Module number 

Numbering every expression in a file.

The parser leaves every Expr holding ExprId::UNSET, and this pass replaces those with 0, 1, … in one walk of the finished tree. Keeping it separate from the parser is what makes the ids a property of the file: they follow source order, not the order the parser happened to build things in, so re-parsing the same text gives the same numbers and a parser change that reorders its own work does not renumber anything.

The walk is deliberately exhaustive and deliberately written without a catch-all arm. Every variant of ExprKind is named here, so a variant added to the tree later stops compiling in this file rather than quietly leaving its children unnumbered — which is the one failure this pass can have, and the one that would be hardest to notice downstream.

Recursion here is bounded by the parser’s MAX_NESTING_DEPTH: a tree only exists because the parser built it, and a walker spends less stack per level than the parser did.

Functions

number_unit

Assigns every expression in unit an id unique within it.