Description
Explanation of the problem
Here is some sample code:
struct Foo;
impl Foo {
fn foo(&mut self, buf: &[u8]) {
self.write(buf);
}
}
impl std::rt::io::Writer for Foo {
fn write(&mut self, buf: &[u8]) {
}
fn flush(&mut self) {
}
}
fn main() {
}
Expected (by me) behaviour: code should compile happily.
Actual behaviour: well, here is the compiler's opinion of this code:
foo.rs:5:8: 5:24 error: type `&mut Foo` does not implement any method in scope named `write`
foo.rs:5 self.write(buf);
^~~~~~~~~~~~~~~~
error: aborting due to previous error
First thought: huh? But I implemented that method just there, see?
If I add use std::rt::io::Writer; to the top of the file, it will work. This is, as I now realise, because only traits used in the current scope are searched for a matching method; this is a necessary evil, lest such things as conflicts arise, yet it need not be this evil, where it gives no real indication of the problem (sure, the "in scope" bit hints obliquely at the problem, but not at all clearly).
Now, what should be done
In such a case, where a method is not found in the current scope, the compiler should scan all impl blocks that it knows about for that type and see if it can find any that matches the description, and then suggest them (hopefully it).
For example, the error message might then read
error: type
&mut Foodoes not implement any method in scope namedwrite; you probably want touse std::rt::io::Writer;