8000 Improve diagnostics for 'let…else' without 'guard' · Issue #81728 · swiftlang/swift · GitHub
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Improve diagnostics for 'let…else' without 'guard' #81728

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jrose-signal opened this issue May 23, 2025 · 4 comments
Open

Improve diagnostics for 'let…else' without 'guard' #81728

jrose-signal opened this issue May 23, 2025 · 4 comments
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8000 compiler The Swift compiler itself diagnostics QoI Bug: Diagnostics Quality of Implementation feature A feature request or implementation parser Area → compiler: The legacy C++ parser swift 6.2

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@jrose-signal
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Motivation

The following code produces a generic "consecutive statements on a line must be separated by ';'" error:

let specific = general as? Specific else {
  return nil
}

However, there's a reason somewhat might write this: this is the Rust syntax for pattern-match-or-exit-scope (though in Rust it's closer to guard case than guard let). Even without that justification, else is not normally the start of a statement anyway, so better recovery is possible.

Proposed solution

If a let or var decl is followed by else, ideally recover as if the user had written guard let instead. If that's not possible, at least add a dedicated diagnostic for the else rather than just "consecutive statements…" followed by "expected expression".

Alternatives considered

No response

Additional information

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@jrose-signal jrose-signal added feature A feature request or implementation triage needed This issue needs more specific labels labels May 23, 2025
@AnthonyLatsis AnthonyLatsis added parser Area → compiler: The legacy C++ parser compiler The Swift compiler itself diagnostics QoI Bug: Diagnostics Quality of Implementation swift 6.2 and removed triage needed This issue needs more specific labels labels May 26, 2025
@AnthonyLatsis
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New parser:

stdin:1:37: error: extraneous code at top level
1 | let specific = general as? Specific else {
  |                                     `- error: extraneous code at top level
2 |   return nil
3 | }

@jrose-signal
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Ah, I meant in a function body, at top level in a non-script file that's probably reasonable.

@AnthonyLatsis
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2 | func f() {
3 |   let specific = general as? Specific else {
  |                                       `- error: unexpected code in function
4 |     return nil
5 |   }

@hamishknight Does the new parser have a parse-as-library setting?

@hamishknight
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Does the new parser have a parse-as-library setting?

Not AFAIK, parsing a SourceFileSyntax always just allows top-level code currently. The "extraneous code at top level" error is because it's failing to parse else as anything useful and just treating it as unexpected

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compiler The Swift compiler itself diagnostics QoI Bug: Diagnostics Quality of Implementation feature A feature request or implementation parser Area → compiler: The legacy C++ parser swift 6.2
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