[NLL] "borrowed value must be valid for lifetime '_#2r..."

[nikomatsakis]

In NLL, there is one rule of diagnostics:

Thou must-eth never print-eth a region.

Unfortunately, we break it. Twice. Here is one example:

#![feature(nll)]

fn gimme(x: &(u32,)) -> &u32 { &x.0 }

fn main() {
    let v = 22;
    let x = gimme(&(v,));
    println!("{:?}", x);
}

This prints the following:

error[E0597]: borrowed value does not live long enough
 --> src/main.rs:7:20
  |
7 |     let x = gimme(&(v,));
  |                    ^^^^ - temporary value only lives until here
  |                    |
  |                    temporary value does not live long enough
  |
  = note: borrowed value must be valid for lifetime '_#2r...

Note the final line. In comparison, the AST-based version gets the following error, which is much better:

error[E0597]: borrowed value does not live long enough
 --> src/main.rs:7:20
  |
7 |     let x = gimme(&(v,));
  |                    ^^^^ - temporary value dropped here while still borrowed
  |                    |
  |                    temporary value does not live long enough
8 |     println!("{:?}", x);
9 | }
  | - temporary value needs to live until here
  |
  = note: consider using a `let` binding to increase its lifetime

We probably want to integrate with the "dump-cause" mode and something like this:

error[E0597]: borrowed value does not live long enough
 --> src/main.rs:7:20
  |
7 |     let x = gimme(&(v,));
  |                    ^^^^ - temporary value only lives until here
  |                    |
  |                    temporary value does not live long enough
 8 |    println!("{}", x);
   |                   ^ reference later used here

[nikomatsakis]

The problem steps from this call to note_and_explain_region:

rust/src/librustc_mir/borrow_check/error_reporting.rs

Lines 527 to 529 in 27a046e

	self.tcx.note_and_explain_region(scope_tree, &mut err,
	"borrowed value must be valid for ",
	borrow.region, "...");

There is another call here:

rust/src/librustc_mir/borrow_check/error_reporting.rs

Lines 505 to 507 in 27a046e

	self.tcx.note_and_explain_region(scope_tree, &mut err,
	"borrowed value must be valid for ",
	borrow.region, "...");

which I guess would be triggered by this test case:

#![feature(nll)]

fn gimme(x: &(u32,)) -> &u32 { &x.0 }

fn main() {
    let x = gimme({ let v = (22,); &v });
    println!("{:?}", x);
}

[nikomatsakis]

In NLL mode, there is already a helper for printing out these messages in NLL mode, it's called explain_why_borrow_containts_point. In fact, when we call note_and_explain_region, we are also calling this method:

rust/src/librustc_mir/borrow_check/error_reporting.rs

Lines 527 to 530 in 27a046e

	self.tcx.note_and_explain_region(scope_tree, &mut err,
	"borrowed value must be valid for ",
	borrow.region, "...");
	self.explain_why_borrow_contains_point(context, borrow, &mut err);

I would say that we almost just want to remove note_and_explain_region altogether -- at least in NLL mode. However, we may also want to expand explain_why_borrow_contains_point. For example, we want to be sure that this example prints something helpful (here, the data must be live for 'a):

#![feature(nll)]
#![allow(warnings)]

fn gimme(x: &(u32,)) -> &u32 { &x.0 }

fn foo<'a>(x: &'a (u32,)) -> &'a u32 {
    let v = 22;
    gimme(&(v,))
}

fn main() {
}

[nikomatsakis]

cc @rust-lang/wg-compiler-nll -- in general, I think we need someone to "take charge" of the "dump-cause" code, which is the code that tries to label the "third use" in NLL, indicating why a borrow lasts for a certain time. This code is currently only enabled when you pass -Znll-dump-cause to the compiler, because the inference is kind of expensive. So there are a few parts to this: this issue mostly about expanding and improving the output, but #46590 is about making it fast.

[nikomatsakis]

I'd be happy to work closely with someone on this! I don't think it's particularly hard, will just take a bit of focus.

[nikomatsakis]

@spastorino has indicated some interest here...

[nikomatsakis]

So I think there are a lot of moving parts. Here is an attempt at making a list:

• If NLL is enabled, do not invoke note_and_explain_region
• Modify -Zdump-nll-cause to not print the overwhelming debug output here. This way we should I believe at least get nice-ish output for our original example.
• Extend explain_why_borrow_contains_point to also work for "universal lifetimes" like the 'a in the example at the end of this comment.
• Figure out how to enable causal information all the time (but that is make causal tracking lazy #46590).