Improve diagnostic for const ctors in array repeat expressions

1 files changed, 20 insertions, 9 deletions

diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
index 073a2a2b1a0..edae9e10818 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
@@ -27,6 +27,7 @@ use rustc_infer::infer::error_reporting::TypeErrCtxt;
 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use rustc_infer::infer::{DefineOpaqueTypes, InferOk, LateBoundRegionConversionTime};
 use rustc_middle::hir::map;
+use rustc_middle::traits::IsConstable;
 use rustc_middle::ty::error::TypeError::{self, Sorts};
 use rustc_middle::ty::{
     self, suggest_arbitrary_trait_bound, suggest_constraining_type_param, AdtKind,
@@ -2832,20 +2833,30 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                     ));
                 }
             }
-            ObligationCauseCode::RepeatElementCopy { is_const_fn } => {
+            ObligationCauseCode::RepeatElementCopy { is_constable, elt_type, elt_span, elt_stmt_span } => {
                 err.note(
                     "the `Copy` trait is required because this value will be copied for each element of the array",
                 );
-
-                if is_const_fn {
-                    err.help(
-                        "consider creating a new `const` item and initializing it with the result \
-                        of the function call to be used in the repeat position, like \
-                        `const VAL: Type = const_fn();` and `let x = [VAL; 42];`",
-                    );
+                let value_kind = match is_constable {
+                    IsConstable::Fn => Some("the result of the function call"),
+                    IsConstable::Ctor => Some("the result of the constructor"),
+                    _ => None
+                };
+                let sm = tcx.sess.source_map();
+                if let Some(value_kind) = value_kind &&
+                    let Ok(snip) = sm.span_to_snippet(elt_span)
+                {
+                    let help_msg = format!(
+                        "consider creating a new `const` item and initializing it with {value_kind} \
+                        to be used in the repeat position");
+                    let indentation = sm.indentation_before(elt_stmt_span).unwrap_or_default();
+                    err.multipart_suggestion(help_msg, vec![
+                        (elt_stmt_span.shrink_to_lo(), format!("const ARRAY_REPEAT_VALUE: {elt_type} = {snip};\n{indentation}")),
+                        (elt_span, "ARRAY_REPEAT_VALUE".to_string())
+                    ], Applicability::MachineApplicable);
                 }
 
-                if self.tcx.sess.is_nightly_build() && is_const_fn {
+                if self.tcx.sess.is_nightly_build() && matches!(is_constable, IsConstable::Fn|IsConstable::Ctor) {
                     err.help(
                         "create an inline `const` block, see RFC #2920 \
                          <https://github.com/rust-lang/rfcs/pull/2920> for more information",