> One of the first things compilers do is throw away the original source code; outside of initial front-end codegen, there is no reference to the original AST

> One of the first things compilers do is throw away the original source code; outside of initial front-end codegen, there is no reference to the original AST That doesn't have to be an immutable fact for all implementations of compilers for all languages. The Rust compiler has multiple IRs (AST, HIR, THIR, MIR & LLVM IR), and for all but the last one (codegen), we keep around, if not a reference to the HIR node that allows us to navigate its context, at least a Span that points at the user's code (sometimes a whole struct or linked list of "facts" we've collected so far, each containing Spans). rustc also throws away information to reduce memory consumption, much to my dismay, but we still perform later checks to re evaluate (sometimes as an approximation) what the reason for a certain "decision" was, so that we can talk to the user in terms they understand, and not in terms of compiler internals. The ideal design would be to have a single implementation of all logic with two modes of operation: the low memory consumption one (what you state as the way compilers work), and a "track everything without discarding metadata until the end" mode, that can provide better human readable output.