this.context.mapResolve(file);
+  }
+
+  get from() {
+    return this.context.from;
+  }
+};

lib/index.js

@@ -8,14 +8,51 @@
   The above copyright notice and this permission notice shall be
   included in all copies or substantial portions of this Source Code Form.
 */
+const assert = require('assert').strict;
+
 const Input = require('postcss/lib/input');
 
 const Parser = require('./ValuesParser');
+const SubInput = require('./SubInput');
 const { stringify } = require('./ValuesStringifier');
 
+const NEWLINE = '\n'.charCodeAt(0);
+const FEED = '\f'.charCodeAt(0);
+const CR = '\r'.charCodeAt(0);
+
+function positionAfter(node, chunks) {
+  let { line } = node.source.start;
+  let { column } = node.source.start;
+  for (const chunk of chunks) {
+    for (let i = 0; i < chunk.length; i++) {
+      const code = chunk.charCodeAt(i);
+      if (
+        code === NEWLINE ||
+        code === FEED ||
+        (code === CR && chunk.charCodeAt(i + 1) !== NEWLINE)
+      ) {
+        column = 1;
+        line += 1;
+      } else {
+        column += 1;
+      }
+    }
+  }
+
+  return { line, column };
+}
+
 module.exports = {
   parse(css, options) {
-    const input = new Input(css, options);
+    let input;
+    if (options.context) {
+      assert(options.lineInContext);
+      assert(options.columnInContext);
+      input = new SubInput(css, options.context, options.lineInContext, options.columnInContext);
+    } else {
+      input = new Input(css, options);
+    }
+
     const parser = new Parser(input, options);
 
     parser.parse();
@@ -32,6 +69,24 @@ module.exports = {
     return parser.root;
   },
 
+  parseDeclValue(decl) {
+    const { line, column } = positionAfter(decl, [decl.prop, decl.raws.between]);
+    return module.exports.parse(decl.value, {
+      context: decl.source.input,
+      lineInContext: line,
+      columnInContext: column
+    });
+  },
+
+  parseAtRuleParams(rule) {
+    const { line, column } = positionAfter(rule, ['@', rule.name, rule.raws.afterName]);
+    return module.exports.parse(rule.value, {
+      context: rule,
+      lineInContext: line,
+      columnInContext: column
+    });
+  },
+
   stringify,
 
   nodeToString(node) {