Auto merge of #66170 - ecstatic-morse:hir-const-check, r=Centril,oli-obk

Add a HIR pass to check consts for `if`, `loop`, etc.

Resolves #66125.

This PR adds a HIR pass to check for high-level control flow constructs that are forbidden in a const-context. The MIR const-checker is unable to provide good spans for these since they are lowered to control flow primitives (e.g., `Goto` and `SwitchInt`), and these often don't map back to the underlying statement as a whole. This PR is intended only to improve diagnostics once `if` and `match` become commonplace in constants (behind a feature flag). The MIR const-checker will continue to operate unchanged, and will catch anything this check might miss.

In this implementation, the HIR const-checking pass is run much earlier than the MIR one, so it will supersede any errors from the latter. I will need some mentoring if we wish to change this, since I'm not familiar with the diagnostics system. Moving this pass into the same phase as the MIR const-checker could also help keep backwards compatibility for items like `const _: () = loop { break; };`, which are currently (erroneously?) accepted by the MIR const-checker (see #62272).

r? @Centril
cc @eddyb (since they filed #62272)

1 files changed, 5 insertions, 0 deletions

diff --git a/src/librustc/query/mod.rs b/src/librustc/query/mod.rs
index bd7b77b0abb..9bd2a933c1c 100644
--- a/src/librustc/query/mod.rs
+++ b/src/librustc/query/mod.rs
@@ -329,6 +329,11 @@ rustc_queries! {
             desc { |tcx| "checking for unstable API usage in {}", key.describe_as_module(tcx) }
         }
 
+        /// Checks the const bodies in the module for illegal operations (e.g. `if` or `loop`).
+        query check_mod_const_bodies(key: DefId) -> () {
+            desc { |tcx| "checking consts in {}", key.describe_as_module(tcx) }
+        }
+
         /// Checks the loops in the module.
         query check_mod_loops(key: DefId) -> () {
             desc { |tcx| "checking loops in {}", key.describe_as_module(tcx) }