ISLE's prototype compiler was built with the simplifying assumption that all constants are integers. "Symbolic constants" (imported opaque values) like $MYVALUE were added later. It's possible to define bool as a primitive type, but the syntax for bool constants, #t and #f, results in generated code that uses integer values 1 and 0 because... everything is (was) an integer. This is clearly suboptimal! In #3572 @alexcrichton used the clever workaround of $false, leveraging the support for arbitrary passthrough of constant names to actually get a false in the code.

We should (i) add a ConstBool alongside ConstInt in the IR (PatternInst and ExprInst), and (ii) codegen these properly as Rust bool types. We might also consider at some point making our "primitive" type hierarchy a bit richer and distinguishing ints and bools, but that's lower-priority.