Fixed: Pprof compatibility with Ruby 2.0

lib/ddtrace/profiling/pprof/builder.rb

@@ -67,7 +67,7 @@ def build_locations(backtrace_locations, length)
           if omitted > 0
             desc = omitted == 1 ? DESC_FRAME_OMITTED : DESC_FRAMES_OMITTED
             locations << @locations.fetch(
-              '',
+              ''.freeze,
               0,
               "#{omitted} #{desc}",
               &method(:build_location)

lib/ddtrace/profiling/pprof/message_set.rb

@@ -17,9 +17,11 @@ def initialize(seed = 0, &block)
         # If they match an existing message, it will return the
         # matching object. If it doesn't match, it will yield to
         # the block with the next ID & args given.
-        def fetch(*args)
+        def fetch(*args, &block)
           key = @key_block ? @key_block.call(*args) : args.hash
-          @items[key] ||= yield(@sequence.next, *args)
+          # TODO: Ruby 2.0 doesn't like yielding here... switch when 2.0 is dropped.
+          # rubocop:disable Performance/RedundantBlockCall
+          @items[key] ||= block.call(@sequence.next, *args)
         end
 
         def length

lib/ddtrace/profiling/pprof/string_table.rb

@@ -7,7 +7,7 @@ module Pprof
       class StringTable
         def initialize
           @sequence = Utils::Sequence.new
-          @ids = { '' => @sequence.next }
+          @ids = { ''.freeze => @sequence.next }
         end
 
         def fetch(string)

spec/ddtrace/profiling/pprof/template_spec.rb

@@ -17,7 +17,7 @@
         before do
           expect(described_class)
             .to receive(:new)
-            .with([described_class::DEFAULT_MAPPINGS.first].to_h)
+            .with(Hash[[described_class::DEFAULT_MAPPINGS.first]])
             .and_return(template)
         end