Account for type param in '_ constraint for returned trait suggestion

Lines 1 to 6 in f7a1f97

    
           use std::any::Any; 
        
           fn foo<T: Any>(value: &T) -> Box<dyn Any> { 
        
               Box::new(value) as Box<dyn Any> 
        
               //~^ ERROR cannot infer an appropriate lifetime 
        
           }

we currently output

rust/src/test/ui/issues/issue-16922.stderr

Lines 1 to 12 in f7a1f97

    
           error[E0759]: cannot infer an appropriate lifetime 
        
             --> $DIR/issue-16922.rs:4:14 
        
              | 
        
           LL | fn foo<T: Any>(value: &T) -> Box<dyn Any> { 
        
              |                       -- this data with an anonymous lifetime `'_`... 
        
           LL |     Box::new(value) as Box<dyn Any> 
        
              |              ^^^^^ ...is captured here, requiring it to live as long as `'static` 
        
              | 
        
           help: to declare that the trait object captures data from argument `value`, you can add an explicit `'_` lifetime bound 
        
              | 
        
           LL | fn foo<T: Any>(value: &T) -> Box<dyn Any + '_> { 
        
              |                                          ^^^^

The suggestion is incomplete and will not result in compilable code. Because the lifetime is coming from an argument of type T, which is an unconstrained type parameter, T itself will not have the needed lifetime and causes the 'static obligation that we're trying to reverse by suggesting '_. The appropriate changes to the code would be either fn foo<'a, T: Any + 'a>(value: &'a T) -> Box<dyn Any + 'a> or fn foo(value: &dyn Any) -> Box<dyn Any + '_>, depending on whether T appears in any other argument or not.

The suggested code works since 1.64.

Apply structured suggestion that allows test to work since 1.64

Rollup merge of rust-lang#116244 - estebank:issue-73497, r=b-naber

Unrolled build for rust-lang#116244

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Account for type param in `'_` constraint for returned trait suggestion #73497

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	use std::any::Any;

	fn foo<T: Any>(value: &T) -> Box<dyn Any> {
	Box::new(value) as Box<dyn Any>
	//~^ ERROR cannot infer an appropriate lifetime
	}

	error[E0759]: cannot infer an appropriate lifetime
	--> $DIR/issue-16922.rs:4:14
	\|
	LL \| fn foo<T: Any>(value: &T) -> Box<dyn Any> {
	\| -- this data with an anonymous lifetime `'_`...
	LL \| Box::new(value) as Box<dyn Any>
	\| ^^^^^ ...is captured here, requiring it to live as long as `'static`
	\|
	help: to declare that the trait object captures data from argument `value`, you can add an explicit `'_` lifetime bound
	\|
	LL \| fn foo<T: Any>(value: &T) -> Box<dyn Any + '_> {
	\| ^^^^

Account for type param in '_ constraint for returned trait suggestion #73497

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estebank commented on Sep 28, 2023

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Account for type param in `'_` constraint for returned trait suggestion #73497