[ST-NNNN] Add a filePath property to SourceLocation

Author: grynspan

proposals/testing/NNNN-sourcelocation-filepath.md

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+# Add a `filePath` property to `SourceLocation`
+
+* Proposal: [ST-NNNN](NNNN-sourcelocation-filepath.md)
+* Authors: [Jonathan Grynspan](https://github.com/grynspan)
+* Review Manager: TBD
+* Status: **Awaiting review**
+* Bug: rdar://152999195
+* Implementation: [swiftlang/swift-testing#1334](https://github.com/swiftlang/swift-testing/pull/1334), [swiftlang/swift-testing#1472](https://github.com/swiftlang/swift-testing/pull/1472)
+* Review: ([pitch](https://forums.swift.org/t/pitch-add-a-filepath-property-to-sourcelocation-in-swift-testing/83995))
+
+## Introduction
+
+Swift Testing represents the in-source location of tests, issues, errors, etc.
+using a structure called [`SourceLocation`](https://developer.apple.com/documentation/testing/sourcelocation).
+This structure contains the line, column, and file ID for a given location in a
+Swift source file. I propose adding the file _path_ to this structure for use by
+developers and tools.
+
+## Motivation
+
+When we initially designed Swift Testing and `SourceLocation`, our expectation
+was that tools like Visual Studio Code or Xcode would be able to translate Swift
+file IDs to file paths where needed.
+
+At the time, Swift 6 had not yet been introduced and we were working off the
+assumption that `#filePath` would eventually be deprecated (possibly in Swift 6).
+This assumption was based on the history behind `#fileID`:
+
+- [SE-0274](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0274-magic-file.md)
+  originally proposed repurposing `#file` to generate file IDs (and added
+  `#filePath` for use cases that still needed full paths.)
+- [SE-0285](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0285-ease-pound-file-transition.md)
+  modified the plan to leave `#file` as-is in Swift 5 and introduced `#fileID`
+  and planned to have `#file` equal `#fileID` in Swift 6 onward.
+- In September 2024, the Language Steering Group [discussed](https://forums.swift.org/t/file-vs-fileid-in-swift-6/74614/4)
+  `#file`, `#filePath`, and `#fileID`, and decided to maintain the status quo.
+
+Because the future of these magic values (now expression macros) was unclear, we
+opted to provide only a `fileID` property on `SourceLocation`. `SourceLocation`
+_does_ gather a full file path, but does not expose a public property to access
+this value at runtime or later.
+
+It has become clear since we shipped Swift Testing that tools are much better
+equipped to handle file paths than Swift file IDs, and the translation from a
+file ID back to the original path on a developer's system may be non-trivial or
+even impossible (in the case of strict sandboxing around a tool).
+
+## Proposed solution
+
+I propose adding a `filePath` property to `SourceLocation` and to the JSON event
+stream schema.
+
+## Detailed design
+
+The following property is added:
+
+```swift
+extension SourceLocation {
+  /// The path to the source file.
+  public var filePath: String { get set }
+}
+```
+
+Swift Testing's JSON schema (version `"6.3"` onward) is updated to add a
+`"filePath"` key to source location values and to make the `"fileID"` key
+optional:
+
+```diff
+ <source-location> ::= {
+-  "fileID": <string>, ; the Swift file ID of the file
++  ["fileID": <string>,] ; the Swift file ID of the file if available, as per
++                        ; SE-0274 § "Specification of the #file string format"
++  "filePath": <string>, ; the compile-time path to the file
+   "line": <number>,
+   "column": <number>,
+ }
+```
+
+The value of the `"filePath"` key is an absolute file system path. It is _not_
+guaranteed to refer to an existing file (for instance, it may refer to a file on
+a build system that isn't present at runtime). The path style used (POSIX,
+Windows, or otherwise) is implementation-defined.
+
+If the `"fileID"` key is not present when Swift Testing decodes an instance of
+`SourceLocation`, it is synthesized from `"filePath"` and the module name is
+assumed to be `"__C"`[^cModuleName]. This change allows Swift Testing to support source
+location information generated in other languages like Objective-C or C++. For
+example, a test written in Objective-C could construct a `<source-location>`
+JSON value as follows:
+
+```objc
+id jsonObject = @{
+  @"line": @(__LINE__),
+  @"column": @(__builtin_COLUMN()),
+  @"filePath": @(__file__)
+};
+```
+
+And, assuming `__file__` equals `"/foo/bar/quux.m"`, Swift Testing would infer a
+file ID of `"__C/quux.m"`.
+
+[^cModuleName]: The Swift compiler and runtime assume a module name of `__C` for
+  most imported foreign types.
+
+## Source compatibility
+
+There are no source compatibility concerns.
+
+## Integration with supporting tools
+
+Supporting tools will be able to migrate from the unsupported `"_filePath"` JSON
+key to the new `"filePath"` key. We will continue to emit `"_filePath"` where
+needed until the ecosystem has moved to `"filePath"`.
+
+## Future directions
+
+N/A
+
+## Alternatives considered
+
+- None.