Zev Eisenberg a994527a23 [SE-0489] Better debugDescription for EncodingError and DecodingError (#80941) ...

Now accepted as
[SE-0489](https://github.com/ZevEisenberg/swift-evolution/blob/main/proposals/0489-codable-error-printing.md).

# To Do

- [x] confirm which version of Swift to use for the availability
annotations. Probably 6.3 at time of writing.

# Context

Re: [Swift forum
post](https://forums.swift.org/t/the-future-of-serialization-deserialization-apis/78585/77),
where a discussion about future serialization tools in Swift prompted
Kevin Perry to suggest that some proposed changes could actually be made
in today's stdlib.

# Summary of Changes

Conforms `EncodingError` and `DecodingError` to
`CustomDebugStringConvertible` and adds a more-readable
`debugDescription`.

# Future Directions

This is a pared-down version of some experiments I did in
[UsefulDecode](https://github.com/ZevEisenberg/UsefulDecode). The
changes in this PR are the best I could do without changing the public
interface of `DecodingError` and `EncodingError`, and without modifying
the way the `JSON`/`PropertyList` `Encoder`/`Decoder` in Foundation
generate their errors' debug descriptions.

In the above-linked
[UsefulDecode](https://github.com/ZevEisenberg/UsefulDecode) repo, when
JSON decoding fails, I go back and re-decode the JSON using
`JSONSerialization` in order to provide more context about what failed,
and why. I didn't attempt to make such a change here, but I'd like to
discuss what may be possible.

# Examples

To illustrate the effect of the changes in this PR, I removed my changes
to stdlib/public/core/Codable.swift and ran my new test cases again.
Here are the resulting diffs.

##
`test_encodingError_invalidValue_nonEmptyCodingPath_nilUnderlyingError`

### Before
`invalidValue(234, Swift.EncodingError.Context(codingPath:
[GenericCodingKey(stringValue: "first", intValue: nil),
GenericCodingKey(stringValue: "second", intValue: nil),
GenericCodingKey(stringValue: "2", intValue: 2)], debugDescription: "You
cannot do that!", underlyingError: nil))`

### After
`EncodingError.invalidValue: 234 (Int). Path: first.second[2]. Debug
description: You cannot do that!`

## `test_decodingError_valueNotFound_nilUnderlyingError`

### Before
`valueNotFound(Swift.String, Swift.DecodingError.Context(codingPath:
[GenericCodingKey(stringValue: "0", intValue: 0),
GenericCodingKey(stringValue: "firstName", intValue: nil)],
debugDescription: "Description for debugging purposes", underlyingError:
nil))`

### After
`DecodingError.valueNotFound: Expected value of type String but found
null instead. Path: [0].firstName. Debug description: Description for
debugging purposes`

## `test_decodingError_keyNotFound_nonNilUnderlyingError`

### Before
`keyNotFound(GenericCodingKey(stringValue: "name", intValue: nil),
Swift.DecodingError.Context(codingPath: [GenericCodingKey(stringValue:
"0", intValue: 0), GenericCodingKey(stringValue: "address", intValue:
nil), GenericCodingKey(stringValue: "city", intValue: nil)],
debugDescription: "Just some info to help you out", underlyingError:
Optional(main.GenericError(name: "hey, who turned out the lights?"))))`

### After
`DecodingError.keyNotFound: Key \'name\' not found in keyed decoding
container. Path: [0].address.city. Debug description: Just some info to
help you out. Underlying error: GenericError(name: "hey, who turned out
the lights?")`

## `test_decodingError_typeMismatch_nilUnderlyingError`

### Before
`typeMismatch(Swift.String, Swift.DecodingError.Context(codingPath:
[GenericCodingKey(stringValue: "0", intValue: 0),
GenericCodingKey(stringValue: "address", intValue: nil),
GenericCodingKey(stringValue: "city", intValue: nil),
GenericCodingKey(stringValue: "birds", intValue: nil),
GenericCodingKey(stringValue: "1", intValue: 1),
GenericCodingKey(stringValue: "name", intValue: nil)], debugDescription:
"This is where the debug description goes", underlyingError: nil))`

### After
`DecodingError.typeMismatch: expected value of type String. Path:
[0].address.city.birds[1].name. Debug description: This is where the
debug description goes`

## `test_decodingError_dataCorrupted_nonEmptyCodingPath`

### Before
`dataCorrupted(Swift.DecodingError.Context(codingPath:
[GenericCodingKey(stringValue: "first", intValue: nil),
GenericCodingKey(stringValue: "second", intValue: nil),
GenericCodingKey(stringValue: "2", intValue: 2)], debugDescription:
"There was apparently some data corruption!", underlyingError:
Optional(main.GenericError(name: "This data corruption is getting out of
hand"))))`

