should_implement_trait triggers on incompatible signatures

[mahkoh]

... and suggests implementing FromStr which has an incompatible signature.

[ebroto]

Is the signature considered incompatible because the return type differs? After a quick look (I may be mistaken) this seems to be the intended behavior (and should probably be allowed by the OP in this case).

Here, the return type is associated to OutType::Any...

rust-clippy/clippy_lints/src/methods/mod.rs

Line 3304 in e214ea8

("from_str", 1, &FN_HEADER, SelfKind::No, OutType::Any, "std::str::FromStr"),

... which means "not the unit type" according to this snippet ...

rust-clippy/clippy_lints/src/methods/mod.rs

Lines 3434 to 3446 in e214ea8

	impl OutType {
	fn matches(self, cx: &LateContext<'_, '_>, ty: &hir::FnRetTy<'_>) -> bool {
	let is_unit = |ty: &hir::Ty<'_>| SpanlessEq::new(cx).eq_ty_kind(&ty.kind, &hir::TyKind::Tup(&[]));
	match (self, ty) {
	(Self::Unit, &hir::FnRetTy::DefaultReturn(_)) => true,
	(Self::Unit, &hir::FnRetTy::Return(ref ty)) if is_unit(ty) => true,
	(Self::Bool, &hir::FnRetTy::Return(ref ty)) if is_bool(ty) => true,
	(Self::Any, &hir::FnRetTy::Return(ref ty)) if !is_unit(ty) => true,
	(Self::Ref, &hir::FnRetTy::Return(ref ty)) => matches!(ty.kind, hir::TyKind::Rptr(_, _)),
	_ => false,
	}
	}
	}

[mahkoh]

The return type of FromStr is Self which is not compatible with &Self. In particular, Self might be unsized (just like str).

[Alexendoo]

Another example from #9638 is when the orphan rules prevent an impl, e.g. you can't impl FromIterator<String> for Option<LocalType> or impl FromStr for Option<LocalType>