10823: fix: better `Fn` traits formatting r=jonas-schievink a=veber-alex

This makes it so formatting `Fn` traits properly handles:

1. An `Fn` trait with only a single argument -> removes the trailing comma.
2. An `Fn` trait which returns nothing (an empty tuple) -> don't show `-> ()` as the return type.

before:
![before](https://user-images.githubusercontent.com/29788806/142745038-44ac68ce-de42-4396-a809-ffdb883be699.png)

after:
![after](https://user-images.githubusercontent.com/29788806/142745040-485feaa2-cc21-4a05-9576-5410ea355029.png)




Co-authored-by: Alex Veber <alexveber@gmail.com>
This commit is contained in:
bors[bot] 2021-11-21 20:01:07 +00:00 committed by GitHub
commit 183ef048f6
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@ -1164,11 +1164,24 @@ impl HirDisplay for Path {
// Do we actually format expressions?
if generic_args.desugared_from_fn {
// First argument will be a tuple, which already includes the parentheses.
// If the tuple only contains 1 item, write it manually to avoid the trailing `,`.
if let hir_def::path::GenericArg::Type(TypeRef::Tuple(v)) =
&generic_args.args[0]
{
if v.len() == 1 {
write!(f, "(")?;
v[0].hir_fmt(f)?;
write!(f, ")")?;
} else {
generic_args.args[0].hir_fmt(f)?;
}
}
if let Some(ret) = &generic_args.bindings[0].type_ref {
if !matches!(ret, TypeRef::Tuple(v) if v.is_empty()) {
write!(f, " -> ")?;
ret.hir_fmt(f)?;
}
}
return Ok(());
}