E0210: clarify "at least one type"

[Natureshadow]

Code

pub trait StreamParser {
    fn new(stream: Box<dyn BufRead>) -> Self;
    fn next_triple(&mut self) -> Result<Option<Arc<Triple>>, SourceError>;
}

pub struct NTriplesParser();

impl StreamParser for NTriplesParser { .. }; // left out for conciseness

pub struct StreamParserIter<P: StreamParser>(P);

impl<P> Iterator for StreamParserIter<P>
where
    P: StreamParser,
{
    type Item = Arc<Triple>;

    fn next(&mut self) -> Option<Self::Item> {
        // FIXME handle errors somehow
        self.0.next_triple().ok()?
    }
}

impl<P> IntoIterator for P
where
    P: StreamParser,
{
    type Item = Arc<Triple>;
    type IntoIter = StreamParserIter<P>;

    fn into_iter(self) -> Self::IntoIter {
        StreamParserIter(self)
    }
}

Current output

error[E0210]: type parameter `P` must be used as the type parameter for some local type (e.g., `MyStruct<P>`)
  --> core/src/sources/graphs/parsers.rs:32:6
   |
32 | impl<P> IntoIterator for P
   |      ^ type parameter `P` must be used as the type parameter for some local type
   |
   = note: implementing a foreign trait is only possible if at least one of the types for which it is implemented is local
   = note: only traits defined in the current crate can be implemented for a type parameter

Desired output

note: implementing a foreign trait is only possible if all of the types for which it is implemented are local

Rationale and extra context

In the above example, clearly one type that satisfies P: StreamParser is local. The real essence here is that some other crate might have at least one type that implements StreamParser. So, in fact, all types hit by the generic trait bound must be local.

Other cases

No response

Rust Version

rustc 1.80.0 (051478957 2024-07-21)
binary: rustc
commit-hash: 051478957371ee0084a7c0913941d2a8c4757bb9
commit-date: 2024-07-21
host: x86_64-unknown-linux-gnu
release: 1.80.0
LLVM version: 18.1.7

Anything else?

No response