Preserve original formatting when rending default expressions in docs with Span::source_text

[Veetaha]

A note for the community from the maintainers

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[Veetaha]

I tried using Span::source_text() to get the original source code of the default expression, but it always returns the span of the first token instead. The span() implementation of syn::Expr does the following:

1. Convert syn::Expr into a TokenStream
2. Join all spans in the TokenStream with join_spans
3. join_spans, in turn calls proc_macro2::Span::join(), but it only works on nightly.

So there is no good way to obtain the original source code with the original formatting currently on stable (rust-lang/rust#54725).