NLL migration mode is accepting code that both NLL and AST-borrowck reject!

[pnkfelix]

D'oh: while investigating #52979, I found a case where we are downgrading the NLL errors to warnings (due to the migration mode added in #52681), but AST-borrowck issues an error for the test! (This may explain #53004, at least in part...)

Namely, ui/borrowck/issue-45983.rs. Here is the test:

rust/src/test/ui/borrowck/issue-45983.rs

Lines 11 to 19 in e415b5e

	fn give_any<F: for<'r> FnOnce(&'r ())>(f: F) {
	f(&());
	}
	fn main() {
	let x = None;
	give_any(|y| x = Some(y));
	//~^ ERROR borrowed data cannot be stored outside of its closure
	}

Here is the AST-borrowck output (a hard error):

rust/src/test/ui/borrowck/issue-45983.stderr

Lines 1 to 11 in e415b5e

	error: borrowed data cannot be stored outside of its closure
	--> $DIR/issue-45983.rs:17:27
	|
	LL | let x = None;
	| - borrowed data cannot be stored into here...
	LL | give_any(|y| x = Some(y));
	| --- ^ cannot be stored outside of its closure
	| |
	| ...because it cannot outlive this closure
	error: aborting due to previous error

And here is the NLL output (another hard error):

rust/src/test/ui/borrowck/issue-45983.nll.stderr

Lines 1 to 25 in e415b5e

	warning: not reporting region error due to nll
	--> $DIR/issue-45983.rs:17:27
	|
	LL | give_any(|y| x = Some(y));
	| ^
	error: borrowed data escapes outside of closure
	--> $DIR/issue-45983.rs:17:18
	|
	LL | let x = None;
	| - `x` is declared here, outside of the closure body
	LL | give_any(|y| x = Some(y));
	| - ^^^^^^^^^^^ `y` escapes the closure body here
	| |
	| `y` is a reference that is only valid in the closure body
	error[E0594]: cannot assign to `x`, as it is not declared as mutable
	--> $DIR/issue-45983.rs:17:18
	|
	LL | let x = None;
	| - help: consider changing this to be mutable: `mut x`
	LL | give_any(|y| x = Some(y));
	| ^^^^^^^^^^^ cannot assign
	error: aborting due to 2 previous errors

But here is the output from -Z borrowck=migrate -Z two-phase-borrows:

warning: not reporting region error due to nll
  --> ../src/test/ui/borrowck/issue-45983.rs:17:27
   |
17 |     give_any(|y| x = Some(y));
   |                           ^

warning: borrowed data escapes outside of closure
  --> ../src/test/ui/borrowck/issue-45983.rs:17:18
   |
16 |     let x = None;
   |         - `x` is declared here, outside of the closure body
17 |     give_any(|y| x = Some(y));
   |               -  ^^^^^^^^^^^ `y` escapes the closure body here
   |               |
   |               `y` is a reference that is only valid in the closure body
   |
   = warning: This error has been downgraded to a warning for backwards compatibility with previous releases.
           It represents potential unsoundness in your code.
           This warning will become a hard error in the future.

warning[E0594]: cannot assign to `x`, as it is not declared as mutable
  --> ../src/test/ui/borrowck/issue-45983.rs:17:18
   |
16 |     let x = None;
   |         - help: consider changing this to be mutable: `mut x`
17 |     give_any(|y| x = Some(y));
   |                  ^^^^^^^^^^^ cannot assign
   |
   = warning: This error has been downgraded to a warning for backwards compatibility with previous releases.
           It represents potential unsoundness in your code.
           This warning will become a hard error in the future.

ui/borrowck/issue-45983.rs is one particular instance of this, but now that I've found one, I'm certain there are more.

This is pretty bad. Very high priority to fix.

[matthewjasper]

This code looks suspicious:

rust/src/librustc/infer/error_reporting/mod.rs

Lines 306 to 312 in 59fa6bd

	if will_later_be_reported_by_nll && self.tcx.use_mir_borrowck() {
	// With `#![feature(nll)]`, we want to present a nice user
	// experience, so don't even mention the errors from the
	// AST checker.
	if self.tcx.features().nll {
	return;
	}

[pnkfelix]

Ha I should have just left this to @matthewjasper to fix, since we seem to have independently identified the culprit.

[matthewjasper]

I think I've already assigned myself to enough issues.