Fix: optional_data_string_conversion should detect String.init and leading-dot .init usages #6372

nadeemnali · 2025-12-09T21:23:57Z

Summary

The optional_data_string_conversion SwiftSyntax rule only detected direct calls like:

String(decoding: data, as: UTF8.self)

but it missed two common variants:

Type-qualified initializer: String.init(decoding: data, as: UTF8.self)
Leading dot initializer used with an explicit type annotation: let text: String = .init(decoding: data, as: UTF8.self)

This change updates the rule to detect both String.init(...) and leading-dot .init(...) when the contextual type is String. It also adds examples to the rule description and a unit test that verifies the rule's description examples.

Changes

Update OptionalDataStringConversionRule visitor to:
- keep existing detection for direct String(...) calls,
- detect String.init(...) (MemberAccessExpr with base String and member init),
- detect leading-dot .init(...) when assigned to a variable with an explicit String type annotation.
Add a test file which runs the rule's description examples.

Why

This fixes the bug reported where
let text: String = .init(decoding: data, as: UTF8.self) did not produce the expected lint violation.

Test plan

Unit test included: Tests/SwiftLintBuiltInRulesTests/OptionalDataStringConversionRuleTests.swift
The test verifies the rule's nonTriggering and triggering examples contained in the RuleDescription.

SwiftLintBot · 2025-12-09T21:31:47Z

	1 Warning
⚠️	This PR may need tests.

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SimplyDanny

Thanks for preparing the PR! Please consider my comments.

SimplyDanny · 2025-12-09T22:14:53Z

Source/SwiftLintBuiltInRules/Rules/Lint/OptionalDataStringConversionRule.swift

               expr.declName.baseName.description == "self" {
                violations.append(node.positionAfterSkippingLeadingTrivia)
            }
        }


We should rather implement visitPost(_ node: FunctionCallExprSyntax) instead. It can check all cases at the same time. If the called name is either String, String.init or .init and the arguments match, it might trigger.

The var x: String = .init(...) case is very special and I don't expect it to appear often. While String.init is safe, .init could appear everywhere (think of f(.init(...)). We might add an option for that (as I've already pointed out in #6359.

Tests/BuiltInRulesTests/OptionalDataStringConversionRuleTests.swift

SimplyDanny · 2025-12-09T22:16:25Z

Source/SwiftLintBuiltInRules/Rules/Lint/OptionalDataStringConversionRule.swift

            Example("String(bytes: data, encoding: .utf8)"),
            Example("String(UTF8.self)"),
            Example("String(a, b, c, UTF8.self)"),
            Example("String(decoding: data, encoding: UTF8.self)"),


Please add more cases that shouldn't trigger, e.g. let text: Int = .init(decoding: data, as: UTF8.self). 🤷‍♂️

I'm missing this example. Please add it.

…ersion rule

SimplyDanny · 2025-12-17T18:53:40Z

CHANGELOG.md


+### Bug Fixes
+
+* optional_data_string_conversion: Detect `String.init(...)` and leading-dot `.init(...)` usages when converting `Data` to `String`. This fixes cases such as `String.init(decoding: data, as: UTF8.self)` and `let text: String = .init(decoding: data, as: UTF8.self)` which previously did not trigger the rule. (Fixes https://github.com/realm/SwiftLint/issues/6359)


Please add this entry to the current "Main" section at the top of the file.

SimplyDanny · 2025-12-17T18:56:25Z

CHANGELOG.md

+* optional_data_string_conversion: Detect `String.init(...)` and leading-dot `.init(...)` usages when converting `Data` to `String`. This fixes cases such as `String.init(decoding: data, as: UTF8.self)` and `let text: String = .init(decoding: data, as: UTF8.self)` which previously did not trigger the rule. (Fixes https://github.com/realm/SwiftLint/issues/6359)
+  [Nadeem Ali](https://github.com/nadeemnali)


Suggested change

* optional_data_string_conversion: Detect `String.init(...)` and leading-dot `.init(...)` usages when converting `Data` to `String`. This fixes cases such as `String.init(decoding: data, as: UTF8.self)` and `let text: String = .init(decoding: data, as: UTF8.self)` which previously did not trigger the rule. (Fixes https://github.com/realm/SwiftLint/issues/6359)

[Nadeem Ali](https://github.com/nadeemnali)

* Add detection of cases such as `String.init(decoding: data, as: UTF8.self)` and

`let text: String = .init(decoding: data, as: UTF8.self)` to `optional_data_string_conversion` rule.

