Suggest array::from_fn for array initialization

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

@@ -3152,6 +3152,10 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                         ],
                         Applicability::MachineApplicable,
                     );
+                } else {
+                    // 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/const-generics/issues/issue-61336-2.stderr

@@ -5,6 +5,8 @@ LL |     [x; { N }]
    |      ^ the trait `Copy` is not implemented for `T`
    |
    = 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 restricting type parameter `T`
    |
 LL | fn g<T: std::marker::Copy, const N: usize>(x: T) -> [T; N] {

tests/ui/const-generics/issues/issue-61336.stderr

@@ -5,6 +5,8 @@ LL |     [x; N]
    |      ^ the trait `Copy` is not implemented for `T`
    |
    = 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 restricting type parameter `T`
    |
 LL | fn g<T: std::marker::Copy, const N: usize>(x: T) -> [T; N] {

tests/ui/consts/const-blocks/migrate-fail.stderr

@@ -6,6 +6,8 @@ LL |         let arr: [Option<Bar>; 2] = [x; 2];
    |
    = note: required for `Option<Bar>` to implement `Copy`
    = 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 `Bar` with `#[derive(Copy)]`
    |
 LL + #[derive(Copy)]
@@ -20,6 +22,8 @@ LL |         let arr: [Option<Bar>; 2] = [x; 2];
    |
    = note: required for `Option<Bar>` to implement `Copy`
    = 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 `Bar` with `#[derive(Copy)]`
    |
 LL + #[derive(Copy)]

tests/ui/consts/const-blocks/nll-fail.stderr

@@ -6,6 +6,8 @@ LL |         let arr: [Option<Bar>; 2] = [x; 2];
    |
    = note: required for `Option<Bar>` to implement `Copy`
    = 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 `Bar` with `#[derive(Copy)]`
    |
 LL + #[derive(Copy)]
@@ -20,6 +22,8 @@ LL |         let arr: [Option<Bar>; 2] = [x; 2];
    |
    = note: required for `Option<Bar>` to implement `Copy`
    = 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 `Bar` with `#[derive(Copy)]`
    |
 LL + #[derive(Copy)]

tests/ui/repeat-expr/repeat-to-run-dtor-twice.stderr

@@ -5,6 +5,8 @@ LL |     let _ = [ a; 5 ];
    |               ^ the trait `Copy` is not implemented for `Foo`
    |
    = 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 `Foo` 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`.