重构模块从编译器创建不同类型的请求?

Question

为什么在重构此示例代码以在调整大小图像库周围使用包装器时，我会收到不同类型的编译器请求。

基本上调整大小似乎总是u16输出，那么为什么我的更改要求u8原始代码从未需要此转换？

extern crate resize;
use resize::Type;
use resize::Pixel;
pub struct Resize{
w1:usize,
h1:usize,
w2:usize,
h2:usize,
src: Vec<u16>,
}
impl Resize {
pub fn new(
ow:usize, 
oh:usize, 
dw:usize, 
dh:usize,
inc: Vec<u16>
) -> Resize {
Resize {
w1:ow,
h1:oh,
w2:dw,
h2:dh,
src:inc,
}
}
pub fn run(&self) -> Vec<u16> {
let mut dst = vec![0;self.w2*self.h2];
resize::resize(
self.w1, 
self.h1, 
self.w2, 
self.h2, 
Pixel::RGBA64, 
Type::Lanczos3, 
self.src.as_slice(), 
&mut dst,
);
dst
}
}

编译器错误消息

error[E0308]: mismatched types
--> src/main.rs:24:45
|
24 |     let image = Resize::new(w1, h1, w2, h2, src);
|                                             ^^^ expected u16, found u8
|
= note: expected type `std::vec::Vec<u16>`
found type `std::vec::Vec<u8>`
= help: here are some functions which might fulfill your needs:
- .to_vec()
error[E0308]: mismatched types
--> src/main.rs:30:54
|
30 |     encoder.write_header().unwrap().write_image_data(dst).unwrap();
|                                                      ^^^ expected u8, found u16
|
= note: expected type `&[u8]`
found type `&[u16]`

Answer 1

TL;DR：src变量的预期类型是Vec<u16>，但由于类型推断，它被设置为Vec<u8>。

---

变量src在行中分配：

let mut src = vec![0;info.buffer_size()];

它稍后通过调用以下命令推断类型Vec<u8>：

reader.next_frame(&mut src).unwrap();

或

resize::resize(w1, h1, w2, h2, Gray8, Triangle, &src, &mut dst);

因为这些函数需要类型&mut [u8]，而不是&mut [u16]。

如果我们在分配src时显式设置预期的类型Vec<u16>：

let mut src: Vec<u16> = vec![0; info.buffer_size()];

现在调用next_frame()和resize()将失败，因为类型不匹配，但编译器会给我们更好的提示，说明出了什么问题以及我们可以做些什么。

error[E0308]: mismatched types
--> src/main.rs:65:23
|
65 |     reader.next_frame(&mut src).unwrap();
|                       ^^^^^^^^ expected slice, found struct `std::vec::Vec`
|
= note: expected type `&mut [u8]`
found type `&mut std::vec::Vec<u16>`
= help: here are some functions which might fulfill your needs:
- .as_mut_slice()
error[E0308]: mismatched types
--> src/main.rs:74:53
|
74 |     resize::resize(w1, h1, w2, h2, Gray8, Triangle, &src, &mut dst);
|                                                     ^^^^ expected slice, found struct `std::vec::Vec`
|
= note: expected type `&[u8]`
found type `&std::vec::Vec<u16>`
= help: here are some functions which might fulfill your needs:
- .as_slice()