Only put Display-like bounds on type variables

[JelteF]

Only put Display-like bounds on type variables

Resolves #363
Requires #377
Related to #371

Synopsis

The problem is that the Display derive adds a bounds for all types that
are used in the format string. But this is not necessary for types that
don't contain a type variable. And adding those bounds can result in
errors like for the following code:

#[derive(Display, Debug)]
#[display("{inner:?}")]
#[display(bounds(T: Display))]
struct OptionalBox<T> {
    inner: Option<Box<T>>,
}

#[derive(Display, Debug)]
#[display("{next}")]
struct ItemStruct {
    next: OptionalBox<ItemStruct>,
}

That code would generate the following error:

error[E0275]: overflow evaluating the requirement `ItemStruct: derive_more::Display`

Solution

This makes sure we don't add unnecessary bounds for Display-like
derives. It does so in the same way as #371 did for the Debug derive: By
only adding bounds when the type contains a type variable.

Checklist

• Tests are added/updated (if required)

Changelog is not updated because the 0.99 did not have this issue.

[tyranron]

@JelteF this seems fine. The only change I've added is extracting the duplicate code of syn::Type/syn::Path traversal to the fmt module shared by both expansions.