Also implement AsUniformValue for textures directly

[strohel]

In addition to implementing if for references, i.e.
impl<'a> AsUniformValue for &'a {TextureType}.

The method as_uniform_value(&self) takes self by reference, so there
should be no need to require the second-level reference.

This allows a longer lifetime to be automatically inferred. Fixes
compilation of the texture_2d_as_uniform_value_lifetime() test from the
previous commit.

This commit doesn't remove the original impl above in order not to introduce
breaking changes. Removing would mean every texture passing in uniform! {}
would have to change.

Should fix #1400. We should also be able to close #1054 (resolved in a slightly different manner).
Related closed issues/PRs: #1629, #1632.

Also related: #1850 - not fixed in this PR, but a similar change to impl for Sampler could fix it.
CC @slightknack.

[slightknack]

Thank you @strohel! We're currently using this fork for a portion our pipeline, this change makes it easier to work with uniforms in an idiomatic Rust manner. Let's get this merged!

[est31]

This commit doesn't remove the original impl above in order not to introduce
breaking changes. Removing would mean every texture passing in uniform! {}
would have to change.

Why would it need a change? As far as I'm aware, when a Trait is implemented for T, then it's also implemented for &'a T, no?

[strohel]

Why would it need a change? As far as I'm aware, when a Trait is implemented for T, then it's also implemented for &'a T, no?

Yes when used with a dot operator (method call resolution), but not when used as other function argument, per https://doc.rust-lang.org/nightly/nomicon/coercions.html:

Note that we do not perform coercions when matching traits (except for receivers, see the next page). If there is an impl for some type U and T coerces to U, that does not constitute an implementation for T. For example, the following will not type check, even though it is OK to coerce t to &T and there is an impl for &T:
(...)

The culprit here is impl<'n, T> UniformsStorage<'n, T, EmptyUniforms> where T: AsUniformValue { pub fn new(name: &'n str, value: T) } used from uniform!{}. If I remove the AsUniformValue impl for references, I get:

error[E0277]: the trait bound `&glium::texture::CompressedSrgbTexture2d: AsUniformValue` is not satisfied
   --> examples/image.rs:148:18
    |
148 |             tex: &opengl_texture
    |                  -^^^^^^^^^^^^^^
    |                  |
    |                  the trait `AsUniformValue` is not implemented for `&glium::texture::CompressedSrgbTexture2d`
    |                  help: consider removing the leading `&`-reference
    |
    = help: the following implementations were found:
              <glium::texture::CompressedSrgbTexture2d as AsUniformValue>

Passing textures by value might not be an option when the caller does not want to lose ownership.

---

If anyone has ideas how to resolve this without keeping the 2 similar AsUniformValue implementations, I'll happily try them out.

[est31]

@strohel fair enough.

[est31]

Thanks!

[strohel]

Thanks, @est31, that was quick!

[strohel]

@est31, can we please request cutting a patch release with this fix? It would allow us to publish a package that depends on it.

[est31]

@strohel sure, will do!