Can't pass Content argument to next template

[drehren]

Hi, consider the following two templates

first_template.rs.html:

@use super::next_template_html;

@(content: Content)

@:next_template_html(content)

next_template.rs.html:

@(content: Content)

<div>
  <p>Here be content!</p>
@:content()
</div>

Compilation fails with the following:

error[E0277]: expected a `FnOnce<(&mut &mut W,)>` closure, found `impl FnOnce(&mut W) -> io::Result<()>`
  --> ructe_ex\target\debug\build\ructe_next-a19889e173553d57\out\templates\template_first_template_html.rs:12:42
   |
12 | next_template_html(_ructe_out_.by_ref(), content)?;
   | ------------------                       ^^^^^^^ expected an `FnOnce<(&mut &mut W,)>` closure, found `impl FnOnce(&mut W) -> io::Result<()>`
   | |
   | required by a bound introduced by this call
   |
   = note: expected a closure with arguments `(&mut W,)`
              found a closure with arguments `(&mut &mut W,)`
note: required by a bound in `next_template_html`
  --> ructe_ex\target\debug\build\ructe_next-a19889e173553d57\out\templates\template_next_template_html.rs:9:98
   |
9  |              pub fn next_template_html<W>(#[allow(unused_mut)] mut _ructe_out_: W, content: impl FnOnce(&mut W) -> io::Result<()>) -> io::Result<()>
   |                                                                                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `next_template_html`

For more information about this error, try `rustc --explain E0277`.

Is it possible to pass Content from a template to the next ?

Thanks !

[kaj]

Yes, I have also seen some problem related to this in the last release, after merging #125 .

@kornelski , could you take a look at this, could the type for content be written in a better way to solve this?

[kornelski]

The problem here is that by_ref() is required, but it also increases level of indirection. Then every time you nest template calls, the by_ref() adds one more &mut, but the sub-templates should ideally all only require &mut W.

So when calling a template, there needs to be something anti-by-ref that decreases levels of indirection.

This hack works:

@use super::contentcallnext_html;

@(content: Content)

@:contentcallnext_html((|w|content(*w)))

Here's a minimal example:

use std::io::{self, Write};

pub fn contentcall_html<W>(#[allow(unused_mut)] mut _ructe_out_: W, content: impl FnOnce(&mut W) -> io::Result<()>) -> io::Result<()>
where W: Write {
    contentcallnext_html(_ructe_out_.by_ref(), |w| content(*w))?;
    Ok(())
}

pub fn contentcallnext_html<W>(#[allow(unused_mut)] mut _ructe_out_: W, content: impl FnOnce(&mut W) -> io::Result<()>) -> io::Result<()>
where W: Write {
    content(_ructe_out_.by_ref())?;
    Ok(())
}

@kaj Are you able to detect when argument to @:sub_template is a Content? If so, wrap arg as |w| arg(*w).

[kornelski]

I wonder if ructe could define its own trait that calls things, so that dealing with W vs &mut W vs &dyn W distinction could be left to Rust's auto-deref method resolution magic.

[kaj]

Another workaround is to change the first_template.rs.html like this:

@use super::next_template_html;

@(content: Content)

@:next_template_html({@:content()})

I'm considering wether I shold document this a required workaround or basically revert #125 , as I still havn't found any other solution.

[drehren]

I am happy with a the workaround.
I think that documenting it would be good as it seems to properly "blend" with how a ructe template should look.