Introduce Efficient Node Localization in Syntax Tree for Unit Tests

[Matejkob]

Description

Currently, there is no mechanism to efficiently refer to specific node in syntax tree for the need of unit test. For example to create Diagnostic.highlights and attach them to a specific node in a syntax tree within diagnostic formatters unit tests. The absence of such a feature makes it cumbersome to identify and highlight syntax nodes for diagnostic or verification purposes.

Proposed Solution by @ahoppen:

Introduce a NodeLocator struct to facilitate the process. Below is a proposed implementation idea:

/// Refers to the first node in a syntax tree after `marker` that satisfies `match`.
///
/// For example, if you have
/// ```swift
/// var x: Int
/// 1️⃣func foo() {}
/// ```
///
/// Then `NodeLocator(marker: "1️⃣")` refers to the `CodeBlockItemSyntax` that
/// contains the function declaration and
/// `NodeLocator(marker: "1️⃣", matches: { $0.is(FunctionDeclSyntax.self) })`
/// refers to the function within.
struct NodeLocator {
  let marker: String
  let match: (Syntax) -> Bool = { _ in true }

  func findNode(in tree: Syntax) -> Syntax? {
    // TODO: Implement
  }
}

Note

Before starting implementation, let's discuss this approach here.

This feature could be a significant step towards enabling more precise and easier-to-write unit tests, especially those involving diagnostic highlights.

[Matejkob]

For the current needs, I've implemented unit tests to examine how the range of source code to highlight is calculated. The relevant tests can be found here:

https://github.com/apple/swift-syntax/blob/b6fcad4496cce446faa8e4f79b7558a33710a842/Tests/SwiftDiagnosticsTest/SyntaxHighlightRangeCalculatorTests.swift#L76-L90

https://github.com/apple/swift-syntax/blob/b6fcad4496cce446faa8e4f79b7558a33710a842/Tests/SwiftDiagnosticsTest/SyntaxHighlightRangeCalculatorTests.swift#L276-L296

And this approach is far from optimal.

[ahoppen]

Tracked in Apple’s issue tracker as rdar://116256883