Add new invisible_characters rule #6424

kapitoshka438 · 2026-01-08T20:51:09Z

Summary

Adds new invisible_characters lint rule that detects invisible characters in string literals:
- zero-width space U+200B
- zero-width non-joiner U+200C
- FEFF formatting character U+FEFF
Reports all occurrences with precise violation positions pointing to each invisible character
Includes early exit optimization to avoid unnecessary iteration when no invisible characters are present

SwiftLintBot · 2026-01-08T21:01:32Z

	19 Messages
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kapitoshka438 · 2026-01-10T21:36:47Z

@SimplyDanny please review. I don't check NULL control character: \u0000 because it gets lost when copypasting so it's almost impossible to have it in string literals by accident.

SimplyDanny

Thanks for the contribution!

I wonder if this rule should catch even more characters of this kind, some maybe optionally. We could check what other linters do.

Source/SwiftLintBuiltInRules/Rules/Lint/InvisibleCharactersRule.swift

Source/SwiftLintBuiltInRules/Rules/Lint/InvisibleCharacterRule.swift

CHANGELOG.md

kapitoshka438 · 2026-01-23T22:43:18Z

Thanks for the contribution!

I wonder if this rule should catch even more characters of this kind, some maybe optionally. We could check what other linters do.

This tool led me to another character (zero-width non-joiner U+200C). I've added processing it to this PR.
ESLint provides a rule that prevents using some control characters in regular expressions, but I think this is not our case because those characters are visible.
There is also zero-width joiner character U+200D, but it is used to construct complex symbols, such as some emoji example

SimplyDanny

Would it be wrong to have the rule auto-correct the findings by just removing the violating characters?

We could also think about making the rule configurable, so that people can add their own characters with descriptions. That's what the "tool" you cited allows as well.

Source/SwiftLintBuiltInRules/Rules/Lint/InvisibleCharacterRule.swift

kapitoshka438 · 2026-01-26T22:55:24Z

Would it be wrong to have the rule auto-correct the findings by just removing the violating characters?

We could also think about making the rule configurable, so that people can add their own characters with descriptions. That's what the "tool" you cited allows as well.

The second commit makes the rule correctable. I've also made some performance optimizations.
As for custom configurations, let me think about this. I'm sure there may be more symbols we haven't accounted for, but I'm not sure users should be forced to set descriptions for them. Each symbol has a name, and the purpose of this rule is to control the presence of specific special symbols, not just any symbols.

Source/SwiftLintBuiltInRules/Rules/Lint/InvisibleCharacterRule.swift

Tests/IntegrationTests/Resources/default_rule_configurations.yml

Source/SwiftLintBuiltInRules/Rules/Lint/InvisibleCharacterRule.swift

SimplyDanny · 2026-01-27T09:06:45Z

I'm sure there may be more symbols we haven't accounted for, but I'm not sure users should be forced to set descriptions for them.

Fine for me to skip the description or have it optional.

kapitoshka438 · 2026-01-27T18:58:31Z

I'm sure there may be more symbols we haven't accounted for, but I'm not sure users should be forced to set descriptions for them.

Fine for me to skip the description or have it optional.

I've added the ability to configure additional characters. I thought about it for a long time and initially decided not to allow disabling the validation of default characters. The hardest part was coming up with names for the parameter and variables 😅. But I'm still having doubts. Please review )

…olating characters

SimplyDanny

This looks very good now! Thank you for all the updates.

I posted a suggestion for the option's name. Let me know what you think about it.

SimplyDanny · 2026-01-27T21:53:02Z

Source/SwiftLintBuiltInRules/Rules/RuleConfigurations/InvisibleCharacterConfiguration.swift

+        key: "include_hex_codes",
+        postprocessor: { $0.formUnion(defaultCharacters) }
+    )
+    private(set) var violatingCharacters = Set<String>()


If you make UnicodeScalar conform to AcceptableByConfigurationElement, it should end up in a Set<UnicodeScalar> right away. This avoids the conversion in violatingScalars() and the definition of defaultCharacters.

