Rollup merge of #130714 - compiler-errors:try-structurally-resolve-const, r=BoxyUwU

Introduce `structurally_normalize_const`, use it in `rustc_hir_typeck`

Introduces `structurally_normalize_const` to typecking to separate the "eval a const" step from the "try to turn a valtree into a target usize" in HIR typeck, where we may still have infer vars and stuff around.

I also changed `check_expr_repeat` to move a double evaluation of a const into a single one. I'll leave inline comments.

r? ```@BoxyUwU```

I hesitated to really test this on the new solver where it probably matters for unevaluated consts. If you're worried about the side-effects, I'd be happy to craft some more tests 😄

compiler/rustc_hir/src/hir.rs

@@ -1668,10 +1668,17 @@ pub enum ArrayLen<'hir> {
 }
 
 impl ArrayLen<'_> {
-    pub fn hir_id(&self) -> HirId {
+    pub fn span(self) -> Span {
         match self {
-            ArrayLen::Infer(InferArg { hir_id, .. }) | ArrayLen::Body(ConstArg { hir_id, .. }) => {
-                *hir_id
+            ArrayLen::Infer(arg) => arg.span,
+            ArrayLen::Body(body) => body.span(),
+        }
+    }
+
+    pub fn hir_id(self) -> HirId {
+        match self {
+            ArrayLen::Infer(InferArg { hir_id, .. }) | ArrayLen::Body(&ConstArg { hir_id, .. }) => {
+                hir_id
             }
         }
     }

compiler/rustc_hir_typeck/src/expr.rs

@@ -1491,8 +1491,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         expr: &'tcx hir::Expr<'tcx>,
     ) -> Ty<'tcx> {
         let tcx = self.tcx;
-        let count = self.lower_array_length(count);
-        if let Some(count) = count.try_eval_target_usize(tcx, self.param_env) {
+        let count_span = count.span();
+        let count = self.try_structurally_resolve_const(count_span, self.lower_array_length(count));
+
+        if let Some(count) = count.try_to_target_usize(tcx) {
             self.suggest_array_len(expr, count);
         }
 
@@ -1520,19 +1522,24 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             return Ty::new_error(tcx, guar);
         }
 
-        self.check_repeat_element_needs_copy_bound(element, count, element_ty);
+        // If the length is 0, we don't create any elements, so we don't copy any.
+        // If the length is 1, we don't copy that one element, we move it. Only check
+        // for `Copy` if the length is larger, or unevaluated.
+        // FIXME(min_const_generic_exprs): We could perhaps defer this check so that
+        // we don't require `<?0t as Tr>::CONST` doesn't unnecessarily require `Copy`.
+        if count.try_to_target_usize(tcx).is_none_or(|x| x > 1) {
+            self.enforce_repeat_element_needs_copy_bound(element, element_ty);
+        }
 
         let ty = Ty::new_array_with_const_len(tcx, t, count);
-
         self.register_wf_obligation(ty.into(), expr.span, ObligationCauseCode::WellFormed(None));
-
         ty
     }
 
-    fn check_repeat_element_needs_copy_bound(
+    /// Requires that `element_ty` is `Copy` (unless it's a const expression itself).
+    fn enforce_repeat_element_needs_copy_bound(
         &self,
         element: &hir::Expr<'_>,
-        count: ty::Const<'tcx>,
         element_ty: Ty<'tcx>,
     ) {
         let tcx = self.tcx;
@@ -1565,9 +1572,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             _ => traits::IsConstable::No,
         };
 
-        // If the length is 0, we don't create any elements, so we don't copy any. If the length is 1, we
-        // don't copy that one element, we move it. Only check for Copy if the length is larger.
-        if count.try_eval_target_usize(tcx, self.param_env).is_none_or(|len| len > 1) {
         let lang_item = self.tcx.require_lang_item(LangItem::Copy, None);
         let code = traits::ObligationCauseCode::RepeatElementCopy {
             is_constable,
@@ -1586,7 +1590,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         };
         self.require_type_meets(element_ty, element.span, code, lang_item);
     }
-    }
 
     fn check_expr_tuple(
         &self,
@@ -2800,9 +2803,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         len: ty::Const<'tcx>,
     ) {
         err.span_label(field.span, "unknown field");
-        if let (Some(len), Ok(user_index)) =
-            (len.try_eval_target_usize(self.tcx, self.param_env), field.as_str().parse::<u64>())
-        {
+        if let (Some(len), Ok(user_index)) = (
+            self.try_structurally_resolve_const(base.span, len).try_to_target_usize(self.tcx),
+            field.as_str().parse::<u64>(),
+        ) {
             let help = "instead of using tuple indexing, use array indexing";
             let applicability = if len < user_index {
                 Applicability::MachineApplicable

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -1470,6 +1470,33 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
     }
 
