1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
|
//@ run-pass
#![allow(unused_must_use)]
#![allow(dead_code)]
#![allow(unused_imports)]
use std::fmt;
use std::io::prelude::*;
use std::io::Cursor;
use std::slice;
use std::marker::PhantomData;
trait Encoder {
type Error;
}
trait Encodable<S: Encoder> {
fn encode(&self, s: &mut S) -> Result<(), S::Error>;
}
struct JsonEncoder<'a>(PhantomData<&'a mut ()>);
impl Encoder for JsonEncoder<'_> {
type Error = ();
}
struct AsJson<'a, T> {
inner: &'a T,
}
impl<'a, T: for<'r> Encodable<JsonEncoder<'r>>> fmt::Display for AsJson<'a, T> {
/// Encodes a json value into a string
fn fmt(&self, _f: &mut fmt::Formatter<'_>) -> fmt::Result {
Ok(())
}
}
fn as_json<T>(t: &T) -> AsJson<'_, T> {
AsJson { inner: t }
}
struct OpaqueEncoder(Vec<u8>);
impl Encoder for OpaqueEncoder {
type Error = ();
}
struct Foo {
baz: bool,
}
impl<S: Encoder> Encodable<S> for Foo {
fn encode(&self, _s: &mut S) -> Result<(), S::Error> {
Ok(())
}
}
struct Bar {
froboz: usize,
}
impl<S: Encoder> Encodable<S> for Bar {
fn encode(&self, _s: &mut S) -> Result<(), S::Error> {
Ok(())
}
}
enum WireProtocol {
JSON,
Opaque,
// ...
}
fn encode_json<T: for<'a> Encodable<JsonEncoder<'a>>>(val: &T, wr: &mut Cursor<Vec<u8>>) {
write!(wr, "{}", as_json(val));
}
fn encode_opaque<T: Encodable<OpaqueEncoder>>(val: &T, wr: Vec<u8>) {
let mut encoder = OpaqueEncoder(wr);
val.encode(&mut encoder);
}
pub fn main() {
let target = Foo { baz: false };
let proto = WireProtocol::JSON;
match proto {
WireProtocol::JSON => encode_json(&target, &mut Cursor::new(Vec::new())),
WireProtocol::Opaque => encode_opaque(&target, Vec::new()),
}
}
|
