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reachable computation: clarify comments around consts

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This commit is contained in:
Ralf Jung 2024-06-11 09:05:36 +02:00
parent aec67e238d
commit be4cd53536
1 changed files with 9 additions and 6 deletions
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15
compiler/rustc_passes/src/reachable.rs
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@ -207,18 +207,21 @@ fn propagate_node(&mut self, node: &Node<'tcx>, search_item: LocalDefId) {
}
hir::ItemKind::Const(_, _, init) => {
// Only things actually ending up in the final constant need to be reachable.
// Everything else is either already available as `mir_for_ctfe`, or can't be used
// by codegen anyway.
// Only things actually ending up in the final constant value are reachable
// for codegen. Everything else is only needed during const-eval, so even if
// const-eval happens in a downstream crate, all they need is
// `mir_for_ctfe`.
match self.tcx.const_eval_poly_to_alloc(item.owner_id.def_id.into()) {
Ok(alloc) => {
let alloc = self.tcx.global_alloc(alloc.alloc_id).unwrap_memory();
self.propagate_from_alloc(alloc);
}
// Reachable generic constants will be inlined into other crates
// unconditionally, so we need to make sure that their
// contents are also reachable.
// We can't figure out which value the constant will evaluate to. In
// lieu of that, we have to consider everything mentioned in the const
// initializer reachable, since it *may* end up in the final value.
Err(ErrorHandled::TooGeneric(_)) => self.visit_nested_body(init),
// If there was an error evaluating the const, nothing can be reachable
// via it, and anyway compilation will fail.
Err(ErrorHandled::Reported(..)) => {}
}
}