WARNING: This is an unofficial (unsupported) patch - stuff might break! - Credit goes to https://github.com/NicoCevallos

eslint-plugin-svelte3+2.7.3.patch

diff --git a/node_modules/eslint-plugin-svelte3/index.js b/node_modules/eslint-plugin-svelte3/index.js
index 86ec523..cba16d2 100644
--- a/node_modules/eslint-plugin-svelte3/index.js
+++ b/node_modules/eslint-plugin-svelte3/index.js
@@ -62,6 +62,15 @@ const get_line_offsets = str => {
 	return offsets;
 };
 
+// find the index of the last element of an array matching a condition
+const find_last_index = (array, cond) => {
+	const idx = array.findIndex(item => !cond(item));
+	return idx === -1 ? array.length - 1 : idx - 1;
+};
+
+// find the last element of an array matching a condition
+const find_last = (array, cond) => array[find_last_index(array, cond)];
+
 // return a new block
 const new_block = () => ({ transformed_code: '', line_offsets: null, translations: new Map() });
 
@@ -100,6 +109,7 @@ Linter.prototype.verify = function(code, config, options) {
 	processor_options.ignore_styles = settings['svelte3/ignore-styles'];
 	processor_options.compiler_options = settings['svelte3/compiler-options'];
 	processor_options.named_blocks = settings['svelte3/named-blocks'];
+	processor_options.svelte_preprocess = settings['svelte3/preprocess'];
 	// call original Linter#verify
 	return verify.call(this, code, config, options);
 };
@@ -110,10 +120,94 @@ const reset = () => {
 		messages: null,
 		var_names: null,
 		blocks: new Map(),
+		pre_line_offsets: null,
+		post_line_offsets: null,
+		mappings: null,
 	};
 };
 reset();
 
+var charToInteger = {};
+var chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=';
+for (var i = 0; i < chars.length; i++) {
+    charToInteger[chars.charCodeAt(i)] = i;
+}
+function decode(mappings) {
+    var generatedCodeColumn = 0; // first field
+    var sourceFileIndex = 0; // second field
+    var sourceCodeLine = 0; // third field
+    var sourceCodeColumn = 0; // fourth field
+    var nameIndex = 0; // fifth field
+    var decoded = [];
+    var line = [];
+    var segment = [];
+    for (var i = 0, j = 0, shift = 0, value = 0, len = mappings.length; i < len; i++) {
+        var c = mappings.charCodeAt(i);
+        if (c === 44) { // ","
+            if (segment.length)
+                line.push(segment);
+            segment = [];
+            j = 0;
+        }
+        else if (c === 59) { // ";"
+            if (segment.length)
+                line.push(segment);
+            segment = [];
+            j = 0;
+            decoded.push(line);
+            line = [];
+            generatedCodeColumn = 0;
+        }
+        else {
+            var integer = charToInteger[c];
+            if (integer === undefined) {
+                throw new Error('Invalid character (' + String.fromCharCode(c) + ')');
+            }
+            var hasContinuationBit = integer & 32;
+            integer &= 31;
+            value += integer << shift;
+            if (hasContinuationBit) {
+                shift += 5;
+            }
+            else {
+                var shouldNegate = value & 1;
+                value >>>= 1;
+                if (shouldNegate) {
+                    value = -value;
+                    if (value === 0)
+                        value = -0x80000000;
+                }
+                if (j == 0) {
+                    generatedCodeColumn += value;
+                    segment.push(generatedCodeColumn);
+                }
+                else if (j === 1) {
+                    sourceFileIndex += value;
+                    segment.push(sourceFileIndex);
+                }
+                else if (j === 2) {
+                    sourceCodeLine += value;
+                    segment.push(sourceCodeLine);
+                }
+                else if (j === 3) {
+                    sourceCodeColumn += value;
+                    segment.push(sourceCodeColumn);
+                }
+                else if (j === 4) {
+                    nameIndex += value;
+                    segment.push(nameIndex);
+                }
+                j++;
+                value = shift = 0; // reset
+            }
+        }
+    }
+    if (segment.length)
+        line.push(segment);
+    decoded.push(line);
+    return decoded;
+}
+
 let default_compiler;
 
 // find the contextual name or names described by a particular node in the AST
@@ -135,7 +229,7 @@ const find_contextual_names = (compiler, node) => {
 };
 
