Suggest arry::from_fn for array initialization

compiler/rustc_hir_typeck/src/expr.rs

@@ -1571,12 +1571,22 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             _ => traits::IsConstable::No,
         };
 
+        let is_repeatable = match element.kind {
+            hir::ExprKind::Call(func, _args) => match *self.node_ty(func.hir_id).kind() {
+                ty::FnDef(_, _) => true,
+                _ => false,
+            },
+            hir::ExprKind::Struct(_, _, _) => true,
+            _ => false,
+        };
+
         // 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).map_or(true, |len| len > 1) {
             let lang_item = self.tcx.require_lang_item(LangItem::Copy, None);
             let code = traits::ObligationCauseCode::RepeatElementCopy {
                 is_constable,
+                is_repeatable,
                 elt_type: element_ty,
                 elt_span: element.span,
                 elt_stmt_span: self

compiler/rustc_middle/src/traits/mod.rs

@@ -305,6 +305,7 @@ pub enum ObligationCauseCode<'tcx> {
         /// If element is a `const fn` or const ctor we display a help message suggesting
         /// to move it to a new `const` item while saying that `T` doesn't implement `Copy`.
         is_constable: IsConstable,
+        is_repeatable: bool,
         elt_type: Ty<'tcx>,
         elt_span: Span,
         /// Span of the statement/item in which the repeat expression occurs. We can use this to

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -3110,6 +3110,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             }
             ObligationCauseCode::RepeatElementCopy {
                 is_constable,
+                is_repeatable,
                 elt_type,
                 elt_span,
                 elt_stmt_span,
@@ -3144,6 +3145,10 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                         ],
                         Applicability::MachineApplicable,
                     );
+                } else if is_repeatable {
+                    // FIXME: we may suggest array::repeat instead
+                    err.help("consider using `core::array::from_fn` to initialize the array");
+                    err.help("see https://doc.rust-lang.org/stable/std/array/fn.from_fn.html# for more information");
                 }
 
                 if self.tcx.sess.is_nightly_build()

tests/ui/array-slice-vec/repeat_empty_ok.stderr

@@ -5,6 +5,8 @@ LL |     let headers = [Header{value: &[]}; 128];
    |                    ^^^^^^^^^^^^^^^^^^ the trait `Copy` is not implemented for `Header<'_>`
    |
    = note: the `Copy` trait is required because this value will be copied for each element of the array
+   = help: consider using `core::array::from_fn` to initialize the array
+   = help: see https://doc.rust-lang.org/stable/std/array/fn.from_fn.html# for more information
 help: consider annotating `Header<'_>` with `#[derive(Copy)]`
    |
 LL + #[derive(Copy)]
@@ -18,6 +20,8 @@ LL |     let headers = [Header{value: &[0]}; 128];
    |                    ^^^^^^^^^^^^^^^^^^^ the trait `Copy` is not implemented for `Header<'_>`
    |
    = note: the `Copy` trait is required because this value will be copied for each element of the array
+   = help: consider using `core::array::from_fn` to initialize the array
+   = help: see https://doc.rust-lang.org/stable/std/array/fn.from_fn.html# for more information
 help: consider annotating `Header<'_>` with `#[derive(Copy)]`
    |
 LL + #[derive(Copy)]

tests/ui/trait-bounds/issue-119530-sugg-from-fn.rs

@@ -0,0 +1,5 @@
+fn foo() -> String { String::new() }
+
+fn main() {
+    let string_arr = [foo(); 64]; //~ ERROR the trait bound `String: Copy` is not satisfied
+}

tests/ui/trait-bounds/issue-119530-sugg-from-fn.stderr

@@ -0,0 +1,13 @@
+error[E0277]: the trait bound `String: Copy` is not satisfied
+  --> $DIR/issue-119530-sugg-from-fn.rs:4:23
+   |
+LL |     let string_arr = [foo(); 64];
+   |                       ^^^^^ the trait `Copy` is not implemented for `String`
+   |
+   = note: the `Copy` trait is required because this value will be copied for each element of the array
+   = help: consider using `core::array::from_fn` to initialize the array
+   = help: see https://doc.rust-lang.org/stable/std/array/fn.from_fn.html# for more information
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.