create a specific error for temporary values that are borrowed too long

Carrying over a comment from #54088:

I feel like the temporary rules are a common source of confusion. I think we should consider using a distinct error code for them, so that --explain can have some extra text to introduce the rules. Maybe good for a follow-up to this PR though.

As a specific example, consider this test:

rust/src/test/ui/nll/borrowed-temporary-error.rs

Lines 18 to 22 in 7ee7207

    
           let x = gimme({ 
        
               let v = 22; 
        
               &(v,) 
        
               //~^ ERROR borrowed value does not live long enough [E0597] 
        
           });

we give this error:

rust/src/test/ui/nll/borrowed-temporary-error.stderr

Lines 1 to 10 in 7ee7207

    
           error[E0597]: borrowed value does not live long enough 
        
             --> $DIR/borrowed-temporary-error.rs:20:10 
        
              | 
        
           LL |         &(v,) 
        
              |          ^^^^ temporary value does not live long enough 
        
           LL |         //~^ ERROR borrowed value does not live long enough [E0597] 
        
           LL |     }); 
        
              |       - temporary value only lives until here 
        
           LL |     println!("{:?}", x); 
        
              |                      - borrow later used here

but I think we should say something more like this:

error[E9999]: temporary value borrowed for too long
  --> $DIR/borrowed-temporary-error.rs:20:10
   |
LL |         &(v,)
   |          ^^^^ creates a temporary which is freed while still in use
LL |         //~^ ERROR borrowed value does not live long enough [E0597]
LL |     });
   |       - temporary value is freed at the end of this statement
LL |     println!("{:?}", x);
   |                      - but the borrow is later used here

Moreover, the extended error for E9999 ought to explain the temporary rules.

Here are some notes on what we might say in the extended error:

When you write &expr and expr is not a "path" rooted in some variable (e.g., &foo.bar) but rather something that creates a new value (e.g., &foo()), a temporary will be created:

let x = foo();
let y = &x;

If this temporary expression is assigned directly into a let, or into a tuple or struct that is assigned into a let, it lives until the end of the block:

Example:

{ 
    let x = &foo(); // <-- temporary is created here
    ...
} // <-- and then freed here

Otherwise, the temporary is freed at the end of the statement. This most commonly occurs with function call arguments:

let p = bar(&foo()); // <-- temporary is freed after this statement

If p here borrows from the temporary, then you are going to get an error, because the temporary gets freed while still in use. You can add a temporary to extend the lifetime:

let tmp_ref = &foo(); // freed at end of block
let p = bar(tmp_ref); 
...

I can take it if no one already started

Auto merge of #54164 - mikhail-m1:54131, r=pnkfelix

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create a specific error for temporary values that are borrowed too long #54131

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	let x = gimme({
	let v = 22;
	&(v,)
	//~^ ERROR borrowed value does not live long enough [E0597]
	});

	error[E0597]: borrowed value does not live long enough
	--> $DIR/borrowed-temporary-error.rs:20:10
	\|
	LL \| &(v,)
	\| ^^^^ temporary value does not live long enough
	LL \| //~^ ERROR borrowed value does not live long enough [E0597]
	LL \| });
	\| - temporary value only lives until here
	LL \| println!("{:?}", x);
	\| - borrow later used here

create a specific error for temporary values that are borrowed too long #54131

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nikomatsakis commented on Sep 11, 2018

mikhail-m1 commented on Sep 11, 2018

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