 // extract scripts to lint from component definition
-const preprocess = text => {
+const preprocess = (text, filename) => {
 	const compiler = processor_options.custom_compiler || default_compiler || (default_compiler = require('svelte/compiler'));
 	if (processor_options.ignore_styles) {
 		// wipe the appropriate <style> tags in the file
@@ -152,10 +246,112 @@ const preprocess = text => {
 			return processor_options.ignore_styles(attrs) ? match.replace(/\S/g, ' ') : match;
 		});
 	}
-	// get information about the component
 	let result;
+	let processedResult;
+	let processedModule;
+	let processedInstance;
+	let processedStyle;
+	let processedMarkup;
+	let moduleExt = 'js';
+	let instanceExt = 'js';
+	let moduleEndLine;
+	let processedModuleLineOffset;
+	let instanceEndLine;
+	let processedInstanceLineOffset;
 	try {
-		result = compiler.compile(text, { generate: false, ...processor_options.compiler_options });
+		// run preprocessor if present
+		if (processor_options.svelte_preprocess) {
+			const result = processor_options.svelte_preprocess(text, filename);
+			if (result) {
+				state.pre_line_offsets = get_line_offsets(text);
+				processedResult = result.code;
+				state.post_line_offsets = get_line_offsets(processedResult);
+				if (result.mappings) {
+					state.mappings = decode(result.mappings);
+				}
+
+				if (result.module) {
+					processedModule = result.module;
+					moduleExt = result.module.ext;
+				}
+				if (result.instance) {
+					processedInstance = result.instance;
+					instanceExt = result.instance.ext;
+				}
+
+				processedStyle = result.style;
+
+				processedMarkup = result.markup;
+
+				processor_options.named_blocks = true;
+			}
+		}
+		// get information about the component
+		result = compiler.compile(processedResult || text, { generate: false, ...processor_options.compiler_options });
+		if (processedResult) {
+			const { html, css, instance, module } = result.ast;
+
+			let moduleDiff = processedModule ? processedModule.diff : 0;
+			let instanceDiff = processedInstance ? processedInstance.diff : 0;
+			let styleDiff = processedStyle ? processedStyle.diff : 0;
+			let markupDiff = processedMarkup ? processedMarkup.diff : 0;
+
+			let modulePreOffset = 0;
+			let modulePostOffset = 0;
+			if (module) {
+				if (module.start > html.start) {
+					modulePreOffset += markupDiff;
+				}
+				if (css && module.start > css.start) {
+					modulePreOffset += styleDiff;
+				}
+				if (instance && module.start > instance.start) {
+					modulePreOffset += instanceDiff;
+				}
+
+				modulePostOffset = modulePreOffset + moduleDiff;
+			}
+
+			let instancePreOffset = 0;
+			let instancePostOffset = 0;
+			if (instance) {
+				if (instance.start > html.start) {
+					instancePreOffset += markupDiff;
+				}
+				if (css && instance.start > css.start) {
+					instancePreOffset += styleDiff;
+				}
+				if (module && instance.start > module.start) {
+					instancePreOffset += moduleDiff;
+				}
+
+				instancePostOffset = instancePreOffset + instanceDiff;
+			}
+
+			if (module && processedModule) {
+				moduleEndLine = module.content.loc.end.line;
+				processedModuleLineOffset = processedModule.ast.loc.end.line - moduleEndLine;
+				module.content.body = processedModule.ast.body;
+
+				module.start += modulePreOffset;
+				module.end += modulePostOffset;
+
+				module.content.start += modulePreOffset;
+				module.content.end += modulePostOffset;
+			}
+
+			if (instance && processedInstance) {
+				instanceEndLine = instance.content.loc.end.line;
+				processedInstanceLineOffset = processedInstance.ast.loc.end.line - instanceEndLine;
+				instance.content.body = processedInstance.ast.body;
+
+				instance.start += instancePreOffset;
+				instance.end += instancePostOffset;
+
+				instance.content.start += instancePreOffset;
+				instance.content.end += instancePostOffset;
+			}
+		}
 	} catch ({ name, message, start, end }) {
 		// convert the error to a linting message, store it, and return
 		state.messages = [
@@ -176,27 +372,40 @@ const preprocess = text => {
 	state.var_names = new Set(vars.map(v => v.name));
 
