Add Generate enum associated value accessors code action

[william-laverty]

Description

Adds a syntax-based code action that generates computed properties for enums with associated values.

For each case with associated values, generates:

• asX: T? — extracts the associated value via pattern matching, or nil for other cases
• isX: Bool — returns true if the value matches the case

Before:

enum Value {
    case text(String)
    case number(Int)
}

After:

enum Value {
    case text(String)
    case number(Int)

    var asText: String? {
        if case let .text(v) = self { return v }
        return nil
    }

    var isText: Bool {
        if case .text = self { return true }
        return false
    }

    var asNumber: Int? {
        if case let .number(v) = self { return v }
        return nil
    }

    var isNumber: Bool {
        if case .number = self { return true }
        return false
    }
}

Handles single and multiple associated values (tuples). Skips generation for accessors that already exist. Inspired by rust-analyzer's generate_enum_as_method.

Tests

• Test for enum with two cases having associated values
• Test that no action is offered for enums without associated values

Resolves #2522

[ahoppen]

Could you run swift-format on your PR as described in CONTRIBUTING.md

[ahoppen]

We have .only for this

Suggested change

	let element = caseDecl.elements.first,
	let element = caseDecl.elements.only,

[ahoppen]

Would it be possible to implement this as a SyntaxRefactoringCodeActionProvider?

[ahoppen]

We should stop at a CodeBlockSyntax same as in other code actions so we don’t offer this while the cursor is inside a function implementation. We should probably also stop at DeclSyntax so we don’t offer this while you’re in a nested type.

[ahoppen]

You should throw a RefactoringNotApplicableError if the refactoring action doesn’t apply because that will hide it in SourceKit-LSP.

[ahoppen]

We should attempt to pick more descriptive variable names here. If the associated value has a label, we should pick that. Otherwise, I’d use value2 instead of v2.

[ahoppen]

We should infer the indentation of the source file (see how other code actions do this) instead of hardcoding it to 4 spaces.

[ahoppen]

I think this would be a lot more readable as a multi-line string literal.

[ahoppen]

You need to pass exhaustive: true if you want to check that a code action doesn’t exist.

[ahoppen]

Is there any reason why we shouldn’t generate the is accessors for cases without associated values?