mirror of https://github.com/rust-lang/rust
38 lines
1.2 KiB
Rust
38 lines
1.2 KiB
Rust
// Variant of diverging-fallback-control-flow that tests
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// the specific case of a free function with an unconstrained
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// return type. This captures the pattern we saw in the wild
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// in the objc crate, where changing the fallback from `!` to `()`
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// resulted in unsoundness.
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//
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//@ check-pass
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//@ revisions: nofallback fallback
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#![cfg_attr(fallback, feature(never_type, never_type_fallback))]
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#![allow(unit_bindings)]
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fn make_unit() {}
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trait UnitReturn {}
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impl UnitReturn for i32 {}
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impl UnitReturn for () {}
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fn unconstrained_return<T: UnitReturn>() -> T {
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unsafe {
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let make_unit_fn: fn() = make_unit;
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let ffi: fn() -> T = std::mem::transmute(make_unit_fn);
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ffi()
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}
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}
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fn main() {
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// In Ye Olde Days, the `T` parameter of `unconstrained_return`
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// winds up "entangled" with the `!` type that results from
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// `panic!`, and hence falls back to `()`. This is kind of unfortunate
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// and unexpected. When we introduced the `!` type, the original
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// idea was to change that fallback to `!`, but that would have resulted
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// in this code no longer compiling (or worse, in some cases it injected
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// unsound results).
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let _ = if true { unconstrained_return() } else { panic!() };
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}
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