Merge pull request #303 from crystal-ameba/Sija/lint-literal-assignments-in-expressions-rule

Add `Lint/LiteralAssignmentsInExpressions` rule

spec/ameba/rule/lint/literal_assignments_in_expressions_spec.cr

@@ -0,0 +1,47 @@
+require "../../../spec_helper"
+
+LITERAL_SAMPLES = {
+  nil, true, 42, 4.2, 'c', "foo", :foo, /foo/,
+  0..42, [1, 2, 3], {1, 2, 3},
+  {foo: :bar}, {:foo => :bar},
+}
+
+module Ameba::Rule::Lint
+  subject = LiteralAssignmentsInExpressions.new
+
+  describe LiteralAssignmentsInExpressions do
+    it "passes if the assignment value is not a literal" do
+      expect_no_issues subject, <<-CRYSTAL
+        if a = b
+          :ok
+        end
+
+        unless a = b.presence
+          :ok
+        end
+
+        :ok if a = b
+        :ok unless a = b
+
+        case {a, b}
+        when {0, 1} then :gt
+        when {1, 0} then :lt
+        end
+        CRYSTAL
+    end
+
+    {% for literal in LITERAL_SAMPLES %}
+      it %(reports if the assignment value is a {{ literal }} literal) do
+        expect_issue subject, <<-CRYSTAL, literal: {{ literal.stringify }}
+          raise "boo!" if foo = {{ literal }}
+                        # ^{literal}^^^^^^ error: Detected assignment with a literal value in control expression
+          CRYSTAL
+
+        expect_issue subject, <<-CRYSTAL, literal: {{ literal.stringify }}
+          raise "boo!" unless foo = {{ literal }}
+                            # ^{literal}^^^^^^ error: Detected assignment with a literal value in control expression
+          CRYSTAL
+      end
+    {% end %}
+  end
+end

src/ameba/rule/lint/literal_assignments_in_expressions.cr

@@ -0,0 +1,43 @@
+module Ameba::Rule::Lint
+  # A rule that disallows assignments with literal values
+  # in control expressions.
+  #
+  # For example, this is considered invalid:
+  #
+  # ```
+  # if foo = 42
+  #   do_something
+  # end
+  # ```
+  #
+  # And most likely should be replaced by the following:
+  #
+  # ```
+  # if foo == 42
+  #   do_something
+  # end
+  # ```
+  #
+  # YAML configuration example:
+  #
+  # ```
+  # Lint/LiteralAssignmentsInExpressions:
+  #   Enabled: true
+  # ```
+  class LiteralAssignmentsInExpressions < Base
+    include AST::Util
+
+    properties do
+      description "Disallows assignments with literal values in control expressions"
+    end
+
+    MSG = "Detected assignment with a literal value in control expression"
+
+    def test(source, node : Crystal::If | Crystal::Unless | Crystal::Case | Crystal::While | Crystal::Until)
+      return unless (cond = node.cond).is_a?(Crystal::Assign)
+      return unless literal?(cond.value, include_paths: true)
+
+      issue_for cond, MSG
+    end
+  end
+end