Fix auto-imports

This change adds support for auto-imports.

The virtual code now contains an empty import of the JSX runtime. This
import was chosen, because it must exist anyway. This is immediately
followed by an empty code mapping, meaning TypeScript always has a place
to insert auto-imports.

Since JSX components can be injected, they are sometimes prefixed with
`_components.` in the virtual code. To support auto-import completions,
an additional mapping is now made to an expression containing merely the
identifier. As a result, the editor now shows auto-import completions,
unless `MDXProvidedComponents` is defined. I don’t know why the
existence of `MDXProvidedComponents` matters, but this probably matches
the expectation of users anyway.

The auto-imports will not be followed by a blank line. This can lead to
a syntax error in case no other imports exist yet. This is not ideal,
but easy and straight-forward to resolve manually.

Closes #452

packages/language-server/test/completion.test.js

@@ -47,7 +47,7 @@ test('support completion in ESM', async () => {
       original: {
         data: {
           fileName: fixturePath('node16/completion.mdx'),
-          offset: 81,
+          offset: 108,
           originalItem: {name: 'Boolean'},
           uri: String(
             URI.from({
@@ -110,7 +110,7 @@ test('support completion in JSX', async () => {
       original: {
         data: {
           fileName: fixturePath('node16/completion.mdx'),
-          offset: 119,
+          offset: 146,
           originalItem: {name: 'Boolean'},
           uri: String(
             URI.from({

packages/language-service/lib/virtual-code.js

@@ -28,6 +28,7 @@ const jsPrefix = (
   jsxImportSource
 ) => `${tsCheck ? '// @ts-check\n' : ''}/* @jsxRuntime automatic
 @jsxImportSource ${jsxImportSource} */
+import '${jsxImportSource}/jsx-runtime'
 `
 
 /**
@@ -294,6 +295,21 @@ function processExports(mdx, node, mapping, esm) {
   return esm + '\n'
 }
 
+/**
+ * Pad the generated offsets of a Volar code mapping.
+ *
+ * @param {CodeMapping} mapping
+ *   The mapping whose generated offsets to pad.
+ * @param {number} padding
+ *   The padding to append to the generated offsets.
+ * @returns {undefined}
+ */
+function padOffsets(mapping, padding) {
+  for (let i = 0; i < mapping.generatedOffsets.length; i++) {
+    mapping.generatedOffsets[i] += padding
+  }
+}
+
 /**
  * @param {string} mdx
  * @param {Root} ast
@@ -302,6 +318,13 @@ function processExports(mdx, node, mapping, esm) {
  * @returns {VirtualCode[]}
  */
 function getEmbeddedCodes(mdx, ast, checkMdx, jsxImportSource) {
+  let hasAwait = false
+  let esm = jsPrefix(checkMdx, jsxImportSource)
+  let jsx = ''
+  let jsxVariables = ''
+  let markdown = ''
+  let nextMarkdownSourceStart = 0
+
   /** @type {CodeMapping[]} */
   const jsMappings = []
 
@@ -311,9 +334,11 @@ function getEmbeddedCodes(mdx, ast, checkMdx, jsxImportSource) {
    * @type {CodeMapping}
    */
   const esmMapping = {
-    sourceOffsets: [],
-    generatedOffsets: [],
-    lengths: [],
+    // The empty mapping makes sure there’s always a valid mapping to insert
+    // auto-imports.
+    sourceOffsets: [0],
+    generatedOffsets: [esm.length],
+    lengths: [0],
     data: {
       completion: true,
       format: true,
@@ -343,6 +368,20 @@ function getEmbeddedCodes(mdx, ast, checkMdx, jsxImportSource) {
     }
   }
 
+  const jsxVariablesMapping = {
+    sourceOffsets: [],
+    generatedOffsets: [],
+    lengths: [],
+    data: {
+      completion: true,
+      format: false,
+      navigation: true,
+      semantic: true,
+      structure: true,
+      verification: true
+    }
+  }
+
   /**
    * The Volar mapping that maps all markdown content to the virtual markdown file.
    *
@@ -364,13 +403,6 @@ function getEmbeddedCodes(mdx, ast, checkMdx, jsxImportSource) {
 
   /** @type {VirtualCode[]} */
   const virtualCodes = []
-
-  let hasAwait = false
-  let esm = jsPrefix(checkMdx, jsxImportSource)
-  let jsx = ''
-  let markdown = ''
-  let nextMarkdownSourceStart = 0
-
   const visitors = createVisitors()
 
   for (const child of ast.children) {
@@ -482,6 +514,17 @@ function getEmbeddedCodes(mdx, ast, checkMdx, jsxImportSource) {
 
       jsx =
         addOffset(jsxMapping, mdx, jsx, newIndex, name.start) + '_components.'
+      if (node.name && node.type === 'JSXOpeningElement') {
+        jsxVariables =
+          addOffset(
+            jsxVariablesMapping,
+            mdx,
+            jsxVariables + '// @ts-ignore\n',
+            name.start,
+            name.end
+          ) + '\n'
+      }
+
       newIndex = name.start
     }
 
@@ -624,6 +667,16 @@ function getEmbeddedCodes(mdx, ast, checkMdx, jsxImportSource) {
           jsx = addOffset(jsxMapping, mdx, jsx + jsxIndent, start, lastIndex)
           if (isInjectableComponent(node.name, programScope)) {
             jsx += '_components.'
+            if (node.name) {
+              jsxVariables =
+                addOffset(
+                  jsxVariablesMapping,
+                  mdx,
+                  jsxVariables + '// @ts-ignore\n',
+                  lastIndex,
+                  lastIndex + node.name.length
+                ) + '\n'
+            }
           }
 
           if (node.name) {
@@ -741,12 +794,12 @@ function getEmbeddedCodes(mdx, ast, checkMdx, jsxImportSource) {
   updateMarkdownFromOffsets(mdx.length, mdx.length)
   esm += componentStart(hasAwait, programScope)
 
-  for (let i = 0; i < jsxMapping.generatedOffsets.length; i++) {
-    jsxMapping.generatedOffsets[i] += esm.length
-  }
-
+  padOffsets(jsxMapping, esm.length)
   esm += jsx + componentEnd
 
+  padOffsets(jsxVariablesMapping, esm.length)
+  esm += jsxVariables
+
   if (esmMapping.sourceOffsets.length > 0) {
     jsMappings.push(esmMapping)
   }
@@ -755,6 +808,10 @@ function getEmbeddedCodes(mdx, ast, checkMdx, jsxImportSource) {
     jsMappings.push(jsxMapping)
   }
 
+  if (jsxVariablesMapping.sourceOffsets.length > 0) {
+    jsMappings.push(jsxVariablesMapping)
+  }
+
   virtualCodes.unshift(
     {
       id: 'jsx',