If T is a trait, its trait objects ~T, @T and &T do not implement T. There
is an implementation of Writer for @Writer, but not for ~Writer or &Writer
which is why you're seeing that error.
Steven Fackler
On Fri, Oct 18, 2013 at 11:27 PM, Oren Ben-Kiki <o...@ben-kiki.org> wrote:
> Ugh, I was too optimistic. Yes, I can write my code using `MyWriter`, but
> I can't cast any @Writer (such as `io::stdout()`) to it. I guess I should
> just use `@Writer` everywhere for now :-(
>
> This raises the question of how come the compiler is smart enough to
> figure out a `@Writer` has the trait `WriterUtil`, but isn't smart enough
> to figure out a `&Writer` has the trait...
>
>
> On Sat, Oct 19, 2013 at 9:08 AM, Oren Ben-Kiki <o...@ben-kiki.org> wrote:
>
>> I run into the following problem (the code below is a toy example).
>>
>> ```
>> use std::io::Writer; // Makes no difference if added/removed.
>>
>> trait PrintWithSpice {
>> fn print(&self, writer: &Writer, spice: bool);
>> }
>>
>> struct Bar {
>> bar: ~PrintWithSpice,
>> }
>>
>> impl Bar {
>> pub fn print(&self, writer: &Writer) {
>> self.bar.print(writer, false);
>> Bar::print_via_borrowed(writer, &self.bar);
>> }
>>
>> fn print_via_borrowed(writer: &Writer, data: &PrintWithSpice) {
>> // Invoking the `print` function via the borrowed pointer to the
>> `PrintWithSpice` trait:
>> // Works fine, as expected..
>> data.print(writer, true);
>> }
>> }
>>
>> struct Foo {
>> foo: bool
>> }
>>
>> impl PrintWithSpice for Foo {
>> fn print(&self, writer: &Writer, spice: bool) {
>> // Invoking the `write_str` function via the borrowed pointer to
>> the `Writer` trait:
>> // error: failed to find an implementation of trait
>> std::io::Writer for &std::io::Writer<no-bounds>
>> // What is going on?
>> writer.write_str(format!("foo: {:b} spice: {:b}", self.foo,
>> spice));
>> }
>> }
>> ```
>>
>> I didn't understand what the compiler is complaining about. "failed to
>> find an implementation of Foo for &Foo<no-bounds>"? A Foo is a Foo, no?
>> Calling a function via a borrowed pointer to a trait should just work (it
>> does a few lines above).
>>
>> After digging I discovered what the compiler really meant (I think). The
>> `write_str` method is defined for `WriterUtils` rather than for `Writer`.
>> So, if I replace `Writer` by `WriterUtil` in the above code, it compiles
>> fine.
>>
>> So, I ended up defining `trait MyWriter: Writer + WriterUtil` and I am
>> using that instead of `Writer` all over my code. I can see doing that as a
>> workaround, but it doesn't smell right to me.
>>
>> So:
>>
>> * Why is the compiler complaining about not finding an implementation for
>> `Writer` when the method I invoke is from `WriterUtil`?
>>
>> * Since there is an `impl<T: Writer> for WriterUtil`, shouldn't the
>> compiler be "sufficiently smart" to deduce that the code is valid in the
>> 1st place?
>>
>> * Until the compiler is "sufficiently smart" (or, if there is a good
>> reason why it would never be), shouldn't we rename `Writer` to
>> `BasicWriter` and define `trait Writer: BasicWriter, WriterUtil {}` so
>> `&Writer` would become more usable?
>>
>
>
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