Segementation fault when calling a method

[ulysseB]

The following code generates a segfault when executed.

trait Foo<T> {
    fn print<'a>(&'a self) where T: 'a { println!("foo"); }
}

impl<'a> Foo<&'a ()> for () { }

trait Bar: for<'a> Foo<&'a ()> { }

impl Bar for () {}

fn main() {
    (&() as &Bar).print(); // Segfault
}

rust version:

rustc 1.16.0-nightly (9761b17d5 2017-01-21)
binary: rustc
commit-hash: 9761b17d558d0c980d4d2d5a0baba54f2bc6b63a
commit-date: 2017-01-21
host: x86_64-unknown-linux-gnu
release: 1.16.0-nightly
LLVM version: 3.9

[jonas-schievink]

Removing where T: 'a makes it work. The binary seems not to include the called method.

[jonas-schievink]

Suspicious code:

rust/src/librustc/traits/mod.rs

Lines 619 to 639 in 1283c02

	// Some methods cannot be called on an object; skip those.
	if !tcx.is_vtable_safe_method(trait_ref.def_id(), &trait_method) {
	debug!("get_vtable_methods: not vtable safe");
	return None;
	}
	// the method may have some early-bound lifetimes, add
	// regions for those
	let substs = Substs::for_item(tcx, def_id,
	|_, _| tcx.mk_region(ty::ReErased),
	|def, _| trait_ref.substs().type_for_def(def));
	// It's possible that the method relies on where clauses that
	// do not hold for this particular set of type parameters.
	// Note that this method could then never be called, so we
	// do not want to try and trans it, in that case (see #23435).
	let predicates = tcx.item_predicates(def_id).instantiate_own(tcx, substs);
	if !normalize_and_test_predicates(tcx, predicates.predicates) {
	debug!("get_vtable_methods: predicates do not hold");
	return None;
	}

(seems to test whether T: 'erased holds to decide if the method can ever be called)

EDIT: 'erased doesn't matter

[nikomatsakis]

@nagisa says this is a regression, as 1.10 (at least) executed this successfully. @eddyb says he made some recent improvements (#34419) might be the problem here, but they don't look that related.

[nikomatsakis]

triage: P-high

Regression.

[arielb1]

This patch fixes the issue:

diff --git a/src/librustc/traits/mod.rs b/src/librustc/traits/mod.rs
index 4893e24091..71d67b106a 100644
--- a/src/librustc/traits/mod.rs
+++ b/src/librustc/traits/mod.rs
@@ -628,6 +628,7 @@ pub fn get_vtable_methods<'a, 'tcx>(
                                           |_, _| tcx.mk_region(ty::ReErased),
                                           |def, _| trait_ref.substs().type_for_def(def));
 
+            let substs = tcx.erase_late_bound_regions_and_normalize(&ty::Binder(substs));
             // It's possible that the method relies on where clauses that
             // do not hold for this particular set of type parameters.
             // Note that this method could then never be called, so we

[nikomatsakis]

Fix in #39887