That's a good point, thanks. I've made changes.

SimplyDanny · 2026-01-27T21:55:26Z

Source/SwiftLintBuiltInRules/Rules/RuleConfigurations/InvisibleCharacterConfiguration.swift

+    @ConfigurationElement(key: "severity")
+    private(set) var severityConfiguration = SeverityConfiguration<Parent>.error
+    @ConfigurationElement(
+        key: "include_hex_codes",


We often use additional_ if the values extend a set of defaults. I'm not sure about "hex codes". Would "code points" be clearer? Then we would end up with additional_code_points. Not sure either ...

Sounds reasonable for me. Done in the last commit.

SimplyDanny · 2026-01-27T21:56:32Z

Source/SwiftLintBuiltInRules/Rules/Lint/InvisibleCharacterRule.swift

+        description: """
+            Disallows invisible characters like zero-width space (U+200B), \
+            zero-width non-joiner (U+200C), and FEFF formatting character (U+FEFF) \
+            in string literals as they can cause hard-to-debug issues


Suggested change

in string literals as they can cause hard-to-debug issues

in string literals as they can cause hard-to-debug issues.

kapitoshka438 force-pushed the invisible_characters branch from 01a6b0a to 21d78ba Compare January 8, 2026 21:01

kapitoshka438 force-pushed the invisible_characters branch 2 times, most recently from 3561c44 to a89ae33 Compare January 8, 2026 21:52

kapitoshka438 force-pushed the invisible_characters branch 2 times, most recently from 9382226 to ceccd1d Compare January 19, 2026 12:47

SimplyDanny requested changes Jan 23, 2026

View reviewed changes

kapitoshka438 force-pushed the invisible_characters branch 3 times, most recently from 3a37574 to 708d32e Compare January 23, 2026 21:53

kapitoshka438 requested a review from SimplyDanny January 23, 2026 22:54

SimplyDanny reviewed Jan 24, 2026

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Source/SwiftLintBuiltInRules/Rules/Lint/InvisibleCharacterRule.swift Outdated Show resolved Hide resolved

Source/SwiftLintBuiltInRules/Rules/Lint/InvisibleCharacterRule.swift Outdated Show resolved Hide resolved

kapitoshka438 force-pushed the invisible_characters branch from 708d32e to 09b260e Compare January 25, 2026 19:26

SimplyDanny linked an issue Jan 25, 2026 that may be closed by this pull request

Rule Request: invisible_characters #6045

Open

2 tasks

kapitoshka438 force-pushed the invisible_characters branch from 09b260e to eef6f03 Compare January 26, 2026 22:51

sm-eminiakhmetov added 2 commits January 27, 2026 02:41

Add new invisible_character rule

90a205f

Make the rule correctable

60290ce

kapitoshka438 force-pushed the invisible_characters branch from eef6f03 to 60290ce Compare January 26, 2026 23:41

SimplyDanny reviewed Jan 27, 2026

View reviewed changes

Corrections optimizations

9723c56

kapitoshka438 requested a review from SimplyDanny January 27, 2026 18:58

kapitoshka438 force-pushed the invisible_characters branch from 9c58429 to c336b5f Compare January 27, 2026 19:09

Added include_hex_codes configuration element to allow adding more vi…

01b29ad

…olating characters

kapitoshka438 force-pushed the invisible_characters branch from c336b5f to 01b29ad Compare January 27, 2026 19:13

SimplyDanny reviewed Jan 27, 2026

View reviewed changes

Conform UnicodeScalar to AcceptableByConfigurationElement

ab30195

kapitoshka438 requested a review from SimplyDanny January 27, 2026 23:28

	in string literals as they can cause hard-to-debug issues
	in string literals as they can cause hard-to-debug issues.

Add new invisible_characters rule #6424

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Add new invisible_characters rule #6424

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