From 0000b35288f7f7d929dd9afe7a5f4b47305f8bff Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Mon, 20 Nov 2023 03:27:41 +0000
Subject: [PATCH] Document DefiningAnchor a bit more
---
compiler/rustc_middle/src/traits/mod.rs | 21 +++++++++++++++++----
1 file changed, 17 insertions(+), 4 deletions(-)
diff --git a/compiler/rustc_middle/src/traits/mod.rs b/compiler/rustc_middle/src/traits/mod.rs
index 6cd75e08727..8af4f57d5e8 100644
--- a/compiler/rustc_middle/src/traits/mod.rs
+++ b/compiler/rustc_middle/src/traits/mod.rs
@@ -956,13 +956,26 @@ pub enum CodegenObligationError {
FulfillmentError,
}
+/// Defines the treatment of opaque types in a given inference context.
+///
+/// This affects both what opaques are allowed to be defined, but also whether
+/// opaques are replaced with inference vars eagerly in the old solver (e.g.
+/// in projection, and in the signature during function type-checking).
#[derive(Debug, PartialEq, Eq, Clone, Copy, Hash, HashStable, TypeFoldable, TypeVisitable)]
pub enum DefiningAnchor {
- /// `DefId` of the item.
+ /// Define opaques which are in-scope of the `LocalDefId`. Also, eagerly
+ /// replace opaque types in `replace_opaque_types_with_inference_vars`.
Bind(LocalDefId),
- /// When opaque types are not resolved, we `Bubble` up, meaning
- /// return the opaque/hidden type pair from query, for caller of query to handle it.
+ /// In contexts where we don't currently know what opaques are allowed to be
+ /// defined, such as (old solver) canonical queries, we will simply allow
+ /// opaques to be defined, but "bubble" them up in the canonical response or
+ /// otherwise treat them to be handled later.
+ ///
+ /// We do not eagerly replace opaque types in `replace_opaque_types_with_inference_vars`,
+ /// which may affect what predicates pass and fail in the old trait solver.
Bubble,
- /// Used to catch type mismatch errors when handling opaque types.
+ /// Do not allow any opaques to be defined. This is used to catch type mismatch
+ /// errors when handling opaque types, and also should be used when we would
+ /// otherwise reveal opaques (such as [`Reveal::All`] reveal mode).
Error,
}