Function escape_dollars 

pub fn escape_dollars(input: TokenStream) -> TokenStream

Replaces every $ with $orig_dollar

Eg, where the template says $fname, we emit $orig_dollar fname. When this is found in the macro_rules expander part of a precanned template, macro_rules doesn’t expand it because orig_dollar isn’t one of the arguments to the macro.

Then, we spot these when parsing the template, and disregard them. That is done by syntax::deescape_orig_dollar.

See doc/implementation.md for why this is needed.

This has the weird result that there’s sometimes (namely, when using an adhoc, rather than precanned template) an undocumented orig_dollar expansion keyword, with strange behaviour. No-one is likely to notice this.

Additionally, if we’re turning $crate into $orig_dollar crate, we change the keyword crate to _dd_intern_crate (and ${crate} likewise), with the span of the original. This is necessary to avoid clippy seeing the bare crate and thinking the user should have written $crate (whereas, in fact, they did), and emitting a spurious lint crate_in_macro_def. $_dd_intern_crate is an internal alias for d-d’s $crate.

Alternative tactics we rejected:

• Pass a literal dollar sign $ into the template pattern macro, capture it with a macro rules parameter $dollar:tt, and write $dollar in the template. This gets the span wrong: the span is that of the literal dollar, which came from the call site, not the template.

• Use a different syntax in precanned templates: have escape_dollars convert to that syntax, and the template parsing notice this case and de-escape the whole template again at the start. This involves processing the whole template twice for no reason. (And it would involve inventing an additional, different, and probably weird, syntax.)

• As above but do the de-escaping on the fly. Currently, though, the information about the template context is not available to the parser. We’d have to pass it in as a thread local, or as an extra generic on SubstContext (producing two monomorphised copies of the whole template engine).