[ruby/irb] Reduce internal operations' exposure to benchmarking

(https://github.com/ruby/irb/pull/618)

* Test last value is assigned with measure enabled

* Remove unnecessary `result` variable

`Context#evaluate` always assigns the result of the evaluation to `_` so
we don't need to do it in `Irb#eval_input`.

* Move benchmarking logic into `Context#evaluate`

Current location of the benchmarking logic is too high up and includes
operations like command loading and argument transformation, which should
be excluded. So this commit moves it into `Context#evaluate` to reduce the
noise.

We don't move it further down to `Workspace#evaluate` because `Context`
is an argument of the measure block, which is not available in `Workspace`.

1 files changed, 28 insertions, 0 deletions

diff --git a/test/irb/test_cmd.rb b/test/irb/test_cmd.rb
index c8c163c82d..71df60db14 100644
--- a/test/irb/test_cmd.rb
+++ b/test/irb/test_cmd.rb
@@ -252,6 +252,34 @@ module TestIRB
       assert_empty(c.class_variables)
     end
 
+    def test_measure_keeps_previous_value
+      conf = {
+        PROMPT: {
+          DEFAULT: {
+            PROMPT_I: '> ',
+            PROMPT_S: '> ',
+            PROMPT_C: '> ',
+            PROMPT_N: '> '
+          }
+        },
+        PROMPT_MODE: :DEFAULT,
+        MEASURE: false
+      }
+
+      c = Class.new(Object)
+      out, err = execute_lines(
+        "measure\n",
+        "3\n",
+        "_\n",
+        conf: conf,
+        main: c
+      )
+
+      assert_empty err
+      assert_match(/\ATIME is added\.\n=> nil\nprocessing time: .+\n=> 3\nprocessing time: .+\n=> 3/, out)
+      assert_empty(c.class_variables)
+    end
+
     def test_measure_enabled_by_rc
       conf = {
         PROMPT: {