Change:
Minimised example: Both the and the fail in the same way, with an ICE: Adding bounds to and on the trait or in the fn’s where clause doesn’t fix it, but changing to does fix it. (In the particular case that I was dealing with, the lifetime could_ be changed to where this was a problem, but it would only have taken a slight variation of it for me to be in a situation where I needed a non- implementation. There were, of course, other arguments which were using the .) The whole thing around there seems a bit fragile and I’m not quite sure what’s supposed to work and what’s not; if we just take an and transmute it to , for example, we get “error: cannot transmute to or from a type that contains type parameters in its interior [E0139]”. Anyway, an ICE is always a bug, so here’s the report!
No longer repros. compiles now. Fails with: But no ICE.

Discussion:
<ins> // Copyright 2015 The Rust Project Developers. See the COPYRIGHT // file at the top-level directory of this distribution and at // http://rust-lang.org/COPYRIGHT. // // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your // option. This file may not be copied, modified, or distributed // except according to those terms. fn f<F, R>(_: F) where F: Fn() -> R { } fn main() { f(|| -> ! { () }); //~^ ERROR: computation may converge in a function marked as diverging [E0270] } </ins>

<ins> // Copyright 2015 The Rust Project Developers. See the COPYRIGHT // file at the top-level directory of this distribution and at // http://rust-lang.org/COPYRIGHT. // // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your // option. This file may not be copied, modified, or distributed // except according to those terms. fn foo(x: i32) { |y| x + y //~^ ERROR: mismatched types: //~| expected `()`, //~|     found closure //~| (expected (), //~|     found closure) [E0308] } fn main() { let x = foo(5)(2); //~^ ERROR: expected function, found `()` } </ins>

<ins> // Copyright 2015 The Rust Project Developers. See the COPYRIGHT // file at the top-level directory of this distribution and at // http://rust-lang.org/COPYRIGHT. // // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your // option. This file may not be copied, modified, or distributed // except according to those terms. trait Trait<'a> { type A; type B; } fn foo<'a, T: Trait<'a>>(value: T::A) { let new: T::B = unsafe { std::mem::transmute(value) }; //~^ ERROR: cannot transmute to or from a type that contains type parameters in its interior [E0139] } fn main() { } </ins>

<ins> // Copyright 2015 The Rust Project Developers. See the COPYRIGHT // file at the top-level directory of this distribution and at // http://rust-lang.org/COPYRIGHT. // // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your // option. This file may not be copied, modified, or distributed // except according to those terms. struct GradFn<F: Fn() -> usize>(F); fn main() { let GradFn(x_squared) : GradFn<_> = GradFn(|| -> usize { 2 }); let _  = x_squared(); } </ins>

<ins> // Copyright 2015 The Rust Project Developers. See the COPYRIGHT // file at the top-level directory of this distribution and at // http://rust-lang.org/COPYRIGHT. // // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your // option. This file may not be copied, modified, or distributed // except according to those terms. struct Foo; impl Foo { fn putc(&self, b: u8) { } fn puts(&self, s: &str) { for byte in s.bytes() { self.putc(byte) } } } fn main() {} </ins>

<ins> // Copyright 2015 The Rust Project Developers. See the COPYRIGHT // file at the top-level directory of this distribution and at // http://rust-lang.org/COPYRIGHT. // // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your // option. This file may not be copied, modified, or distributed // except according to those terms. trait Trait<'a> { type A; type B; } fn foo<'a, T: Trait<'a>>(value: T::A) { let new: T::B = unsafe { std::mem::transmute_copy(&value) }; } fn main() { } </ins>