type_alias_impl_trait is unusable #3079

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1. RustyYato commented on Feb 14, 2021

   

   RustyYato

   on Feb 14, 2021

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   That isn't a complete snippet, you also need a defining use, i.e. a function return type.

   #![feature(type_alias_impl_trait)]
   
   type Adder = impl Fn(usize) -> usize;
   
   fn adder() -> Adder { /* snip */ }
2. Sherlock-Holo commented on Feb 14, 2021

   

   Sherlock-Holo

   on Feb 14, 2021

   Author

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   I try this

   #![feature(type_alias_impl_trait)]
   
   fn main() {}
   
   type Adder = impl Fn(usize) -> usize;
   
   fn doit() -> Adder {
       func
   }
   
   fn func(n: usize) -> usize {
       n
   }

   now cargo check doesn't report errors, but why I have to use it?
   If I write type RR<T> = Result<T, Infallible>; cargo check won't force me to use it
3. RustyYato commented on Feb 14, 2021

   

   RustyYato

   on Feb 14, 2021

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   I think this will be easier to understand via analogy.

   1 - does not compile type_alias_impl_trait

   #![feature(type_alias_impl_trait)]
   
   fn main() {}
   
   type Adder = impl Fn(usize) -> usize;

   is similar to

   let Adder: impl Fn(usize) -> usize;

   Note how Adder is declared, but has no value. Now, Rust enforces that all top-level declarations must have a value. So leaving it how doesn't work.

   2 - compiles type_alias_impl_trait

   #![feature(type_alias_impl_trait)]
   
   fn main() {}
   
   type Adder = impl Fn(usize) -> usize;
   
   fn doit() -> Adder {
       func
   }
   
   fn func(n: usize) -> usize {
       n
   }

   is similar to

   let Adder: impl Fn(usize) -> usize;
   
   Adder = func;
   
   // snip

   Note how Adder now has a value. So this is fine, even though the value is provided in a different spot from the declaration.

   3 - compiles type alias

   type RR<T> = Result<T, Infallible>;

   is similar to

   let Adder: impl Fn(usize) -> usize = func;

   Note how the declaration and definition are done at the same time. So this is also fine. There are two big differences between normal variables, and top-level declarations:

   • Top-level declarations can be in an arbitrary order, where as normal variables must be declared before or at the same time as the they are defined/used.
   • Top-level declarations must have a defined value (even if unused), whereas normal variables are allowed to be undefined (as long as they aren't used).

   👍React with 👍2Reacted by Lucius Hu and cupofc0t
4. scottmcm commented on Mar 31, 2021

   

   scottmcm

   on Mar 31, 2021

   Member

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   It looks like this is working as expected, then, so I'll close this issue.
5. scottmcm

   closed this as completedon Mar 31, 2021