Defer repeat expr Copy check

Author: BoxyUwU

compiler/rustc_hir_typeck/src/expr.rs

@@ -1853,12 +1853,15 @@ 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);
         }
 
@@ -1868,7 +1871,7 @@ 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/_impl.rs

@@ -85,33 +85,36 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         })
     }
 
-    /// Resolves type and const variables in `ty` if possible. Unlike the infcx
+    /// Resolves type and const variables in `t` if possible. Unlike the infcx
     /// version (resolve_vars_if_possible), this version will
     /// also select obligations if it seems useful, in an effort
     /// to get more type information.
     // FIXME(-Znext-solver): A lot of the calls to this method should
     // probably be `try_structurally_resolve_type` or `structurally_resolve_type` instead.
     #[instrument(skip(self), level = "debug", ret)]
-    pub(crate) fn resolve_vars_with_obligations(&self, mut ty: Ty<'tcx>) -> Ty<'tcx> {
+    pub(crate) fn resolve_vars_with_obligations<T: TypeFoldable<TyCtxt<'tcx>>>(
+        &self,
+        mut t: T,
+    ) -> T {
         // No Infer()? Nothing needs doing.
-        if !ty.has_non_region_infer() {
+        if !t.has_non_region_infer() {
             debug!("no inference var, nothing needs doing");
-            return ty;
+            return t;
         }
 
-        // If `ty` is a type variable, see whether we already know what it is.
-        ty = self.resolve_vars_if_possible(ty);
-        if !ty.has_non_region_infer() {
-            debug!(?ty);
-            return ty;
+        // If `t` is a type variable, see whether we already know what it is.
+        t = self.resolve_vars_if_possible(t);
+        if !t.has_non_region_infer() {
+            debug!(?t);
+            return t;
         }
 
         // If not, try resolving pending obligations as much as
         // possible. This can help substantially when there are
         // indirect dependencies that don't seem worth tracking
         // precisely.
         self.select_obligations_where_possible(|_| {});
-        self.resolve_vars_if_possible(ty)
+        self.resolve_vars_if_possible(t)
     }
 
     pub(crate) fn record_deferred_call_resolution(
@@ -1454,7 +1457,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         sp: Span,
         ct: ty::Const<'tcx>,
     ) -> ty::Const<'tcx> {
-        // FIXME(min_const_generic_exprs): We could process obligations here if `ct` is a var.
+        let ct = self.resolve_vars_with_obligations(ct);
 
         if self.next_trait_solver()
             && let ty::ConstKind::Unevaluated(..) = ct.kind()
@@ -1510,6 +1513,32 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
     }
 
+    pub(crate) fn structurally_resolve_const(
+        &self,
+        sp: Span,
+        ct: ty::Const<'tcx>,
+    ) -> ty::Const<'tcx> {
+        let ct = self.try_structurally_resolve_const(sp, ct);
+
+        if !ct.is_ct_infer() {
+            ct
+        } else {
+            let e = self.tainted_by_errors().unwrap_or_else(|| {
+                self.err_ctxt()
+                    .emit_inference_failure_err(
+                        self.body_id,
+                        sp,
+                        ct.into(),
+                        TypeAnnotationNeeded::E0282,
+                        true,
+                    )
+                    .emit()
+            });
+            // FIXME: Infer `?ct = {const error}`?
+            ty::Const::new_error(self.tcx, e)
+        }
+    }
+
     pub(crate) fn with_breakable_ctxt<F: FnOnce() -> R, R>(
         &self,
         id: HirId,

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -115,6 +115,31 @@ 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(..) {
+            // We want to emit an error if the const is not structurally resolveable as otherwise
+            // we can find up conservatively proving `Copy` which may infer the repeat expr count
+            // to something that never required `Copy` in the first place.
+            let count =
+                self.structurally_resolve_const(element.span, self.normalize(element.span, count));
+
+            // Avoid run on "`NotCopy: Copy` is not implemented" errors when the repeat expr count
+            // is erroneous/unknown. The user might wind up specifying a repeat count of 0/1.
+            if count.references_error() {
+                continue;
+            }
+
+            // 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_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

@@ -199,6 +199,15 @@ fn typeck_with_inspect<'tcx>(
         fcx.write_ty(id, expected_type);
     };
 
+    // Whether to check repeat exprs before/after inference fallback is somewhat arbitrary of a decision
+    // as neither option is strictly more permissive than the other. However, we opt to check repeat exprs
+    // first as errors from not having inferred array lengths yet seem less confusing than errors from inference
+    // fallback arbitrarily inferring something incompatible with `Copy` inference side effects.
+    //
+    // This should also be forwards compatible with moving repeat expr checks to a custom goal kind or using
+    // marker traits in the future.
+    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()),
         }