Fix Lint::FormatParameterMismatch cop

The cop failed to process format string in invalid format and raised exception. There are several cases when format string contains valid segments but these segments aren't compatible.

For instance numbered format cannot be mixed with unnumbered or named. Such format string will lead to ArgumentError when code is executed but sometimes it's needed e.g. in tests to check how the code such invalid format strings.

CHANGELOG.md

@@ -8,6 +8,7 @@
 * [#8081](https://github.com/rubocop-hq/rubocop/issues/8081): Fix a false positive for `Lint/SuppressedException` when empty rescue block in `do` block. ([@koic][])
 * [#8096](https://github.com/rubocop-hq/rubocop/issues/8096): Fix a false positive for `Lint/SuppressedException` when empty rescue block in defs. ([@koic][])
 * [#8108](https://github.com/rubocop-hq/rubocop/issues/8108): Fix infinite loop in `Layout/HeredocIndentation` auto-correct. ([@jonas054][])
+* [#8042](https://github.com/rubocop-hq/rubocop/pull/8042): Fix raising error in `Lint::FormatParameterMismatch` when it handles invalid format strings and add new offense. ([@andrykonchin][])
 
 ## 0.85.0 (2020-06-01)
 
@@ -4571,3 +4572,4 @@
 [@jschneid]: https://github.com/jschneid
 [@ric2b]: https://github.com/ric2b
 [@burnettk]: https://github.com/burnettk
+[@andrykonchin]: https://github.com/andrykonchin

lib/rubocop/cop/lint/format_parameter_mismatch.rb

@@ -43,6 +43,7 @@ def offending_node?(node)
           num_of_format_args, num_of_expected_fields = count_matches(node)
 
           return false if num_of_format_args == :unknown
+          return false if num_of_expected_fields == :unknown
 
           matched_arguments_count?(num_of_expected_fields, num_of_format_args)
         end
@@ -113,6 +114,8 @@ def expected_fields_count(node)
           return :unknown unless node.str_type?
 
           format_string = RuboCop::Cop::Utils::FormatString.new(node.source)
+
+          return :unknown unless format_string.valid?
           return 1 if format_string.named_interpolation?
 
           max_digit_dollar_num = format_string.max_digit_dollar_num

lib/rubocop/cop/utils/format_string.rb

@@ -97,6 +97,10 @@ def format_sequences
           @format_sequences ||= parse
         end
 
+        def valid?
+          !mixed_formats?
+        end
+
         def named_interpolation?
           format_sequences.any?(&:name)
         end
@@ -114,6 +118,20 @@ def parse
             )
           end
         end
+
+        def mixed_formats?
+          formats = format_sequences.map do |seq|
+            if seq.name
+              :named
+            elsif seq.max_digit_dollar_num
+              :numbered
+            else
+              :unnumbered
+            end
+          end
+
+          formats.uniq.size > 1
+        end
       end
     end
   end

spec/rubocop/cop/lint/format_parameter_mismatch_spec.rb

@@ -171,6 +171,12 @@
     end
   end
 
+  context 'when format is invalid' do
+    it 'does not register an offense' do
+      expect_no_offenses("format('%s %2$s', 'foo', 'bar')")
+    end
+  end
+
   # Regression: https://github.com/rubocop-hq/rubocop/issues/3869
   context 'when passed an empty array' do
     it 'does not register an offense' do

spec/rubocop/cop/utils/format_string_spec.rb

@@ -80,4 +80,36 @@ def format_sequences(string)
     it_behaves_like 'named format sequence', '%+0<num>8.2f'
     it_behaves_like 'named format sequence', '%+08<num>.2f'
   end
+
+  describe '#valid?' do
+    it 'returns true when there are only unnumbered formats' do
+      fs = described_class.new('%s %d')
+      expect(fs.valid?).to eq true
+    end
+
+    it 'returns true when there are only numbered formats' do
+      fs = described_class.new('%1$s %2$d')
+      expect(fs.valid?).to eq true
+    end
+
+    it 'returns true when there are only named formats' do
+      fs = described_class.new('%{foo}s')
+      expect(fs.valid?).to eq true
+    end
+
+    it 'returns false when there are unnumbered and numbered formats' do
+      fs = described_class.new('%s %1$d')
+      expect(fs.valid?).to eq false
+    end
+
+    it 'returns false when there are unnumbered and named formats' do
+      fs = described_class.new('%s %{foo}d')
+      expect(fs.valid?).to eq false
+    end
+
+    it 'returns false when there are numbered and named formats' do
+      fs = described_class.new('%1$s %{foo}d')
+      expect(fs.valid?).to eq false
+    end
+  end
 end