### After
`DecodingError.dataCorrupted: Data was corrupted. Path: first.second[2].
Debug description: There was apparently some data corruption!.
Underlying error: GenericError(name: "This data corruption is getting
out of hand")`

## `test_decodingError_valueNotFound_nonNilUnderlyingError`

### Before
`valueNotFound(Swift.Int, Swift.DecodingError.Context(codingPath:
[GenericCodingKey(stringValue: "0", intValue: 0),
GenericCodingKey(stringValue: "population", intValue: nil)],
debugDescription: "Here is the debug description for value-not-found",
underlyingError: Optional(main.GenericError(name: "these aren\\\'t the
droids you\\\'re looking for"))))`

### After
`DecodingError.valueNotFound: Expected value of type Int but found null
instead. Path: [0].population. Debug description: Here is the debug
description for value-not-found. Underlying error: GenericError(name:
"these aren\\\'t the droids you\\\'re looking for")`

##
`test_encodingError_invalidValue_emptyCodingPath_nonNilUnderlyingError`

### Before
`invalidValue(345, Swift.EncodingError.Context(codingPath: [],
debugDescription: "You cannot do that!", underlyingError:
Optional(main.GenericError(name: "You really cannot do that"))))`

### After
`EncodingError.invalidValue: 345 (Int). Debug description: You cannot do
that!. Underlying error: GenericError(name: "You really cannot do
that")`

## `test_decodingError_typeMismatch_nonNilUnderlyingError`

### Before
`typeMismatch(Swift.String, Swift.DecodingError.Context(codingPath:
[GenericCodingKey(stringValue: "0", intValue: 0),
GenericCodingKey(stringValue: "address", intValue: nil),
GenericCodingKey(stringValue: "1", intValue: 1),
GenericCodingKey(stringValue: "street", intValue: nil)],
debugDescription: "Some debug description", underlyingError:
Optional(main.GenericError(name: "some generic error goes here"))))`

### After
`DecodingError.typeMismatch: expected value of type String. Path:
[0].address[1].street. Debug description: Some debug description.
Underlying error: GenericError(name: "some generic error goes here")`

## `test_encodingError_invalidValue_emptyCodingPath_nilUnderlyingError`

### Before
`invalidValue(123, Swift.EncodingError.Context(codingPath: [],
debugDescription: "You cannot do that!", underlyingError: nil))`

### After
`EncodingError.invalidValue: 123 (Int). Debug description: You cannot do
that!`

## `test_decodingError_keyNotFound_nilUnderlyingError`

### Before
`keyNotFound(GenericCodingKey(stringValue: "name", intValue: nil),
Swift.DecodingError.Context(codingPath: [GenericCodingKey(stringValue:
"0", intValue: 0), GenericCodingKey(stringValue: "address", intValue:
nil), GenericCodingKey(stringValue: "city", intValue: nil)],
debugDescription: "How would you describe your relationship with your
debugger?", underlyingError: nil))`

### After
`DecodingError.keyNotFound: Key \'name\' not found in keyed decoding
container. Path: [0]address.city. Debug description: How would you
describe your relationship with your debugger?`

## `test_decodingError_dataCorrupted_emptyCodingPath`

### Before
`dataCorrupted(Swift.DecodingError.Context(codingPath: [],
debugDescription: "The given data was not valid JSON", underlyingError:
Optional(main.GenericError(name: "just some data corruption"))))`

### After
`DecodingError.dataCorrupted: Data was corrupted. Debug description: The
given data was not valid JSON. Underlying error: GenericError(name:
"just some data corruption")`

##
`test_encodingError_invalidValue_nonEmptyCodingPath_nonNilUnderlyingError`

### Before
`invalidValue(456, Swift.EncodingError.Context(codingPath:
[GenericCodingKey(stringValue: "first", intValue: nil),
GenericCodingKey(stringValue: "second", intValue: nil),
GenericCodingKey(stringValue: "2", intValue: 2)], debugDescription: "You
cannot do that!", underlyingError: Optional(main.GenericError(name: "You
really cannot do that"))))`

### After
`EncodingError.invalidValue: 456 (Int). Path: first.second[2]. Debug
description: You cannot do that!. Underlying error: GenericError(name:
"You really cannot do that")`

2025-12-01 11:34:00 -05:00

..

Inputs/macOS

[Debug] Add pointer based stringForPrintObject (#84742)

2025-11-18 09:32:28 -08:00

macOS

[SE-0489] Better debugDescription for EncodingError and DecodingError (#80941)

2025-12-01 11:34:00 -05:00