prompt: make it possible to set name dynamically

Fixes #1738 (Possible to make prompt_name dynamic?)

The user-facing API is the following:

```rb
Pry.config.prompt_name = Pry.lazy { rand(100) }

[1] 80(main)>
[2] 87(main)>
[3] 30(main)>
```

lib/pry/config.rb

@@ -1,5 +1,31 @@
 class Pry
+  # The Pry config.
+  # @api public
   class Config < Pry::BasicObject
+    # Wraps a block so it can have a name.
+    #
+    # @example
+    #   proc1 = proc {}
+    #   proc2 = Pry::Config::Lazy.new(&proc {})
+    #
+    #   proc1.is_a?(Pry::Config::Lazy)
+    #   #=> false
+    #   proc2.is_a?(Pry::Config::Lazy)
+    #   #=> true
+    #
+    # @api private
+    # @since v0.12.0
+    class Lazy
+      def initialize(&block)
+        @block = block
+      end
+
+      # @return [Object]
+      def call
+        @block.call
+      end
+    end
+
     include Behavior
 
     def self.shortcuts

lib/pry/prompt.rb

@@ -31,7 +31,7 @@ class Pry
           format(
             DEFAULT_TEMPLATE,
             in_count: _pry_.input_ring.count,
-            name: _pry_.config.prompt_name,
+            name: prompt_name(_pry_.config.prompt_name),
             context: Pry.view_clip(context),
             nesting: (nesting > 0 ? ":#{nesting}" : ''),
             separator: separator
@@ -44,7 +44,7 @@ class Pry
         proc do |context, _nesting, _pry_|
           format(
             SHELL_TEMPLATE,
-            name: _pry_.config.prompt_name,
+            name: prompt_name(_pry_.config.prompt_name),
             context: Pry.view_clip(context),
             pwd: Dir.pwd,
             separator: separator
@@ -59,12 +59,18 @@ class Pry
           format(
             NAV_TEMPLATE,
             in_count: _pry_.input_ring.count,
-            name: _pry_.config.prompt_name,
+            name: prompt_name(_pry_.config.prompt_name),
             tree: tree.join(' / '),
             stack_size: _pry_.binding_stack.size - 1
           )
         end
       end
+
+      def prompt_name(name)
+        return name unless name.is_a?(Pry::Config::Lazy)
+
+        name.call
+      end
     end
 
     # The default Pry prompt, which includes the context and nesting level.

lib/pry/pry_class.rb

@@ -406,6 +406,24 @@ Readline version #{Readline::VERSION} detected - will not auto_resize! correctly
   ensure
     Thread.current[:pry_critical_section] -= 1
   end
+
+  # Wraps a block in a named block called `Pry::Config::Lazy`. This is used for
+  # dynamic config values, which are calculated every time
+  # {Pry::Config::Lazy#call} is called.
+  #
+  # @example
+  #   # pryrc
+  #   Pry.config.prompt_name = Pry.lazy { rand(100) }
+  #
+  #   # Session
+  #   [1] 96(main)>
+  #   [2] 19(main)>
+  #   [3] 80(main)>
+  #
+  # @return [#call]
+  def self.lazy(&block)
+    Pry::Config::Lazy.new(&block)
+  end
 end
 
 Pry.init

spec/prompt_spec.rb

@@ -1,6 +1,6 @@
 require_relative 'helper'
 
-describe "Prompts" do
+describe Pry::Prompt do
   describe "one-parameter prompt proc" do
     it 'should get full config object' do
       config = nil
@@ -67,4 +67,21 @@ describe "Prompts" do
       expect(p1.is_a?(Pry)).to eq true
     end
   end
+
+  it "can compute prompt name dynamically" do
+    config = nil
+    redirect_pry_io(InputTester.new("def hello", "exit-all")) do
+      Pry.start(self, prompt: proc { |v| config = v })
+    end
+
+    enum = Enumerator.new do |y|
+      count = 100
+      loop { y << count += 1 }
+    end
+    config._pry_.config.prompt_name = Pry.lazy { enum.next }
+
+    proc = subject::DEFAULT.first
+    expect(proc.call(Object.new, 1, config._pry_)).to eq('[1] 101(#<Object>):1> ')
+    expect(proc.call(Object.new, 1, config._pry_)).to eq('[1] 102(#<Object>):1> ')
+  end
 end