"try using a variant of the expected type" suggesting a peculiar syntax

[Patryk27]

Hi,

I've got following code:

#[derive(Eq, PartialEq)]
pub struct Foo;

impl Foo {
    fn new() -> Self { Foo }
}

fn main() {
    if None == Foo::new() {
            // ^^^^^^^^^^
            // |
            // expected enum `std::option::Option`, found struct `Foo`
            // help: try using a variant of the expected type: `Some(<Foo>::new())`
    }
}

(playground link)

The suggested <Foo>::new() syntax is valid, but - quite frankly - spooky (and it seems to be this way from the very beginning of this feature). Could it suggest just Foo::new()?

I'd like to tackle this myself, but I'd need at least a hint where to look :-)

[csmoe]

improvement needed inside print_qpath:

rust/src/librustc/hir/print.rs

Lines 1491 to 1536 in f54911c

	pub fn print_qpath(&mut self,
	qpath: &hir::QPath,
	colons_before_params: bool)
	{
	match *qpath {
	hir::QPath::Resolved(None, ref path) => {
	self.print_path(path, colons_before_params)
	}
	hir::QPath::Resolved(Some(ref qself), ref path) => {
	self.s.word("<");
	self.print_type(qself);
	self.s.space();
	self.word_space("as");
	for (i, segment) in path.segments[..path.segments.len() - 1].iter().enumerate() {
	if i > 0 {
	self.s.word("::")
	}
	if segment.ident.name != kw::PathRoot {
	self.print_ident(segment.ident);
	self.print_generic_args(segment.generic_args(),
	segment.infer_args,
	colons_before_params);
	}
	}
	self.s.word(">");
	self.s.word("::");
	let item_segment = path.segments.last().unwrap();
	self.print_ident(item_segment.ident);
	self.print_generic_args(item_segment.generic_args(),
	item_segment.infer_args,
	colons_before_params)
	}
	hir::QPath::TypeRelative(ref qself, ref item_segment) => {
	self.s.word("<");
	self.print_type(qself);
	self.s.word(">");
	self.s.word("::");
	self.print_ident(item_segment.ident);
	self.print_generic_args(item_segment.generic_args(),
	item_segment.infer_args,
	colons_before_params)
	}
	}
	}

[Patryk27]

Thanks! I'll try to fix it tomorrow :-)

[estebank]

@Patryk27 consider also modifying the following to use span_to_snippet and only use pprint if that method returns Err:

rust/src/librustc_typeck/check/demand.rs

Line 175 in fa0f7d0

let expr_text = print::to_string(print::NO_ANN, |s| s.print_expr(expr));

That way we ensure that we suggest the code as it is in the user's code whenever that is possible, regardless of formatting they might be using and the idiosyncrasies of pprint.