Commits on Jun 4, 2020

1. Add -Z span-debug to allow for easier debugging of proc macros …

   Currently, the `Debug` impl for `proc_macro::Span` just prints out
   the byte range. This can make debugging proc macros (either as a crate
   author or as a compiler developer) very frustrating, since neither the
   actual filename nor the `SyntaxContext` is displayed.
   
   This commit adds a perma-unstable flag `-Z span-debug`. When enabled,
   the `Debug` impl for `proc_macro::Span` simply forwards directly to
   `rustc_span::Span`. Once rust-lang#72618 is merged, this will start displaying
   actual line numbers.
   
   While `Debug` impls are not subject to Rust's normal stability
   guarnatees, we probably shouldn't expose any additional information on
   stable until `#![feature(proc_macro_span)]` is stabilized. Otherwise,
   we would be providing a 'backdoor' way to access information that's
   supposed be behind unstable APIs.

   

   Aaron1011 committed Jun 4, 2020
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