Commits on Aug 20, 2018

1. syntax: Optimize some literal parsing …

   Currently in the `wasm-bindgen` project we have a very very large crate that's
   procedurally generated, `web-sys`. To generate this crate we parse all of a
   browser's WebIDL and we then generate bindings for all of the APIs contained
   within.
   
   The resulting Rust file is 18MB large (wow!) and currently takes a very long
   time to compile in debug mode. On the nightly compiler a *debug* build takes 90s
   for the crate to finish. I was curious what was taking so long and upon
   investigating a *massive* portion of the time was spent in the `lit_token`
   method of the compiler, primarily formatting strings via `format!`.
   
   Upon some more investigation it looks like the `byte_str_lit` was allocating an
   error message once per byte, causing a very large number of allocations to
   happen for large literals, of which wasm-bindgen generates quite a few (some are
   MB large).
   
   This commit fixes the issue by lazily allocating the error message, only doing
   so if the error message is actually needed (which should be never). As a result,
   the debug mode compilation time for our `web-sys` crate decreased from 90s to
   20s, a very nice improvement! (although we've still got some work to do).

   

   alexcrichton committed Aug 20, 2018
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