Closed
Description
openedon Jan 21, 2020
Consider this code:
#![feature(transparent_enums)]
use std::mem::size_of;
fn main() {
#[repr(transparent)]
enum VisibleE8 {
A(u8), B(u8), C(u8), D(u8), E(u8), F(u8), G(u8), H(u8), I(u8), J(u8),
K(u8), L(u8), M(u8), N(u8), O(u8), P(u8), Q(u8), R(u8), S(u8), T(u8),
}
println!("VisibleE8: {}", size_of::<VisibleE8>());
}When you try to compile this, you get two error diagnostics:
error[E0731]: transparent enum needs exactly one variant, but has 20
--> src/main.rs:7:5
|
7 | enum VisibleE8 {
| ^^^^^^^^^^^^^^ needs exactly one variant, but has 20
8 | A(u8), B(u8), C(u8), D(u8), E(u8), F(u8), G(u8), H(u8), I(u8), J(u8),
| ----- ----- ----- ----- ----- ----- ----- ----- ----- -----
9 | K(u8), L(u8), M(u8), N(u8), O(u8), P(u8), Q(u8), R(u8), S(u8), T(u8),
| ----- ----- ----- ----- ----- ----- ----- ----- ----- ----- too many variants in `main::VisibleE8`
error[E0690]: the variant of a transparent enum needs exactly one non-zero-sized field, but has 20
--> src/main.rs:7:5
|
7 | enum VisibleE8 {
| ^^^^^^^^^^^^^^ needs exactly one non-zero-sized field, but has 20
8 | A(u8), B(u8), C(u8), D(u8), E(u8), F(u8), G(u8), H(u8), I(u8), J(u8),
| -- -- -- -- -- -- -- -- -- -- this field is non-zero-sized
| | | | | | | | | |
| | | | | | | | | this field is non-zero-sized
| | | | | | | | this field is non-zero-sized
| | | | | | | this field is non-zero-sized
| | | | | | this field is non-zero-sized
| | | | | this field is non-zero-sized
| | | | this field is non-zero-sized
| | | this field is non-zero-sized
| | this field is non-zero-sized
| this field is non-zero-sized
9 | K(u8), L(u8), M(u8), N(u8), O(u8), P(u8), Q(u8), R(u8), S(u8), T(u8),
| -- -- -- -- -- -- -- -- -- -- this field is non-zero-sized
| | | | | | | | | |
| | | | | | | | | this field is non-zero-sized
| | | | | | | | this field is non-zero-sized
| | | | | | | this field is non-zero-sized
| | | | | | this field is non-zero-sized
| | | | | this field is non-zero-sized
| | | | this field is non-zero-sized
| | | this field is non-zero-sized
| | this field is non-zero-sized
| this field is non-zero-sized
error: aborting due to 2 previous errors
The first diagnostic is exactly describing the problem.
The second diagnostic is irrelevant, and arguably confusing (at least in my mental model of enums, the field count of one variant should be considered independently of the counts for all of the enum's other variants).
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