Update question 5 for universe transition

questions/005-trait-resolution-hrtb.md

@@ -1,32 +1,43 @@
-Answer: 112
-Difficulty: 2
+Answer: error
+Difficulty: 3
 
 # Hint
 
-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.
-
-[hrtb]: https://doc.rust-lang.org/nomicon/hrtb.html
+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+.
 
 # Explanation
 
+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.
+
 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] type `for<'a> fn(&'a T)` for some concrete type `T` that
+outlives `'a`.
+
+[higher-ranked]: https://doc.rust-lang.org/nomicon/hrtb.html
 
 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 closure `a` we infer `_ = u8`, yielding the closure type `fn(u8)` taking
-an argument of type `u8` and returning `()`.
+For the function pointer `a` we infer `_ = u8`, yielding the function pointer
+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 closure type
-`for<'a> fn(&'a u8)`.
+And finally for `c` we infer `_ = u8`, yielding the higher-ranked function
+pointer 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

questions/005-trait-resolution-hrtb.rs

@@ -1,24 +1,27 @@
 trait Trait {
-    fn f(self);
+    fn p(self);
 }
 
 impl<T> Trait for fn(T) {
-    fn f(self) {
+    fn p(self) {
         print!("1");
     }
 }
 
 impl<T> Trait for fn(&T) {
-    fn f(self) {
+    fn p(self) {
         print!("2");
     }
 }
 
+fn f(_: u8) {}
+fn g(_: &u8) {}
+
 fn main() {
-    let a: fn(_) = |_: u8| {};
-    let b: fn(_) = |_: &u8| {};
-    let c: fn(&_) = |_: &u8| {};
-    a.f();
-    b.f();
-    c.f();
+    let a: fn(_) = f;
+    let b: fn(_) = g;
+    let c: fn(&_) = g;
+    a.p();
+    b.p();
+    c.p();
 }