 	// convert warnings to linting messages
-	state.messages = (processor_options.ignore_warnings ? warnings.filter(warning => !processor_options.ignore_warnings(warning)) : warnings).map(({ code, message, start, end }) => ({
-		ruleId: code,
-		severity: 1,
-		message,
-		line: start && start.line,
-		column: start && start.column + 1,
-		endLine: end && end.line,
-		endColumn: end && end.column + 1,
-	}));
+	state.messages = (processor_options.ignore_warnings ? warnings.filter(warning => !processor_options.ignore_warnings(warning)) : warnings).map(({ code, message, start, end }) => {
+		let fixLine = 0;
+
+		if (processedInstanceLineOffset && start && start.line > instanceEndLine ) {
+			fixLine += processedInstanceLineOffset;
+		}
+
+		if (processedModuleLineOffset && start && start.line > moduleEndLine ) {
+			fixLine += processedModuleLineOffset;
+		}
+		return {
+			ruleId: code,
+			severity: 1,
+			message,
+			line: start && start.line + fixLine,
+			column: start && start.column + 1,
+			endLine: end && end.line + fixLine,
+			endColumn: end && end.column + 1,
+		}
+	});
 
 	// build strings that we can send along to ESLint to get the remaining messages
 
 	if (ast.module) {
 		// block for <script context='module'>
 		const block = new_block();
-		state.blocks.set('module.js', block);
+		state.blocks.set(`module.${moduleExt}`, block);
 
 		get_translation(text, block, ast.module.content);
 
 		if (ast.instance) {
-			block.transformed_code += text.slice(ast.instance.content.start, ast.instance.content.end);
+			block.transformed_code += processedInstance
+			? processedInstance.original
+			: text.slice(ast.instance.content.start, ast.instance.content.end);
 		}
 
 		block.transformed_code += references_and_reassignments;
@@ -205,7 +414,7 @@ const preprocess = text => {
 	if (ast.instance) {
 		// block for <script context='instance'>
 		const block = new_block();
-		state.blocks.set('instance.js', block);
+		state.blocks.set(`instance.${instanceExt}`, block);
 
 		block.transformed_code = vars.filter(v => v.injected || v.module).map(v => `let ${v.name};`).join('');
 
@@ -223,11 +432,13 @@ const preprocess = text => {
 
 		const nodes_with_contextual_scope = new WeakSet();
 		let in_quoted_attribute = false;
+		const htmlText = processedResult || text;
+
 		compiler.walk(ast.html, {
 			enter(node, parent, prop) {
 				if (prop === 'expression') {
 					return this.skip();
-				} else if (prop === 'attributes' && '\'"'.includes(text[node.end - 1])) {
+				} else if (prop === 'attributes' && '\'"'.includes(htmlText[node.end - 1])) {
 					in_quoted_attribute = true;
 				}
 				contextual_names.length = 0;
@@ -248,7 +459,7 @@ const preprocess = text => {
 				if (node.expression && typeof node.expression === 'object') {
 					// add the expression in question to the constructed string
 					block.transformed_code += '(';
-					get_translation(text, block, node.expression, { template: true, in_quoted_attribute });
+					get_translation(htmlText, block, node.expression, { template: true, in_quoted_attribute });
 					block.transformed_code += ');';
 				}
 			},
@@ -268,6 +479,31 @@ const preprocess = text => {
 	return [...state.blocks].map(([filename, { transformed_code: text }]) => processor_options.named_blocks ? { text, filename } : text);
 };
 
+const unmap = message => {
+	for (let j = 0; j < 2; j++) {
+		if (message[j ? 'endLine' : 'line']) {
+			const mapping = find_last(state.mappings[message[j ? 'endLine' : 'line'] - 1], ([column]) => column < message[j ? 'endColumn' : 'column']);
+			if (!mapping || mapping[1] !== 0) {
+				return false;
+			}
+			message[j ? 'endLine' : 'line'] = mapping[2] + 1;
+			message[j ? 'endColumn' : 'column'] += mapping[3] - mapping[0];
+		}
+	}
+	if (message.fix) {
+		for (let j = 0; j < 2; j++) {
+			const line = find_last_index(state.post_line_offsets, offset => offset < message.fix.range[j]);
+			const line_offset = state.post_line_offsets[line];
+			const mapping = find_last(state.mappings[line], ([column]) => column < message.fix.range[j] - line_offset);
+			if (!mapping || mapping[1] !== 0) {
+				return false;
+			}
+			message.fix.range[j] += mapping[3] - mapping[0] + state.pre_line_offsets[mapping[2]] - line_offset;
+		}
+	}
+	return true;
+};
+
 // transform a linting message according to the module/instance script info we've gathered
 const transform_message = ({ transformed_code }, { unoffsets, dedent, offsets, range }, message) => {
 	// strip out the start and end of the fix if they are not actually changes
@@ -385,6 +621,9 @@ const postprocess = blocks_messages => {
 			}
 		}
 	}
+	if (state.mappings) {
+		state.messages = state.messages.filter(unmap);
+	}
 
 	// sort messages and return
 	const sorted_messages = state.messages.sort((a, b) => a.line - b.line || a.column - b.column);