Defer repeat expr Copy check

Author: BoxyUwU

compiler/rustc_hir_typeck/src/expr.rs

@@ -1856,12 +1856,15 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             return Ty::new_error(tcx, guar);
         }
 
+        // We defer checking whether the element type is `Copy` as it is possible to have
+        // an inference variable as a repeat count and it seems unlikely that `Copy` would
+        // have inference side effects required for type checking to succeed.
+        if tcx.features().generic_arg_infer() {
+            self.deferred_repeat_expr_checks.borrow_mut().push((element, element_ty, count));
         // 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) {
+        } else if count.try_to_target_usize(self.tcx).is_none_or(|x| x > 1) {
             self.enforce_repeat_element_needs_copy_bound(element, element_ty);
         }
 
@@ -1871,7 +1874,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     }
 
     /// Requires that `element_ty` is `Copy` (unless it's a const expression itself).
-    fn enforce_repeat_element_needs_copy_bound(
+    pub(super) fn enforce_repeat_element_needs_copy_bound(
         &self,
         element: &hir::Expr<'_>,
         element_ty: Ty<'tcx>,

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -119,6 +119,22 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
     }
 
+    pub(in super::super) fn check_repeat_exprs(&self) {
+        let mut deferred_repeat_expr_checks = self.deferred_repeat_expr_checks.borrow_mut();
+        debug!("FnCtxt::check_repeat_exprs: {} deferred checks", deferred_repeat_expr_checks.len());
+        for (element, element_ty, count) in deferred_repeat_expr_checks.drain(..) {
+            let count = self
+                .try_structurally_resolve_const(element.span, self.normalize(element.span, count));
+
+            // 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.
+            if count.try_to_target_usize(self.tcx).is_none_or(|x| x > 1) {
+                self.enforce_repeat_element_needs_copy_bound(element, element_ty);
+            }
+        }
+    }
+
     pub(in super::super) fn check_method_argument_types(
         &self,
         sp: Span,

compiler/rustc_hir_typeck/src/lib.rs

@@ -194,6 +194,8 @@ fn typeck_with_inspect<'tcx>(
         fcx.write_ty(id, expected_type);
     };
 
+    fcx.check_repeat_exprs();
+
     fcx.type_inference_fallback();
 
     // Even though coercion casts provide type hints, we check casts after fallback for

compiler/rustc_hir_typeck/src/typeck_root_ctxt.rs

@@ -62,6 +62,9 @@ pub(crate) struct TypeckRootCtxt<'tcx> {
 
     pub(super) deferred_coroutine_interiors: RefCell<Vec<(LocalDefId, hir::BodyId, Ty<'tcx>)>>,
 
+    pub(super) deferred_repeat_expr_checks:
+        RefCell<Vec<(&'tcx hir::Expr<'tcx>, Ty<'tcx>, ty::Const<'tcx>)>>,
+
     /// Whenever we introduce an adjustment from `!` into a type variable,
     /// we record that type variable here. This is later used to inform
     /// fallback. See the `fallback` module for details.
@@ -96,6 +99,7 @@ impl<'tcx> TypeckRootCtxt<'tcx> {
             deferred_transmute_checks: RefCell::new(Vec::new()),
             deferred_asm_checks: RefCell::new(Vec::new()),
             deferred_coroutine_interiors: RefCell::new(Vec::new()),
+            deferred_repeat_expr_checks: RefCell::new(Vec::new()),
             diverging_type_vars: RefCell::new(Default::default()),
             infer_var_info: RefCell::new(Default::default()),
         }