Add a question about enum dicriminants

code/examples/misc_10.rs

@@ -0,0 +1,16 @@
+#[repr(u32)]
+enum A {
+    Zero,
+    One(),
+    Two {},
+}
+
+fn main() {
+    println!("{}", A::Zero as usize);
+    println!("{}", A::One as usize);
+    println!("{}", A::Two as usize);
+
+    println!("{}", A::Zero {} as usize);
+    println!("{}", A::One {} as usize);
+    println!("{}", A::Two {} as usize);
+}

code/examples/stderr/misc_10.stderr

@@ -0,0 +1,13 @@
+error[E0533]: expected value, found struct variant `A::Two`
+  --> examples/misc_10.rs:11:20
+   |
+11 |     println!("{}", A::Two as usize);
+   |                    ^^^^^^ not a value
+   |
+help: you might have meant to create a new value of the struct
+   |
+11 |     println!("{}", A::Two {} as usize);
+   |                           ++
+
+For more information about this error, try `rustc --explain E0533`.
+error: could not compile `code` (example "misc_10") due to 1 previous error

src/misc/10.md

@@ -0,0 +1,38 @@
+# Misc 10 @m-ou-se @WaffleLapkin
+
+{{#include ../include/quiz-is-wip.md}}
+
+```rust
+{{#include ../../code/examples/misc_10.rs}}
+```
+
+For each `println!`, say what it will print or that it won't compile.
+
+<details>
+<summary>Solution</summary>
+
+Only `A::Two as usize` doesn't compile:
+
+```
+{{#include ../../code/examples/stderr/misc_10.stderr}}
+```
+
+Subtituting the non-compiling line with a dummy print will output something like the following:
+```
+0
+98926091234224
+doesn't compile
+0
+1
+2
+```
+
+There are multiple potentially suprising things:
+
+1. You can `as`-cast an enum with non-unit variants to get its discriminant, [as long as it's fieldless](https://doc.rust-lang.org/reference/items/enumerations.html#r-items.enum.discriminant.coercion.fieldless).
+
+2. `A::One as usize` gives you the *address of the constructor function of `A::One`*, and not `A::One`'s discriminant (the latter can be gotten via `A::One() as usize`).
+
+3. Unit and tuple variants can be constructed with `{}` syntax. This is similar to how unit and tuple structures can also be created with `{}`.
+
+</details>