Implement Diagnostic for Infallible

[caass]

I've been working with some code that looks like this:

trait DiagnosticTryFrom<Input>: Sized {
  type Error: Diagnostic;

  fn try_from_diagnostic(input: Input) -> Result<Self, Self::Error>;
}

And so logically it makes sense to create a blanket implementation on top of TryInto:

impl <I, O> DiagnosticTryFrom<I> for O where O: TryFrom<I>, O::Error: Diagnostic {
  type Error = O::Error;

  fn try_from_diagnostic(input: I) -> Result<Self, Self::Error> {
    O::try_from(input)
  }
}

Which works very well for my own TryFrom implementations -- but not so well when there's a From implementation available. Every From implementation gets a blanket implementation of TryFrom with an error type of Infallible, so I'm not able to implement Diagnostic for that error type due to the orphan rule.

This PR implements Diagnostic for Infallible, so any API relying on TryFrom<T, Error: Diagnostic> can also use From<T> implementations.

[Porges]

FWIW the unsafe is not really needed here, using an empty match match *self {} should (all of the time? most of the time?) result in the same output.

Comparing using the "Show Assembly" feature in Rust Playground:

unreachable	unsafe unreachable	match
use std::convert::Infallible; trait Diagnostic { fn get_a_thing(&self) -> Option<String>; } impl Diagnostic for Infallible { fn get_a_thing(&self) -> Option<String> { unreachable!() } } #[inline(never)] pub fn do_the_thing(x: Infallible) -> Option<String> { x.get_a_thing() }	use std::convert::Infallible; trait Diagnostic { fn get_a_thing(&self) -> Option<String>; } impl Diagnostic for Infallible { fn get_a_thing(&self) -> Option<String> { unsafe { std::hint::unreachable_unchecked() } } } #[inline(never)] pub fn do_the_thing(x: Infallible) -> Option<String> { x.get_a_thing() }	use std::convert::Infallible; trait Diagnostic { fn get_a_thing(&self) -> Option<String>; } impl Diagnostic for Infallible { fn get_a_thing(&self) -> Option<String> { match *self {} } } #[inline(never)] pub fn do_the_thing(x: Infallible) -> Option<String> { x.get_a_thing() }
<core::convert::Infallible as playground::Diagnostic>::get_a_thing: pushq %rax leaq .L__unnamed_1(%rip), %rdi leaq .L__unnamed_2(%rip), %rdx movl $40, %esi callq *core::panicking::panic@GOTPCREL(%rip) playground::do_the_thing: pushq %rax leaq .L__unnamed_1(%rip), %rdi leaq .L__unnamed_2(%rip), %rdx movl $40, %esi callq *core::panicking::panic@GOTPCREL(%rip) .L__unnamed_1: .ascii "internal error: entered unreachable code" .L__unnamed_3: .ascii "src/lib.rs" .L__unnamed_2: .quad .L__unnamed_3 .asciz "\n\000\000\000\000\000\000\000\t\000\000\000\t\000\000"	playground::do_the_thing: ud2	playground::do_the_thing: ud2