Segfault related to trait bounds

[novalis]

I am saying "related to trait bounds" because I have managed to get this in a few different ways, all of which involved trait bounds. But since I don't fully understand how traits work yet, I'm not really sure what is going on. Here's a few examples:

This one spits out a zillion of these before dying:
segme.rs:14:0: 14:17 error: expected Op<S,S>, but found Op<int,S> (expected struct S but found int)
segme.rs:14 impl Bar for S {}

struct S {
    a: u64
}

trait Op<RHS,Result> {
    fn foo(&self, rhs: &RHS) -> Result;
}

pub trait Bar: Op<S,Self> {
}

impl Bar for S {}

impl<T:Bar> Op<int, T> for T {
    fn foo(&self, a: &int) -> T {
        //yes, I know this doesn't make sense
        return 0;
    }
}

fn main() {}

This one just segfaults:

trait Shape {
    fn n_sides(&self) -> uint;
}

trait Quadrilateral : Shape {
    fn sides(&self) -> ~[float, ..4];
}

impl<T:Quadrilateral> Shape for T {
    fn n_sides(&self) -> uint {
        return 4;
    }
}

trait Rectangle : Quadrilateral {
    //only rectangles have equal-sided diagonals
    fn diagonal(&self) -> float {
        let sides = self.sides();
        return (sides[0] * sides[0] + sides[1] * sides[1]).sqrt();
    }
}

struct SquareImpl {
    side : float
}

impl Quadrilateral for SquareImpl {
    fn sides(&self) -> ~[float, ..4] {
        let sides : ~[float, ..4] = ~([self.side, self.side, self.side, self.side]);
        return sides;
    }
}

impl Rectangle for SquareImpl {
    //Rectangle's diagonal is implemented by Rectangle
}

fn main() {}

[novalis]

Note that the segfault is in a different place than #8762:

