44 lines
1.2 KiB
Rust
44 lines
1.2 KiB
Rust
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// Documents that Rust currently does not permit the coercion &mut &mut T -> *mut *mut T
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// Making this compile was a feature request in rust-lang/rust#34117 but this is currently
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// "working as intended". Allowing "deep pointer coercion" seems footgun-prone, and would
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// require proceeding carefully.
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use std::ops::{Deref, DerefMut};
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struct Foo(i32);
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struct SmartPtr<T>(*mut T);
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impl<T> SmartPtr<T> {
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fn get_addr(&mut self) -> &mut *mut T {
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&mut self.0
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}
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}
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impl<T> Deref for SmartPtr<T> {
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type Target = T;
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fn deref(&self) -> &T {
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unsafe { &*self.0 }
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}
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}
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impl<T> DerefMut for SmartPtr<T> {
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fn deref_mut(&mut self) -> &mut T {
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unsafe { &mut *self.0 }
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}
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}
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/// Puts a Foo into the pointer provided by the caller
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fn make_foo(_: *mut *mut Foo) {
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unimplemented!()
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}
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fn main() {
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let mut result: SmartPtr<Foo> = SmartPtr(std::ptr::null_mut());
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make_foo(&mut &mut *result); //~ mismatched types
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//~^ expected `*mut *mut Foo`, found `&mut &mut Foo`
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make_foo(out(&mut result)); // works, but makes one wonder why above coercion cannot happen
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}
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fn out<T>(ptr: &mut SmartPtr<T>) -> &mut *mut T {
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ptr.get_addr()
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}
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