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)

Author: bors

src/librustc/hir/map/mod.rs

@@ -79,6 +79,33 @@ impl<'hir> Entry<'hir> {
         }
     }
 
+    fn fn_sig(&self) -> Option<&'hir FnSig> {
+        match &self.node {
+            Node::Item(item) => {
+                match &item.kind {
+                    ItemKind::Fn(sig, _, _) => Some(sig),
+                    _ => None,
+                }
+            }
+
+            Node::TraitItem(item) => {
+                match &item.kind {
+                    TraitItemKind::Method(sig, _) => Some(sig),
+                    _ => None
+                }
+            }
+
+            Node::ImplItem(item) => {
+                match &item.kind {
+                    ImplItemKind::Method(sig, _) => Some(sig),
+                    _ => None,
+                }
+            }
+
+            _ => None,
+        }
+    }
+
     fn associated_body(self) -> Option<BodyId> {
         match self.node {
             Node::Item(item) => {
@@ -450,6 +477,14 @@ impl<'hir> Map<'hir> {
         }
     }
 
+    pub fn fn_sig_by_hir_id(&self, hir_id: HirId) -> Option<&'hir FnSig> {
+        if let Some(entry) = self.find_entry(hir_id) {
+            entry.fn_sig()
+        } else {
+            bug!("no entry for hir_id `{}`", hir_id)
+        }
+    }
+
     /// Returns the `HirId` that corresponds to the definition of
     /// which this is the body of, i.e., a `fn`, `const` or `static`
     /// item (possibly associated), a closure, or a `hir::AnonConst`.

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) }