+    #[instrument(level = "debug", skip(self, sp), ret)]
+    pub fn try_structurally_resolve_const(&self, sp: Span, ct: ty::Const<'tcx>) -> ty::Const<'tcx> {
+        // FIXME(min_const_generic_exprs): We could process obligations here if `ct` is a var.
+
+        if self.next_trait_solver()
+            && let ty::ConstKind::Unevaluated(..) = ct.kind()
+        {
+            // We need to use a separate variable here as otherwise the temporary for
+            // `self.fulfillment_cx.borrow_mut()` is alive in the `Err` branch, resulting
+            // in a reentrant borrow, causing an ICE.
+            let result = self
+                .at(&self.misc(sp), self.param_env)
+                .structurally_normalize_const(ct, &mut **self.fulfillment_cx.borrow_mut());
+            match result {
+                Ok(normalized_ct) => normalized_ct,
+                Err(errors) => {
+                    let guar = self.err_ctxt().report_fulfillment_errors(errors);
+                    return ty::Const::new_error(self.tcx, guar);
+                }
+            }
+        } else if self.tcx.features().generic_const_exprs {
+            ct.normalize(self.tcx, self.param_env)
+        } else {
+            ct
+        }
+    }
+
     /// Resolves `ty` by a single level if `ty` is a type variable.
     ///
     /// When the new solver is enabled, this will also attempt to normalize

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -1502,7 +1502,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // Create an dummy type `&[_]` so that both &[] and `&Vec<T>` can coerce to it.
         let dummy_ty = if let ty::Array(elem_ty, size) = peeled.kind()
             && let ty::Infer(_) = elem_ty.kind()
-            && size.try_eval_target_usize(self.tcx, self.param_env) == Some(0)
+            && self
+                .try_structurally_resolve_const(provided_expr.span, *size)
+                .try_to_target_usize(self.tcx)
+                == Some(0)
         {
             let slice = Ty::new_slice(self.tcx, *elem_ty);
             Ty::new_imm_ref(self.tcx, self.tcx.lifetimes.re_static, slice)

compiler/rustc_hir_typeck/src/intrinsicck.rs

@@ -101,7 +101,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 }
             }
             Ok(SizeSkeleton::Generic(size)) => {
-                if let Some(size) = size.try_eval_target_usize(tcx, self.param_env) {
+                if let Some(size) =
+                    self.try_structurally_resolve_const(span, size).try_to_target_usize(tcx)
+                {
                     format!("{size} bytes")
                 } else {
                     format!("generic size {size}")

compiler/rustc_hir_typeck/src/pat.rs

@@ -2412,7 +2412,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         len: ty::Const<'tcx>,
         min_len: u64,
     ) -> (Option<Ty<'tcx>>, Ty<'tcx>) {
-        let len = len.try_eval_target_usize(self.tcx, self.param_env);
+        let len = self.try_structurally_resolve_const(span, len).try_to_target_usize(self.tcx);
 
         let guar = if let Some(len) = len {
             // Now we know the length...

compiler/rustc_trait_selection/src/traits/structural_normalize.rs

@@ -46,4 +46,44 @@ impl<'tcx> At<'_, 'tcx> {
             Ok(self.normalize(ty).into_value_registering_obligations(self.infcx, fulfill_cx))
         }
     }
+
+    fn structurally_normalize_const<E: 'tcx>(
+        &self,
+        ct: ty::Const<'tcx>,
+        fulfill_cx: &mut dyn TraitEngine<'tcx, E>,
+    ) -> Result<ty::Const<'tcx>, Vec<E>> {
+        assert!(!ct.is_ct_infer(), "should have resolved vars before calling");
+
+        if self.infcx.next_trait_solver() {
+            let ty::ConstKind::Unevaluated(..) = ct.kind() else {
+                return Ok(ct);
+            };
+
+            let new_infer_ct = self.infcx.next_const_var(self.cause.span);
+
+            // We simply emit an `alias-eq` goal here, since that will take care of
+            // normalizing the LHS of the projection until it is a rigid projection
+            // (or a not-yet-defined opaque in scope).
+            let obligation = Obligation::new(
+                self.infcx.tcx,
+                self.cause.clone(),
+                self.param_env,
+                ty::PredicateKind::AliasRelate(
+                    ct.into(),
+                    new_infer_ct.into(),
+                    ty::AliasRelationDirection::Equate,
+                ),
+            );
+
+            fulfill_cx.register_predicate_obligation(self.infcx, obligation);
+            let errors = fulfill_cx.select_where_possible(self.infcx);
+            if !errors.is_empty() {
+                return Err(errors);
+            }
+
+            Ok(self.infcx.resolve_vars_if_possible(new_infer_ct))
+        } else {
+            Ok(self.normalize(ct).into_value_registering_obligations(self.infcx, fulfill_cx))
+        }
+    }
 }

tests/ui/const-generics/generic_const_exprs/different-fn.stderr

@@ -2,10 +2,10 @@ error[E0308]: mismatched types
   --> $DIR/different-fn.rs:10:5
    |
 LL |     [0; size_of::<Foo<T>>()]
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^ expected `size_of::<T>()`, found `size_of::<Foo<T>>()`
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^ expected `size_of::<T>()`, found `0`
    |
    = note: expected constant `size_of::<T>()`
-              found constant `size_of::<Foo<T>>()`
+              found constant `0`
 
 error: unconstrained generic constant
   --> $DIR/different-fn.rs:10:9