#0  0x00007ffff62b8d51 in middle::typeck::infer::lattice::__extensions__::set_var_to_merged_bounds_63789::_294ad465868072::_0$x2e8$x2dpre () from /usr/local/bin/../lib/librustc-d3cb8c2ccd84a7a7-0.8-pre.so
#1  0x00007ffff62ae412 in middle::typeck::infer::sub::__extensions__::meth_63531::tys::_5bcb7e7ec8e26d49::_0$x2e8$x2dpre ()
   from /usr/local/bin/../lib/librustc-d3cb8c2ccd84a7a7-0.8-pre.so
#2  0x00007ffff62aeb11 in middle::typeck::infer::sub::__extensions__::meth_63538::contratys::_5bcb7e7ec8e26d49::_0$x2e8$x2dpre ()
   from /usr/local/bin/../lib/librustc-d3cb8c2ccd84a7a7-0.8-pre.so
#3  0x00007ffff632f1c0 in middle::typeck::infer::combine::Combine::anon::expr_fn_67001 ()
   from /usr/local/bin/../lib/librustc-d3cb8c2ccd84a7a7-0.8-pre.so
#4  0x00007ffff62c59bb in result::__extensions__::chain_64119::_92c2298aed66c6a::_0$x2e8$x2dpre ()
   from /usr/local/bin/../lib/librustc-d3cb8c2ccd84a7a7-0.8-pre.so
#5  0x00007ffff632f043 in middle::typeck::infer::combine::Combine::substs_66993::_ef3a55d95551ab4::_0$x2e8$x2dpre ()
   from /usr/local/bin/../lib/librustc-d3cb8c2ccd84a7a7-0.8-pre.so
#6  0x00007ffff634d8d0 in middle::typeck::infer::mk_sub_trait_refs::anon::anon::expr_fn_67613 ()
   from /usr/local/bin/../lib/librustc-d3cb8c2ccd84a7a7-0.8-pre.so
#7  0x00007ffff634d2b4 in middle::typeck::infer::__extensions__::try_67607::anon::expr_fn_67609 ()
   from /usr/local/bin/../lib/librustc-d3cb8c2ccd84a7a7-0.8-pre.so
#8  0x00007ffff634bfef in util::common::indent_67575::_5977847d83168476::_0$x2e8$x2dpre ()
   from /usr/local/bin/../lib/librustc-d3cb8c2ccd84a7a7-0.8-pre.so
#9  0x00007ffff634cb16 in middle::typeck::infer::__extensions__::commit_67603::anon::expr_fn_67605 ()
   from /usr/local/bin/../lib/librustc-d3cb8c2ccd84a7a7-0.8-pre.so
#10 0x00007ffff634bfef in util::common::indent_67575::_5977847d83168476::_0$x2e8$x2dpre ()
   from /usr/local/bin/../lib/librustc-d3cb8c2ccd84a7a7-0.8-pre.so
#11 0x00007ffff634c841 in middle::typeck::infer::mk_sub_trait_refs::anon::expr_fn_67601 ()
   from /usr/local/bin/../lib/librustc-d3cb8c2ccd84a7a7-0.8-pre.so
#12 0x00007ffff634bfef in util::common::indent_67575::_5977847d83168476::_0$x2e8$x2dpre ()
   from /usr/local/bin/../lib/librustc-d3cb8c2ccd84a7a7-0.8-pre.so
#13 0x00007ffff61e426d in middle::typeck::infer::mk_sub_trait_refs::_f7ea5d6609058ad::_0$x2e8$x2dpre ()
   from /usr/local/bin/../lib/librustc-d3cb8c2ccd84a7a7-0.8-pre.so
#14 0x00007ffff61e35c1 in middle::typeck::check::vtable::relate_trait_refs::_5b7aa9d4975c2048::_0$x2e8$x2dpre ()
   from /usr/local/bin/../lib/librustc-d3cb8c2ccd84a7a7-0.8-pre.so
#15 0x00007ffff61e5caf in middle::typeck::check::vtable::search_for_vtable::_ce718d4bacc81570::_0$x2e8$x2dpre ()
   from /usr/local/bin/../lib/librustc-d3cb8c2ccd84a7a7-0.8-pre.so
#16 0x00007ffff61e2c84 in middle::typeck::check::vtable::lookup_vtable::_ce718d4bacc81570::_0$x2e8$x2dpre ()
   from /usr/local/bin/../lib/librustc-d3cb8c2ccd84a7a7-0.8-pre.so
#17 0x00007ffff61e1faf in middle::typeck::check::vtable::lookup_vtables_for_param::anon::expr_fn_57896 ()
   from /usr/local/bin/../lib/librustc-d3cb8c2ccd84a7a7-0.8-pre.so
#18 0x00007ffff6111cdc in middle::ty::each_bound_trait_and_supertraits::_fa9077d6aceae27b::_0$x2e8$x2dpre ()
   from /usr/local/bin/../lib/librustc-d3cb8c2ccd84a7a7-0.8-pre.so
#19 0x00007ffff61e14f1 in middle::typeck::check::vtable::lookup_vtables_for_param::_e53c5d2999aba97::_0$x2e8$x2dpre ()
   from /usr/local/bin/../lib/librustc-d3cb8c2ccd84a7a7-0.8-pre.so
#20 0x00007ffff61dfa6d in middle::typeck::check::vtable::lookup_vtables::_419eb61d9f602274::_0$x2e8$x2dpre ()
   from /usr/local/bin/../lib/librustc-d3cb8c2ccd84a7a7-0.8-pre.so
#21 0x00007ffff61e5d63 in middle::typeck::check::vtable::search_for_vtable::_ce718d4bacc81570::_0$x2e8$x2dpre ()
   from /usr/local/bin/../lib/librustc-d3cb8c2ccd84a7a7-0.8-pre.so
#22 0x00007ffff61e2c84 in middle::typeck::check::vtable::lookup_vtable::_ce718d4bacc81570::_0$x2e8$x2dpre ()
   from /usr/local/bin/../lib/librustc-d3cb8c2ccd84a7a7-0.8-pre.so
#23 0x00007ffff61e1faf in middle::typeck::check::vtable::lookup_vtables_for_param::anon::expr_fn_57896 ()
   from /usr/local/bin/../lib/librustc-d3cb8c2ccd84a7a7-0.8-pre.so
#24 0x00007ffff6111cdc in middle::ty::each_bound_trait_and_supertraits::_fa9077d6aceae27b::_0$x2e8$x2dpre ()
   from /usr/local/bin/../lib/librustc-d3cb8c2ccd84a7a7-0.8-pre.so
#25 0x00007ffff61e14f1 in middle::typeck::check::vtable::lookup_vtables_for_param::_e53c5d2999aba97::_0$x2e8$x2dpre ()
   from /usr/local/bin/../lib/librustc-d3cb8c2ccd84a7a7-0.8-pre.so
#26 0x00007ffff61dfa6d in middle::typeck::check::vtable::lookup_vtables::_419eb61d9f602274::_0$x2e8$x2dpre ()
   from /usr/local/bin/../lib/librustc-d3cb8c2ccd84a7a7-0.8-pre.so
...

(and so on, for a zillion frames)

[novalis]

I just realized that the following is bogus (since I won't be able to impl any other Shape):

impl<T:Quadrilateral> Shape for T {
    fn n_sides(&self) -> uint {
        return 4;
    }
}

(but it's still bad that the compiler crashes, of course)

[pzol]

Updated example - s/float/f32/

Compiler error message task 'rustc' has overflowed its stack

trait Shape {
    fn n_sides(&self) -> uint;
}

trait Quadrilateral : Shape {
    fn sides(&self) -> ~[f32, ..4];
}

impl<T:Quadrilateral> Shape for T {
    fn n_sides(&self) -> uint {
        return 4;
    }
}

trait Rectangle : Quadrilateral {
    //only rectangles have equal-sided diagonals
    fn diagonal(&self) -> f32 {
        let sides = self.sides();
        return (sides[0] * sides[0] + sides[1] * sides[1]).sqrt();
    }
}

struct SquareImpl {
    side : f32
}

impl Quadrilateral for SquareImpl {
    fn sides(&self) -> ~[f32, ..4] {
        let sides : ~[f32, ..4] = ~([self.side, self.side, self.side, self.side]);
        return sides;
    }
}

impl Rectangle for SquareImpl {
    //Rectangle's diagonal is implemented by Rectangle
}

fn main() {}

[alexcrichton]

This issue has since been fixed, and I imagine it was probably due to some random change in the internals over the years, so we probably don't need a test for this :)