Add Lint/MissingBlockArgument rule

spec/ameba/rule/lint/missing_block_argument_spec.cr

@@ -0,0 +1,42 @@
+require "../../../spec_helper"
+
+module Ameba::Rule::Lint
+  subject = MissingBlockArgument.new
+
+  describe MissingBlockArgument do
+    it "passes if the block argument is defined" do
+      expect_no_issues subject, <<-CRYSTAL
+        def foo(&)
+          yield 42
+        end
+
+        def bar(&block)
+          yield 24
+        end
+
+        def baz(a, b, c, &block)
+          yield a, b, c
+        end
+        CRYSTAL
+    end
+
+    it "reports if the block argument is missing" do
+      expect_issue subject, <<-CRYSTAL
+        def foo
+          # ^^^ error: Missing anonymous block argument. Use `&` as an argument name to indicate yielding method.
+          yield 42
+        end
+
+        def bar
+          # ^^^ error: Missing anonymous block argument. Use `&` as an argument name to indicate yielding method.
+          yield 24
+        end
+
+        def baz(a, b, c)
+          # ^^^ error: Missing anonymous block argument. Use `&` as an argument name to indicate yielding method.
+          yield a, b, c
+        end
+        CRYSTAL
+    end
+  end
+end

src/ameba/rule/lint/missing_block_argument.cr

@@ -0,0 +1,49 @@
+module Ameba::Rule::Lint
+  # A rule that disallows yielding method definitions without block argument.
+  #
+  # For example, this is considered invalid:
+  #
+  # ```
+  # def foo
+  #   yield 42
+  # end
+  # ```
+  #
+  # And has to be written as the following:
+  #
+  # ```
+  # def foo(&)
+  #   yield 42
+  # end
+  # ```
+  #
+  # YAML configuration example:
+  #
+  # ```
+  # Lint/MissingBlockArgument:
+  #   Enabled: true
+  # ```
+  class MissingBlockArgument < Base
+    include AST::Util
+
+    properties do
+      description "Disallows yielding method definitions without block argument"
+    end
+
+    MSG = "Missing anonymous block argument. Use `&` as an argument " \
+          "name to indicate yielding method."
+
+    def test(source)
+      AST::ScopeVisitor.new self, source
+    end
+
+    def test(source, node : Crystal::Def, scope : AST::Scope)
+      return if !scope.yields? || node.block_arg
+
+      return unless location = node.name_location
+      end_location = name_end_location(node)
+
+      issue_for location, end_location, MSG
+    end
+  end
+end