Fix cacjs#37: better help output for negated options

Not only doesn't show the `(default: true)` for negated options, which
was quite confusing, but merges them with the line describing the
affirmative case.

Added a new test.  Since it relies on help output, it is a bit
brittle, like the other "snapshot"-based ones.  I didn't the same
execa snapshot for this because it would make a test take a lot longer
to run (becasue spawns a process).

* README.md (Options): Mention [no-]foo syntax.

* src/Command.ts (outputHelp): Consider options with negated
counterparts when printing.

* src/__test__/index.test.ts ('negated option help output'): New test.

README.md

@@ -188,6 +188,17 @@ cli
 ```
 
 This will let CAC set the default value of `config` to true, and you can use `--no-config` flag to set it to `false`.
+This also results in the familiar `[no-]foo` syntax in `cli.help()` output:
+
+```
+Usage:
+  $ foo build [project]
+
+Options:
+  --[no-]config <path>   Disable config file/Use a custom config file
+  -h, --help             Display this message
+  -v, --version          Display version number
+```
 
 ### Variadic Arguments
 

src/Command.ts

@@ -184,22 +184,42 @@ class Command {
       ? globalOptions
       : [...this.options, ...(globalOptions || [])]
     if (options.length > 0) {
-      const longestOptionName = findLongest(
-        options.map((option) => option.rawName)
-      )
+      const negatedOpts: { [key: string]: Option } = {}
+      // O(n2), let's hope there aren't a zillion options
+      for (const o1 of options) {
+        if (!o1.negated) continue
+        for (const o2 of options) {
+          if (o1.name == o2.name) negatedOpts[o2.name] = o1
+        }
+      }
+      const longest = options
+        .map((o) => {
+          if (o.negated) return 0
+          const l = o.rawName.length
+          // '5' is the length of '[no-]'. As to the 1, I have no clue...
+          return negatedOpts[o.name] ? l + 5 + 1 : l
+        })
+        .reduce((acc, l) => Math.max(acc, l), 0)
       sections.push({
         title: 'Options',
         body: options
           .map((option) => {
-            return `  ${padRight(option.rawName, longestOptionName.length)}  ${
-              option.description
-            } ${
+            if (option.negated) return
+            let desc = option.description
+            let name = option.rawName
+            if (negatedOpts[option.name]) {
+              const ndesc = negatedOpts[option.name].description
+              if (ndesc) desc = `${ndesc}/${desc}`
+              name = name.replace(/^(--?)/, '$1[no-]')
+            }
+
+            return `  ${padRight(name, longest)}  ${desc} ${
               option.config.default === undefined
                 ? ''
                 : `(default: ${option.config.default})`
             }`
           })
-          .join('\n'),
+          .filter(Boolean).join('\n'),
       })
     }
 

src/__test__/index.test.ts

@@ -103,6 +103,34 @@ test('negated option validation', () => {
   expect(options.config).toBe(false)
 })
 
+test('negated option help output', () => {
+  const cli = cac()
+
+  cli.option('--config <config>', 'Use custom config file')
+  cli.option('--no-config', 'Skip')
+
+  const saved = console.log
+  let output = ''
+  try {
+    console.log = (msg, more) => {
+      if (more) throw new Error('Unexpected multi-arg call to console.log')
+      output += msg
+    }
+    cli.outputHelp()
+    expect(output).toBe(
+      `
+
+Usage:
+  $  <command> [options]
+
+Options:
+  --[no-]config <config>   Skip/Use custom config file `
+    )
+  } finally {
+    console.log = saved
+  }
+})
+
 test('array types without transformFunction', () => {
   const cli = cac()