util: add a "customInspect" option to util.inspect()

For disabling calling the custom `inspect()` function when defined on an object
that is being inspected.

doc/api/util.markdown

@@ -83,6 +83,9 @@ formatted string:
  - `colors` - if `true`, then the output will be styled with ANSI color codes.
    Defaults to `false`. Colors are customizable, see below.
 
+ - `customInspect` - if `false`, then custom `inspect()` functions defined on the
+   objects being inspected won't be called. Defaults to `true`.
+
 Example of inspecting all properties of the `util` object:
 
     var util = require('util');

lib/util.js

@@ -132,6 +132,7 @@ function inspect(obj, opts/* legacy: showHidden, depth, colors*/) {
   if (typeof ctx.showHidden === 'undefined') ctx.showHidden = false;
   if (typeof ctx.depth === 'undefined') ctx.depth = 2;
   if (typeof ctx.colors === 'undefined') ctx.colors = false;
+  if (typeof ctx.customInspect === 'undefined') ctx.customInspect = true;
   if (ctx.colors) ctx.stylize = stylizeWithColor;
   return formatValue(ctx, obj, ctx.depth);
 }
@@ -200,7 +201,7 @@ function arrayToHash(array) {
 function formatValue(ctx, value, recurseTimes) {
   // Provide a hook for user-specified inspect functions.
   // Check that value is an object with an inspect function on it
-  if (value && typeof value.inspect === 'function' &&
+  if (ctx.customInspect && value && typeof value.inspect === 'function' &&
       // Filter out the util module, it's inspect function is special
       value.inspect !== exports.inspect &&
       // Also filter out any prototype objects using the circular check.

test/simple/test-util-inspect.js

@@ -149,3 +149,11 @@ assert(util.inspect(subject, { colors: true }).indexOf('\u001b[32m') !== -1);
 assert(util.inspect(subject, { depth: 2 }).indexOf('c: [Object]') !== -1);
 assert(util.inspect(subject, { depth: 0 }).indexOf('a: [Object]') !== -1);
 assert(util.inspect(subject, { depth: null }).indexOf('{ d: 0 }') !== -1);
+
+// "customInspect" option can enable/disable calling inspect() on objects
+subject = { inspect: function() { return 123; } };
+
+assert(util.inspect(subject, { customInspect: true }).indexOf('123') !== -1);
+assert(util.inspect(subject, { customInspect: true }).indexOf('inspect') === -1);
+assert(util.inspect(subject, { customInspect: false }).indexOf('123') === -1);
+assert(util.inspect(subject, { customInspect: false }).indexOf('inspect') !== -1);