Commits on Jan 20, 2021

1. Split Symbol interner into static and dynamic …

   This improves performance of Symbol interning in several ways.
   The main motivation of this work is to prepare rustc for
   efficient parallel builds.
   
   The Symbol lookup table (mapping from strings to Symbol numbers)
   is now split into two separate tables: a static table and a
   dynamic table. The static table contains strings that are known
   to rustc, including all keywords and all `sym::foo` symbols.
   The dynamic table contains strings that rustc discovers while
   compiling, such as "my_super_obscure_function_name".
   
   Since the static table is known at compile time (that is, when
   rustc itself is being compiled), this table can be stored
   entirely in static data structures. We use the `phf` crate to
   generate this table; `phf` generates perfect hash functions.
   
   This allows rustc to perform Symbol lookups for static symbols
   without any multithreaded synchronization, or accessing any
   dynamic data whatsoever.
   
   I measured the percentage of static symbol lookups in many
   common Rust crates, including rust/compiler, rust/library,
   servo, rand, quote, syn, rust-analyzer, rayon, and rsvg.
   Among these crates, between 35% and 55% of all symbol lookups
   were resolved using the static lookup table.

   Arlie Davis committed Jan 20, 2021
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