Expand description

A pass that promotes borrows of constant rvalues.

The rvalues considered constant are trees of temps, each with exactly one initialization, and holding a constant value with no interior mutability. They are placed into a new MIR constant body in promoted and the borrow rvalue is replaced with a Literal::Promoted using the index into promoted of that constant MIR.

This pass assumes that every use is dominated by an initialization and can otherwise silence errors, if move analysis runs after promotion on broken MIR.


  • A “root candidate” for promotion, which will become the returned value in a promoted MIR, unless it’s a subset of a larger candidate.
  • Collector 🔒
  • A MirPass for promotion.
  • Promoter 🔒
  • Validator 🔒
    Checks whether locals that appear in a promotion context (Candidate) are actually promotable.


  • State of a temporary during collection and promotion.