Create a new plugin for ensuring a single space before a '=>'

This will trigger a warning only for resources with single parameters
such as:

```
file { 'foo':
  ensure⎵⎵=> file,
}
```

README.md

@@ -117,7 +117,7 @@ Any flag that can be specified on the command line can also be specified in the
 Or to specify an allowlist of allowed checks, include a line like:
 
 ```
---only-checks=trailing_whitespace,hard_tabs,duplicate_params,double_quoted_strings,unquoted_file_mode,only_variable_string,variables_not_enclosed,single_quote_string_with_variables,variable_contains_dash,ensure_not_symlink_target,unquoted_resource_title,relative_classname_inclusion,file_mode,resource_reference_without_title_capital,leading_zero,arrow_alignment,variable_is_lowercase,ensure_first_param,resource_reference_without_whitespace,file_ensure,trailing_comma,leading_zero
+--only-checks=trailing_whitespace,hard_tabs,duplicate_params,double_quoted_strings,unquoted_file_mode,only_variable_string,variables_not_enclosed,single_quote_string_with_variables,variable_contains_dash,ensure_not_symlink_target,unquoted_resource_title,relative_classname_inclusion,file_mode,resource_reference_without_title_capital,leading_zero,arrow_alignment,space_before_arrow,variable_is_lowercase,ensure_first_param,resource_reference_without_whitespace,file_ensure,trailing_comma,leading_zero
 ```
 
 Please note that there is an important difference between reading options from the command line and reading options from a configuration file: In the former case the shell interprets one level of quotes. That does not happen in the latter case. So, it would make sense to quote some configuration values on the command line, like so:
@@ -253,6 +253,7 @@ For a complete list of checks, and how to resolve errors on each check, see the
 * Should not exceed an 140-character line width.
   * An exception has been made for `source => 'puppet://...'` lines as splitting these over multiple lines decreases the readability of the manifests.
 * Should align arrows (`=>`) within blocks of attributes.
+* Should contain at most a single space before an arrows(`=>`) where the parameter block contains exactly one parameter.
 
 ### Quoting
 

lib/puppet-lint/plugins/check_whitespace/space_before_arrow.rb

@@ -0,0 +1,72 @@
+# frozen_string_literal: true
+
+# Check the manifest tokens for any arrows (=>) that have too much space
+# before them in situations when a given resource has at most one line with
+# such arrows. For example:
+
+# file {
+#   # too much space after "foo"
+#   foo⎵⎵=>⎵'bar'
+# }
+#
+# file {
+#   # too much space after 'bar'
+#   foo⎵=>⎵{ bar⎵⎵=>⎵'baz' }
+# }
+#
+
+# Linting multi-parameter resources like this:
+#
+# package { 'xxx':
+#   foo => 'bar',
+#   bar  => 'baz',
+# }
+#
+# is handled by the "arrow_alignment") plugin.
+
+PuppetLint.new_check(:space_before_arrow) do
+  def check
+    resource_indexes.each do |res_idx|
+      resource_tokens = res_idx[:tokens]
+      resource_tokens.reject! do |token|
+        Set[:COMMENT, :SLASH_COMMENT, :MLCOMMENT].include?(token.type)
+      end
+
+      first_arrow = resource_tokens.index { |r| r.type == :FARROW }
+      last_arrow = resource_tokens.rindex { |r| r.type == :FARROW }
+      next if first_arrow.nil?
+      next if last_arrow.nil?
+
+      # If this is a single line resource, skip it
+      next unless resource_tokens[first_arrow].line == resource_tokens[last_arrow].line
+
+      resource_tokens.select { |token| token.type == :FARROW }.each do |token|
+        prev_token = token.prev_token
+        next unless prev_token
+        next if prev_token.value == ' '
+
+        line = prev_token.line
+        column = prev_token.column
+
+        notify(
+          :warning,
+          message: "there should be a single space before '=>' on line #{line}, column #{column}",
+          line: line,
+          column: column,
+          token: prev_token,
+        )
+      end
+    end
+  end
+
+  def fix(problem)
+    token = problem[:token]
+
+    if token.type == :WHITESPACE
+      token.value = ' '
+      return
+    end
+
+    add_token(tokens.index(token), PuppetLint::Lexer::Token.new(:WHITESPACE, ' ', 0, 0))
+  end
+end

spec/unit/puppet-lint/plugins/check_whitespace/space_before_arrow_spec.rb

@@ -0,0 +1,126 @@
+require 'spec_helper'
+
+describe 'space_before_arrow' do
+  let(:msg) { "there should be a single space before '=>' on line %d, column %d" }
+
+  context 'with code that should not trigger any warnings' do
+    context 'resource with multiple parameters on different lines' do
+      let(:code) do
+        <<-END
+          file { 'foo':
+            foo => bar,
+            bar  => buzz,
+          }
+        END
+      end
+
+      it 'does not detect any problems' do
+        expect(problems).to be_empty
+      end
+    end
+
+    context 'resource with single param and normal spacing' do
+      let(:code) do
+        <<-END
+          file { 'foo':
+            foo => bar,
+          }
+        END
+      end
+
+      it 'does not detect any problems' do
+        expect(problems).to be_empty
+      end
+    end
+
+    context 'resource with multiple params and normal spacing' do
+      let(:code) do
+        <<-END
+          file { 'foo':
+            foo => { "bar" => "baz" },
+          }
+        END
+      end
+
+      it 'does not detect any problems' do
+        expect(problems).to be_empty
+      end
+    end
+  end
+
+  context 'resource with a single param and simple value with too much space before arrow' do
+    let(:code) do
+      <<-END
+        file { 'foo':
+          foo  => bar,
+        }
+      END
+    end
+
+    context 'with fix disabled' do
+      it 'does not detect any problems' do
+        expect(problems.size).to eq(1)
+      end
+
+      it 'produces 1 warning' do
+        expect(problems).to contain_warning(msg % [2, 14]).on_line(2).in_column(14)
+      end
+    end
+
+    context 'with fix enabled' do
+      before(:each) do
+        PuppetLint.configuration.fix = true
+      end
+
+      after(:each) do
+        PuppetLint.configuration.fix = false
+      end
+
+      it 'detects the problem' do
+        expect(problems.size).to eq(1)
+      end
+
+      it 'fixes the manifest' do
+        expect(problems).to contain_fixed(msg % [2, 14])
+      end
+    end
+  end
+
+  context 'resource with a single param, a hash as value and bad spacing within the hash' do
+    let(:code) do
+      <<-END
+        file { 'foo':
+          foo => { "bar"  => "baz" },
+        }
+      END
+    end
+
+    context 'with fix disabled' do
+      it 'does not detect any problems' do
+        expect(problems.size).to eq(1)
+      end
+
+      it 'produces a warning' do
+        expect(problems).to contain_warning(msg % [2, 25]).on_line(2).in_column(25)
+      end
+    end
+
+    context 'with fix enabled' do
+      before(:each) do
+        PuppetLint.configuration.fix = true
+      end
+
+      after(:each) do
+        PuppetLint.configuration.fix = false
+      end
+
+      it 'detects the problem' do
+        expect(problems.size).to eq(1)
+      end
+
+      it 'fixes the manifest' do
+        expect(problems).to contain_fixed(msg % [2, 25])
+      end
+    end
+  end
+end