Start poking ICEs until they melt

Author: oli-obk

compiler/rustc_middle/src/ty/fast_reject.rs

@@ -221,6 +221,9 @@ impl DeepRejectCtxt {
             (GenericArgKind::Const(obl), GenericArgKind::Const(imp)) => {
                 self.consts_may_unify(obl, imp)
             }
+            (GenericArgKind::Constness(obl), GenericArgKind::Constness(imp)) => {
+                self.constness_may_unify(obl, imp)
+            }
             _ => bug!("kind mismatch: {obligation_arg} {impl_arg}"),
         }
     }
@@ -402,4 +405,19 @@ impl DeepRejectCtxt {
             }
         }
     }
+
+    pub fn constness_may_unify(
+        self,
+        obligation_ct: ty::ConstnessArg,
+        impl_ct: ty::ConstnessArg,
+    ) -> bool {
+        match (obligation_ct, impl_ct) {
+            (_, ty::ConstnessArg::Param) => true,
+            (ty::ConstnessArg::Param, _) => match self.treat_obligation_params {
+                TreatParams::AsPlaceholder => false,
+                TreatParams::AsInfer => true,
+            },
+            _ => obligation_ct == impl_ct,
+        }
+    }
 }

compiler/rustc_middle/src/ty/generics.rs

@@ -238,6 +238,11 @@ impl<'tcx> Generics {
         }
     }
 
+    /// Returns the `ConstnessArg`
+    pub fn has_constness_param(&'tcx self) -> bool {
+        self.params.iter().any(|param| matches!(param.kind, ty::GenericParamDefKind::Constness))
+    }
+
     /// Returns `true` if `params` has `impl Trait`.
     pub fn has_impl_trait(&'tcx self) -> bool {
         self.params.iter().any(|param| {

compiler/rustc_typeck/src/astconv/mod.rs

@@ -573,9 +573,11 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                             // TODO: fall back to `Not`?
                             None => ty::ConstnessArg::Param,
                             Some(context) => {
-                                if tcx.hir().body_const_context(context.expect_local()).is_some() {
-                                    ty::ConstnessArg::Required
+                                if tcx.generics_of(context).has_constness_param() {
+                                    ty::ConstnessArg::Param
                                 } else {
+                                    // TODO: should use `Required` if we're in a const context
+                                    // like `const`/`static` item initializers.
                                     ty::ConstnessArg::Not
                                 }
                             }