8000 Cannot reinitialize inout parameter of type `MutableSpan<T>?` · Issue #81976 · swiftlang/swift · GitHub
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Cannot reinitialize inout parameter of type MutableSpan<T>? #81976

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glessard opened this issue Jun 4, 2025 · 0 comments
Open

Cannot reinitialize inout parameter of type MutableSpan<T>? #81976

glessard opened this issue Jun 4, 2025 · 0 comments
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. ownership Feature: Ownership modifiers and semantics

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@glessard
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glessard commented Jun 4, 2025 •
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Description

An inout parameter of an Optional of a type that is ~Copyable & ~Escapable is very hard to work with. In particular, the idiomatic way to unwrap that parameter doesn't work.

Reproduction

Using swift-DEVELOPMENT-SNAPSHOT-2025-06-01-a (e.g. in godbolt)

func modify<T, U>(
    _ s: inout MutableSpan<T>?,
    _ body: (inout MutableSpan<T>) -> U
) -> U {
    var unwrapped: MutableSpan<T>
    if let uw = s {
        unwrapped = uw
    } else {
        unwrapped = MutableSpan(_unsafeElements: .init(start: nil, count: 0))
    }
    let u = body(&unwrapped)
    s = consume unwrapped
    return u
}

Does not compile. The error is:

<source>:5:9: error: lifetime-dependent variable 'unwrapped' escapes its scope
 3 |     _ body: (inout MutableSpan<T>) -> U
 4 | ) -> U {
 5 |     var unwrapped: MutableSpan<T>
   |         `- error: lifetime-dependent variable 'unwrapped' escapes its scope
 6 |     if let uw = s {
 7 |         unwrapped = uw
 8 |     } else {
 9 |         unwrapped = MutableSpan(_unsafeElements: .init(start: nil, count: 0))
   |                                                   `- note: it depends on the lifetime of this parent value
10 |     }
11 |     let u = body(&unwrapped)
12 |     s = consume unwrapped
   |       `- note: this use causes the lifetime-dependent value to escape
13 |     return u
14 | }
Compiler returned: 1

Expected behavior

This should compile. In fact, modifying line 6 to read if let uw = s.take() { allows the whole thing to compile. That changes things very little, yet satisfies the compiler.

Environment

Swift version 6.2-dev (LLVM 5e7837820b1c886, Swift d5ef256)
Target: x86_64-unknown-linux-gnu
Build config: +assertions

The compiler is called with -enable-experimental-feature LifetimeDependence

Additional information

rdar://152572002
@glessard glessard added bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. ownership Feature: Ownership modifiers and semantics labels Jun 4, 2025
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