传递(迭代器)值到闭包(预期类型参数' I '，发现结构' std::slice::Iter ')的问题是什么?<

Question

我想了解以下错误:

error[E0308]: mismatched types
--> src/tabs.rs:70:44
|
43 | fn detab_go<'a, I, R, W>(
|                 - this type parameter
...
70 |                         f_out, bytes_iter, buf_iter,
|                                            ^^^^^^^^ expected type parameter `I`, found struct `std::slice::Iter`
|
= note: expected type parameter `I`
found struct `std::slice::Iter<'_, u8>`

导致错误的代码看起来像这样(从我一直在研究的宏扩展):

fn detab_go<'a, I, R, W>(
f_out: &mut W,
bytes_iter: BytesIter<R>,
buf_iter: I,
tab_pos_last: usize,
) -> Result<(), Error>
where
I: Iterator<Item = &'a u8>,
R: Read,
W: Write,
{
tailcall::trampoline::run_res(
#[inline(always)]
|(f_out, mut bytes_iter, mut buf_iter, tab_pos_last)| {
Ok(tailcall::trampoline::Finish({
match buf_iter.next() {
Some(byte) => {
if !is_tab_or_newline(*byte) {
write_u8(f_out, *byte)?;
}
return Ok(tailcall::trampoline::Recurse((
f_out, bytes_iter, buf_iter, 1,
)));
}
None => match bytes_iter.next() {
Some(buf_new) => {
let buf_test: Vec<u8> = buf_new?;
let buf_iter = buf_test.iter();
return Ok(tailcall::trampoline::Recurse((
f_out, bytes_iter, buf_iter, 1,
)));
}
None => Ok(()),
},
}
}))
},
(f_out, bytes_iter, buf_iter, tab_pos_last),
)
}

非宏展开的代码不会导致编译错误。它应该具有与宏扩展代码相同的含义(尽管运行时特征略有不同)，看起来像:

fn detab_go<'a, I, R, W>(
f_out: &mut W,
mut bytes_iter: BytesIter<R>,
mut buf_iter: I,
tab_pos_last: usize,
) -> Result<(), Error>
where
I: Iterator<Item = &'a u8>,
R: Read,
W: Write,
{
match buf_iter.next() {
Some(byte) => {
if !is_tab_or_newline(*byte) {
write_u8(f_out, *byte)?;
}
detab_go(
f_out, bytes_iter, buf_iter,
/*&tab_pos_new*/ /*todo!() */ 1,
)
}
None => {
match bytes_iter.next() {
Some(buf_new) => {
let buf_test: Vec<u8> = buf_new?;
let buf_iter = buf_test.iter(); //shadow
detab_go(
f_out, bytes_iter, buf_iter,
/*&tab_pos_new*/ /*todo!()*/ 1,
)
}
None => Ok(()), /* Finished */
}
}
}
}

Answer 1

我认为区别在于递归的类型:直接递归地调用detab_go允许编译器看到detab_go接受任意的Iterator<Item=u8>，而trampoline版本似乎修复了I——不允许不同类型的输入。

也许它有助于使detab_go接受std::slice::Iter<'_, u8>而不是通用的Iterator。