Add Lint/DuplicateWhenCondition rule

spec/ameba/rule/lint/duplicate_when_condition_spec.cr

@@ -0,0 +1,45 @@
+require "../../../spec_helper"
+
+module Ameba::Rule::Lint
+  describe DuplicateWhenCondition do
+    subject = DuplicateWhenCondition.new
+
+    it "passes if there are no duplicated `when` conditions" do
+      expect_no_issues subject, <<-CRYSTAL
+        case x
+        when .nil?
+          do_something
+        when Symbol
+          do_something_else
+        end
+        CRYSTAL
+    end
+
+    it "reports if there are a duplicated `when` conditions" do
+      expect_issue subject, <<-CRYSTAL
+        case x
+        when .foo?, .nil?
+          do_something
+        when .nil?
+           # ^^^^^ error: Duplicate `when` condition detected
+          do_something_else
+        end
+        CRYSTAL
+
+      expect_issue subject, <<-CRYSTAL
+        case
+        when foo?
+          :foo
+        when foo?, bar?
+           # ^^^^ error: Duplicate `when` condition detected
+           :foobar
+        when Time.utc.year == 1996
+          :yo
+        when Time.utc.year == 1996
+           # ^^^^^^^^^^^^^^^^^^^^^ error: Duplicate `when` condition detected
+          :yo
+        end
+        CRYSTAL
+    end
+  end
+end

src/ameba/rule/lint/duplicate_when_condition.cr

@@ -0,0 +1,53 @@
+module Ameba::Rule::Lint
+  # Reports repeated conditions used in case `when` expressions.
+  #
+  # This is considered invalid:
+  #
+  # ```
+  # case x
+  # when .nil?
+  #   do_something
+  # when .nil?
+  #   do_something_else
+  # end
+  # ```
+  #
+  # And this is valid:
+  #
+  # ```
+  # case x
+  # when .nil?
+  #   do_something
+  # when Symbol
+  #   do_something_else
+  # end
+  # ```
+  #
+  # YAML configuration example:
+  #
+  # ```
+  # Lint/DuplicateWhenCondition:
+  #   Enabled: true
+  # ```
+  class DuplicateWhenCondition < Base
+    properties do
+      since_version "1.7.0"
+      description "Reports repeated conditions used in case `when` expressions"
+    end
+
+    MSG = "Duplicate `when` condition detected"
+
+    def test(source, node : Crystal::Case)
+      node.whens.each_with_object(Set(String).new) do |when_node, processed_conditions|
+        when_node.conds.each do |cond|
+          cond_s = cond.to_s
+          if processed_conditions.includes?(cond_s)
+            issue_for cond, MSG
+          else
+            processed_conditions << cond_s
+          end
+        end
+      end
+    end
+  end
+end