::fix_hidden_lifetime_bug

Are you getting one of the two following errors (E700)?

• rust,ignore error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds --> examples/main.rs:13:40 | 13 | fn foo<'a, 'b> (it: &'a mut &'b ()) -> impl 'a + Sized { | ^^^^^^^^^^^^^^^ | note: hidden type `&'a mut &'b ()` captures the lifetime `'b` as defined on the function body at 13:12 --> examples/main.rs:13:12 | 13 | fn foo<'a, 'b> (it: &'a mut &'b ()) -> impl 'a + Sized { | ^^

  Problematic code

  rust,compile_fail fn foo<'a, 'b> (it: &'a mut &'b ()) -> impl 'a + Sized { it }

• rust,ignore error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds --> examples/main.rs:8:45 | 8 | async fn bar<'a> (_: &(), _: Box<dyn Send>) { | ^ | note: hidden type `impl Future` captures lifetime smaller than the function body --> examples/main.rs:8:45 | 8 | async fn bar<'a> (_: &(), _: Box<dyn Send>) { | ^

  Problematic code

  rust,compile_fail async fn bar<'a> (_: &(), _: Box<dyn Send>) { /* … */ }

• rust,ignore error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds --> examples/main.rs:4:57 | 4 | async fn baz<'a> (a: &'static (), b: &'_ (), c: &'_ ()) { | ^ | note: hidden type `impl Future` captures lifetime smaller than the function body --> examples/main.rs:4:57 | 4 | async fn baz<'a> (a: &'static (), b: &'_ (), c: &'_ ()) { | ^

  Problematic code

  rust,compile_fail async fn baz<'a> (a: &'static (), b: &'_ (), c: &'_ ()) { /* … */ }

Then you can can the attribute provided by this crate to automagically generate an equivalent signature that soothes this grumpy compiler

• See [the lifetime bug async issue], as well as this other comment for more info.

  The fix is thus to perform the unsugaring from an async fn to an fn yielding a Future, and then just adding the necessary + Captures<'_> bounds.

Usage

1. cargo add fix_hidden_lifetime_bug, or add the following to your Cargo.toml file:

   toml [dependencies] fix_hidden_lifetime_bug = "x.y.z"

   • where you can find the version using cargo search fix_hidden_lifetime_bug

2. Add the following to your lib.rs file:

   ```rust,ignore

   [macro_use]

   extern crate fixhiddenlifetime_bug; ```

3. Slap a #[fix_hidden_lifetime_bug] on the problematic function:

   ```rust

   use ::fixhiddenlifetime_bug::*;

   [fixhiddenlifetime_bug] // <-- Add this!

   fn foo<'a, 'b>(it: &'a mut &'b ()) -> impl 'a + Sized { it } ```

   ```rust

   use ::fixhiddenlifetime_bug::*;

   [fixhiddenlifetime_bug] // <-- Add this!

   async fn baz<'a>(fst: &'static str, snd: &str, thrd: &str) { /* … */ } ```