[NLL] false edges on infinite loops

[nikomatsakis]

The NLL RFC specifies false edges on infinite loops, so that we get errors in some cases. This is a somewhat disputed clause (see nikomatsakis/nll-rfc#35) that we may change, but it makes sense for now. I'm filing this so we don't forget about it.

[nikomatsakis]

So @mikhail-m1 added the notion of "false edges" in #45384. This seems to be roughly what is needed here. We would want to edit the code that generates a loop construct:

rust/src/librustc_mir/build/expr/into.rs

Lines 149 to 204 in 3e49ada

	ExprKind::Loop { condition: opt_cond_expr, body } => {
	// [block] --> [loop_block] ~~> [loop_block_end] -1-> [exit_block]
	// ^ |
	// | 0
	// | |
	// | v
	// [body_block_end] <~~~ [body_block]
	//
	// If `opt_cond_expr` is `None`, then the graph is somewhat simplified:
	//
	// [block] --> [loop_block / body_block ] ~~> [body_block_end] [exit_block]
	// ^ |
	// | |
	// +--------------------------+
	//
	let loop_block = this.cfg.start_new_block();
	let exit_block = this.cfg.start_new_block();
	// start the loop
	this.cfg.terminate(block, source_info,
	TerminatorKind::Goto { target: loop_block });
	this.in_breakable_scope(
	Some(loop_block), exit_block, destination.clone(),
	move |this| {
	// conduct the test, if necessary
	let body_block;
	if let Some(cond_expr) = opt_cond_expr {
	let loop_block_end;
	let cond = unpack!(
	loop_block_end = this.as_local_operand(loop_block, cond_expr));
	body_block = this.cfg.start_new_block();
	let term = TerminatorKind::if_(this.hir.tcx(), cond,
	body_block, exit_block);
	this.cfg.terminate(loop_block_end, source_info, term);
	// if the test is false, there's no `break` to assign `destination`, so
	// we have to do it; this overwrites any `break`-assigned value but it's
	// always `()` anyway
	this.cfg.push_assign_unit(exit_block, source_info, destination);
	} else {
	body_block = loop_block;
	}
	// The “return” value of the loop body must always be an unit. We therefore
	// introduce a unit temporary as the destination for the loop body.
	let tmp = this.get_unit_temp();
	// Execute the body, branching back to the test.
	let body_block_end = unpack!(this.into(&tmp, body_block, body));
	this.cfg.terminate(body_block_end, source_info,
	TerminatorKind::Goto { target: loop_block });
	}
	);
	exit_block.unit()
	}

and modify the graph in the case where condition is None:

rust/src/librustc_mir/build/expr/into.rs

Line 191 in 3e49ada

body_block = loop_block;

so that we create a FalseEdges that "breaks out".

[nikomatsakis]

Added some mentoring instructions to this NLL issue. Can elaborate as needed!

cc nll folks:

@davidtwco
@chrisvittal
@pnkfelix
@spastorino
@Nashenas88
@gaurikholkar
@qmx
@Pulkit07
@ritiek

[Pulkit07]

I am trying to fix this one. I have already a kind of patch ready, need to dump MIR and see if it's doing the right thing.

[nikomatsakis]

@Pulkit07 nice :)

[Pulkit07]

This might take some time, I getting core dumped error, I need to learn about what's happening before moving things here and there. If this is required to be fixed urgently, I am good with someone else fixing it.

[nikomatsakis]

@Pulkit07 woah, core dumped?! When do you get the error, when bootstrapping?

[Pulkit07]

@nikomatsakis I am getting this error when running tests like src/test/run-pass/auto-loop.rs, src/test/run-pass/break.rs and many others.

[nikomatsakis]

@Pulkit07 even if you don't have time to continue working, it'd be helpful if you could post the diff somewhere of how far you got

[nikomatsakis]

From gitter, here is the diff https://bpaste.net/show/ac4eab0ead8e