Module hashing 

A hash for keys that are already numbers.

Everything this compiler keys a map by is a small integer or two: a Span is a pair of byte offsets, a definition is an index into a table. The standard library hashes with SipHash, which is chosen to survive an attacker choosing the keys. Nothing here has that problem: the keys come out of a file the compiler was handed, and a file that could pick its own spans could do far worse than make a hash map slow.

Measured before it was written, because that is the rule. Reading a name was the most expensive small thing in the language and it is two lookups, one from the span where the name is written to what it refers to and one from that to the value. crates/deed-driver/examples/interpreting.rs says what a name costs and what it costs now.

The mixing is one multiply and one shift, the same shape as the hash used by every compiler that has had this problem, and it is written out here rather than depended on because it is nine lines.

Structs

NumberHasher

Mixes whatever it is given into one word.

Type Aliases

ByNumber

A BuildHasher for maps keyed by numbers.