[Nadeem Ali](https://github.com/nadeemnali)

[#6359](https://github.com/realm/SwiftLint/issues/6359)

SimplyDanny · 2025-12-17T18:56:47Z

Source/SwiftLintBuiltInRules/Rules/Lint/OptionalDataStringConversionRule.swift

            Example("String(bytes: data, encoding: .utf8)"),
            Example("String(UTF8.self)"),
            Example("String(a, b, c, UTF8.self)"),
            Example("String(decoding: data, encoding: UTF8.self)"),


I'm missing this example. Please add it.

SimplyDanny · 2025-12-17T18:58:03Z

Source/SwiftLintBuiltInRules/Rules/Lint/OptionalDataStringConversionRule.swift

+            guard node.arguments.map(\.label?.text) == ["decoding", "as"] else { return }
+
+            // Check that the `as:` argument is `UTF8.self`
+            func isUTF8Self(on call: FunctionCallExprSyntax) -> Bool {


Why do we need this function? It's only called once, isn't it?

SimplyDanny · 2025-12-17T18:59:19Z

Source/SwiftLintBuiltInRules/Rules/Lint/OptionalDataStringConversionRule.swift

+            }
+
+            // Case 2 and 3: `.init` or `String.init`
+            if let member = called.as(MemberAccessExprSyntax.self), member.declName.baseName.text == "init" {


Can be a guard.

SimplyDanny · 2025-12-17T19:03:24Z

Source/SwiftLintBuiltInRules/Rules/Lint/OptionalDataStringConversionRule.swift

+                if member.base == nil {
+                    // Walk ancestors to find a VariableDecl or PatternBinding with a type annotation of `String`.
+                    var parent: Syntax? = node.parent
+                    while let ancestor = parent {


Better to not walk up the tree arbitrarily high. node.parent.parent gives you the binding to check for the String type.

SimplyDanny linked an issue Dec 9, 2025 that may be closed by this pull request

optional_data_string_conversion does not trigger when .init(decoding:as:) is called instead of String(decoding:as:) #6359

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SimplyDanny requested changes Dec 9, 2025

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nadeemnali requested a review from SimplyDanny December 11, 2025 19:05

nadeemnali added 5 commits December 16, 2025 21:19

Detect String.init and leading-dot init in optional_data_string_conv…

cfd5820

…ersion rule

Update unit tests

ae666fe

Update linting issues

35729e9

Update to use visitPost(_ node: FunctionCallExprSyntax)

6dc0266

Updated non-triggering examples for better coverage

4cfbd90

nadeemnali force-pushed the feature/large-tuple-ignore-regex branch from 804ec4c to 4cfbd90 Compare December 16, 2025 21:19

SimplyDanny requested changes Dec 17, 2025

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Fix: optional_data_string_conversion should detect String.init and leading-dot .init usages #6372

Fix: optional_data_string_conversion should detect String.init and leading-dot .init usages #6372

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		### Bug Fixes

		* optional_data_string_conversion: Detect `String.init(...)` and leading-dot `.init(...)` usages when converting `Data` to `String`. This fixes cases such as `String.init(decoding: data, as: UTF8.self)` and `let text: String = .init(decoding: data, as: UTF8.self)` which previously did not trigger the rule. (Fixes https://github.com/realm/SwiftLint/issues/6359)

		* optional_data_string_conversion: Detect `String.init(...)` and leading-dot `.init(...)` usages when converting `Data` to `String`. This fixes cases such as `String.init(decoding: data, as: UTF8.self)` and `let text: String = .init(decoding: data, as: UTF8.self)` which previously did not trigger the rule. (Fixes https://github.com/realm/SwiftLint/issues/6359)
		[Nadeem Ali](https://github.com/nadeemnali)

Fix: optional_data_string_conversion should detect String.init and leading-dot .init usages #6372

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Fix: optional_data_string_conversion should detect String.init and leading-dot .init usages #6372

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nadeemnali commented Dec 9, 2025

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