Add Typing/ProcLiteralReturnTypeRestriction (#522)

* Add `Typing/ProcLiteralReturnTypeRestriction`

* Typing/ProcLiteralReturnTypeRestriction disabled by default

* Update src/ameba/rule/typing/proc_literal_return_type_restriction.cr

Co-authored-by: Sijawusz Pur Rahnama <[email protected]>

* Apply suggestions from code review

Co-authored-by: Sijawusz Pur Rahnama <[email protected]>

* Fix specs

* Move ProcLiteral in node list

* Apply suggestions from code review

Co-authored-by: Vitalii Elenhaupt <[email protected]>

* Apply suggestions from code review

* Update src/ameba/rule/typing/proc_literal_return_type_restriction.cr

* Update src/ameba/rule/typing/proc_literal_return_type_restriction.cr

---------

Co-authored-by: Sijawusz Pur Rahnama <[email protected]>
Co-authored-by: Vitalii Elenhaupt <[email protected]>

spec/ameba/base_spec.cr

@@ -19,6 +19,7 @@ module Ameba::Rule
           Naming
           Performance
           Style
+          Typing
         ]
       end
     end

spec/ameba/rule/typing/proc_literal_return_type_restriction_spec.cr

@@ -0,0 +1,18 @@
+require "../../../spec_helper"
+
+module Ameba::Rule::Typing
+  subject = ProcLiteralReturnTypeRestriction.new
+
+  it "passes if a proc literal has a return type restriction" do
+    expect_no_issues subject, <<-CRYSTAL
+      my_proc = ->(var : String) : Nil { puts var }
+      CRYSTAL
+  end
+
+  it "fails if a proc literal doesn't have a return type restriction" do
+    expect_issue subject, <<-CRYSTAL
+      my_proc = ->(var : String) { puts var }
+              # ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ error: Proc literal should have a return type restriction
+      CRYSTAL
+  end
+end

src/ameba/ast/visitors/node_visitor.cr

@@ -34,6 +34,7 @@ module Ameba::AST
       ModuleDef,
       MultiAssign,
       NilLiteral,
+      ProcLiteral,
       StringInterpolation,
       Unless,
       Until,

src/ameba/rule/typing/proc_literal_return_type_restriction.cr

@@ -0,0 +1,45 @@
+module Ameba::Rule::Typing
+  # A rule that enforces that `Proc` literals have a return type.
+  #
+  # For example, these are considered valid:
+  #
+  # ```
+  # greeter = ->(name : String) : String { "Hello #{name}" }
+  # ```
+  #
+  # ```
+  # task = -> : Task { Task.new("execute this command") }
+  # ```
+  #
+  # And these are invalid:
+  #
+  # ```
+  # greeter = ->(name : String) { "Hello #{name}" }
+  # ```
+  #
+  # ```
+  # task = -> { Task.new("execute this command") }
+  # ```
+  #
+  # YAML configuration example:
+  #
+  # ```
+  # Typing/ProcLiteralReturnTypeRestriction:
+  #   Enabled: false
+  # ```
+  class ProcLiteralReturnTypeRestriction < Base
+    properties do
+      since_version "1.7.0"
+      description "Disallows Proc literals without return type restrictions"
+      enabled false
+    end
+
+    MSG = "Proc literal should have a return type restriction"
+
+    def test(source, node : Crystal::ProcLiteral)
+      return if node.def.return_type
+
+      issue_for node, MSG
+    end
+  end
+end