New rule: hash duplicated key

config/ameba.yml

@@ -18,6 +18,10 @@ EmptyExpression:
   # Disallows empty expressions.
   Enabled: true
 
+HashDuplicatedKey:
+  # Disallows duplicated keys in hash literals.
+  Enabled: true
+
 LargeNumbers:
   # A rule that disallows usage of large numbers without underscore.
   Enabled: true

spec/ameba/rule/hash_duplicated_key_spec.cr

@@ -0,0 +1,41 @@
+require "../../spec_helper"
+
+module Ameba::Rule
+  describe HashDuplicatedKey do
+    subject = HashDuplicatedKey.new
+
+    it "passes if there is no duplicated keys in a hash literals" do
+      s = Source.new %(
+        h = {"a" => 1, :a => 2, "b" => 3}
+        h = {"a" => 1, "b" => 2, "c" => {"a" => 3, "b" => 4}}
+        h = {} of String => String
+      )
+      subject.catch(s).should be_valid
+    end
+
+    it "fails if there is a duplicated key in a hash literal" do
+      s = Source.new %q(
+        h = {"a" => 1, "b" => 2, "a" => 3}
+      )
+      subject.catch(s).should_not be_valid
+    end
+
+    it "fails if there is a duplicated key in the inner hash literal" do
+      s = Source.new %q(
+        h = {"a" => 1, "b" => {"a" => 3, "b" => 4, "a" => 5}}
+      )
+      subject.catch(s).should_not be_valid
+    end
+
+    it "reports rule, location and message" do
+      s = Source.new %q(
+        h = {"a" => 1, "a" => 2}
+      ), "source.cr"
+      subject.catch(s).should_not be_valid
+      error = s.errors.first
+      error.rule.should_not be_nil
+      error.location.to_s.should eq "source.cr:2:13"
+      error.message.should eq "Duplicated keys in hash literal."
+    end
+  end
+end

src/ameba/ast/traverse.cr

@@ -14,6 +14,7 @@ module Ameba::AST
     EnumDef,
     ExceptionHandler,
     Expressions,
+    HashLiteral,
     If,
     InstanceVar,
     LibDef,

src/ameba/rule/hash_duplicated_key.cr

@@ -0,0 +1,40 @@
+module Ameba::Rule
+  # A rule that disallows duplicated keys in hash literals.
+  #
+  # This is considered invalid:
+  #
+  # ```
+  # h = {"foo" => 1, "bar" => 2, "foo" => 3}
+  # ```
+  #
+  # And it has to written as this instead:
+  #
+  # ```
+  # h = {"foo" => 1, "bar" => 2}
+  # ```
+  #
+  # YAML configuration example:
+  #
+  # ```
+  # HashDuplicatedKey:
+  #   Enabled: true
+  # ```
+  #
+  struct HashDuplicatedKey < Base
+    def test(source)
+      AST::Visitor.new self, source
+    end
+
+    def test(source, node : Crystal::HashLiteral)
+      return unless duplicated_keys?(node.entries)
+
+      source.error self, node.location, "Duplicated keys in hash literal."
+    end
+
+    private def duplicated_keys?(entries)
+      entries.map(&.key)
+             .group_by(&.itself)
+             .any? { |_, v| v.size > 1 }
+    end
+  end
+end