Suggest arry::from_fn for array initialization

Author: chenyukang

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

@@ -3144,6 +3144,10 @@ 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/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: considering 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`.