Revert "Update question 5 for universe transition"

The breaking change was reverted in rust-lang/rust#58592.

This reverts commit 6fd0e82.

Author: dtolnay

questions/005-trait-resolution-hrtb.md

@@ -1,43 +1,32 @@
-Answer: error
-Difficulty: 3
+Answer: 112
+Difficulty: 2
 
 # Hint
 
-The answer is different for Rust versions 1.0 through 1.32 vs 1.33+. The answer
-accepted as correct here is the one for compilers 1.33+.
+If you are familiar with [higher-rank trait bound][hrtb] syntax, try desugaring
+all the types in the impl signatures and types in `main` into their fully
+explicit form.
 
-# Explanation
+[hrtb]: https://doc.rust-lang.org/nomicon/hrtb.html
 
-This is a rare example of a Rust program that *used to* compile. This code
-compiles and runs successfully with every Rust version 1.0 through 1.32,
-printing the output `112`. The reasoning on those compilers is as follows.
+# Explanation
 
 The first impl applies to function pointers of type `fn(T)` where `T` is any
 single concrete type. The second impl applies to function pointers of
-[higher-ranked] type `for<'a> fn(&'a T)` for some concrete type `T` that
-outlives `'a`.
-
-[higher-ranked]: https://doc.rust-lang.org/nomicon/hrtb.html
+higher-ranked type `for<'a> fn(&'a T)` for some concrete type `T` that outlives
+`'a`.
 
 Inside of `main`, the compiler is going to use type inference to substitute all
 occurrences of `_` in a type by some concrete type.
 
-For the function pointer `a` we infer `_ = u8`, yielding the function pointer
-type `fn(u8)` taking an argument of type `u8` and returning `()`.
+For the closure `a` we infer `_ = u8`, yielding the closure type `fn(u8)` taking
+an argument of type `u8` and returning `()`.
 
 For `b` we infer `_ = &'x u8` for some concrete lifetime `'x` that will
 ultimately feed into the borrow checker. The type of `b` is `fn(&'x u8)`.
 
-And finally for `c` we infer `_ = u8`, yielding the higher-ranked function
-pointer type `for<'a> fn(&'a u8)`.
+And finally for `c` we infer `_ = u8`, yielding the higher-ranked closure type
+`for<'a> fn(&'a u8)`.
 
 Framed in this way, it follows that the trait method calls at the end of `main`
 print `112`.
-
-The compiler's reasoning changed in Rust version 1.33 as part of the ["universe
-transition"] and this program no longer compiles. Under the new model the first
-impl applies to all three function pointers. If the second impl didn't exist,
-the program would compile and print `111`. But with both impls present these are
-considered conflicting impls and the program fails to compile.
-
-["universe transition"]: https://github.com/rust-lang/rust/issues/56105