Add BlockNode#{first,last}_argument helpers

These methods allow more consistent use of `first_argument` and
`last_argument` across more node types.

changelog/new_add_blocknodefirstlast_argument_helpers.md

@@ -0,0 +1 @@
+* [#x](https://github.com/rubocop/rubocop-ast/pull/x): Add `BlockNode#{first,last}_argument` helpers. ([@sambostock][])

lib/rubocop/ast/node/block_node.rb

@@ -21,6 +21,24 @@ def send_node
         node_parts[0]
       end
 
+      # A shorthand for getting the first argument of this block.
+      # Equivalent to `arguments.first`.
+      #
+      # @return [Node, nil] the first argument of this block,
+      #                     or `nil` if there are no arguments
+      def first_argument
+        arguments[0]
+      end
+
+      # A shorthand for getting the last argument of this block.
+      # Equivalent to `arguments.last`.
+      #
+      # @return [Node, nil] the last argument of this block,
+      #                     or `nil` if there are no arguments
+      def last_argument
+        arguments[-1]
+      end
+
       # The arguments of this block.
       # Note that if the block has destructured arguments, `arguments` will
       # return a `mlhs` node, whereas `argument_list` will return only

spec/rubocop/ast/block_node_spec.rb

@@ -302,4 +302,84 @@
       it { expect(block_node.receiver.source).to eq('foo') }
     end
   end
+
+  describe '#first_argument' do
+    context 'with no arguments' do
+      let(:source) { 'foo { bar }' }
+
+      it { expect(block_node.first_argument).to be_nil }
+    end
+
+    context 'with a single literal argument' do
+      let(:source) { 'foo { |q| bar(q) }' }
+
+      it { expect(block_node.first_argument.source).to eq('q') }
+    end
+
+    context 'with a single splat argument' do
+      let(:source) { 'foo { |*q| bar(q) }' }
+
+      it { expect(block_node.first_argument.source).to eq('*q') }
+    end
+
+    context 'with multiple mixed arguments' do
+      let(:source) { 'foo { |q, *z| bar(q, z) }' }
+
+      it { expect(block_node.first_argument.source).to eq('q') }
+    end
+
+    context 'with destructured arguments' do
+      let(:source) { 'foo { |(q, r), s| bar(q, r, s) }' }
+
+      it { expect(block_node.first_argument.source).to eq('(q, r)') }
+    end
+
+    context '>= Ruby 2.7', :ruby27 do
+      context 'using numbered parameters' do
+        let(:source) { 'foo { _1 }' }
+
+        it { expect(block_node.first_argument).to be_nil }
+      end
+    end
+  end
+
+  describe '#last_argument' do
+    context 'with no arguments' do
+      let(:source) { 'foo { bar }' }
+
+      it { expect(block_node.last_argument).to be_nil }
+    end
+
+    context 'with a single literal argument' do
+      let(:source) { 'foo { |q| bar(q) }' }
+
+      it { expect(block_node.last_argument.source).to eq('q') }
+    end
+
+    context 'with a single splat argument' do
+      let(:source) { 'foo { |*q| bar(q) }' }
+
+      it { expect(block_node.last_argument.source).to eq('*q') }
+    end
+
+    context 'with multiple mixed arguments' do
+      let(:source) { 'foo { |q, *z| bar(q, z) }' }
+
+      it { expect(block_node.last_argument.source).to eq('*z') }
+    end
+
+    context 'with destructured arguments' do
+      let(:source) { 'foo { |q, (r, s)| bar(q, r, s) }' }
+
+      it { expect(block_node.last_argument.source).to eq('(r, s)') }
+    end
+
+    context '>= Ruby 2.7', :ruby27 do
+      context 'using numbered parameters' do
+        let(:source) { 'foo { _1 }' }
+
+        it { expect(block_node.last_argument).to be_nil }
+      end
+    end
+  end
 end