Change:
Swapping the order of the branches compiles fine. cc
Just checked: this fails with a normal closure too, not just a proc.
Compiles fine now but potentially needs a test.

Discussion:
<ins> // Copyright 2014 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. use std::io; use std::vec; pub struct Container<'a> { reader: &'a mut Reader //~ ERROR explicit lifetime bound required } impl<'a> Container<'a> { pub fn wrap<'s>(reader: &'s mut Reader) -> Container<'s> { Container { reader: reader } } pub fn read_to(&mut self, vec: &mut [u8]) { self.reader.read(vec); } } pub fn for_stdin<'a>() -> Container<'a> { let mut r = io::stdin(); Container::wrap(&mut r as &mut Reader) } fn main() { let mut c = for_stdin(); let mut v = vec::Vec::from_elem(10, 0u8); c.read_to(v.as_mut_slice()); } </ins>

<ins> // Copyright 2014 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 blah() -> int { //~ ERROR not all control paths return a value 1i ; //~ NOTE consider removing this semicolon: } fn main() { } </ins>

<ins> // Copyright 2014 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. #![feature(struct_variant)] mod a { pub enum Enum { EnumStructVariant { x: u8, y: u8, z: u8 } } pub fn get_enum_struct_variant() -> () { EnumStructVariant { x: 1, y: 2, z: 3 } //~^ ERROR mismatched types: expected `()`, found `a::Enum` (expected (), found enum a::Enum) } } mod b { mod test { use a; fn test_enum_struct_variant() { let enum_struct_variant = ::a::get_enum_struct_variant(); match enum_struct_variant { a::EnumStructVariant { x, y, z } => { //~^ ERROR error: mismatched types: expected `()`, found a structure pattern } } } } } fn main() {} </ins>

<ins> // Copyright 2014 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 main() { let mut array = [1, 2, 3]; //~^ ERROR cannot determine a type for this local variable: cannot determine the type of this integ let pie_slice = array.slice(1, 2); } </ins>

<ins> // Copyright 2014 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. pub fn foo(params: Option<&[&str]>) -> uint { params.unwrap().head().unwrap().len() } fn main() { let name = "Foo"; let msg = foo(Some(&[name.as_slice()])); //~^ ERROR mismatched types: expected `core::option::Option<&[&str]>` assert_eq!(msg, 3); } </ins>

<ins> // Copyright 2014 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 main() { let v = &[]; //~ ERROR cannot determine a type for this local variable: unconstrained type let it = v.iter(); } </ins>

<ins> // Copyright 2014 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. // error-pattern:explicit failure pub fn main() { fail!(); println!("{}", 1i); } </ins>

<ins> // Copyright 2014 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. // error-pattern:bad input fn main() { Some("foo").unwrap_or(fail!("bad input")).to_string(); } </ins>

<ins> // Copyright 2014 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. pub mod two_tuple { trait T {} struct P<'a>(&'a T + 'a, &'a T + 'a); pub fn f<'a>(car: &'a T, cdr: &'a T) -> P<'a> { P(car, cdr) } } pub mod two_fields { trait T {} struct P<'a> { car: &'a T + 'a, cdr: &'a T + 'a } pub fn f<'a>(car: &'a T, cdr: &'a T) -> P<'a> { P{ car: car, cdr: cdr } } } fn main() {} </ins>

<ins> // Copyright 2014 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 main() { if true { proc(_) {} } else { proc(_: &mut ()) {} }; } </ins>