When failing to import core, suggest std

compiler/rustc_resolve/src/diagnostics.rs

@@ -1858,6 +1858,15 @@ pub(crate) fn report_path_resolution_error(
                         Applicability::MaybeIncorrect,
                     )),
                 )
+            } else if ident.name == sym::core {
+                (
+                    format!("maybe a missing crate `{ident}`?"),
+                    Some((
+                        vec![(ident.span, "std".to_string())],
+                        "try using `std` instead of `core`".to_string(),
+                        Applicability::MaybeIncorrect,
+                    )),
+                )
             } else if self.tcx.sess.is_rust_2015() {
                 (
                     format!("maybe a missing crate `{ident}`?"),

tests/ui/feature-gates/feature-gate-extern_absolute_paths.stderr

@@ -2,9 +2,10 @@ error[E0432]: unresolved import `core`
   --> $DIR/feature-gate-extern_absolute_paths.rs:1:5
    |
 LL | use core::default;
-   |     ^^^^ maybe a missing crate `core`?
-   |
-   = help: consider adding `extern crate core` to use the `core` crate
+   |     ^^^^
+   |     |
+   |     maybe a missing crate `core`?
+   |     help: try using `std` instead of `core`: `std`
 
 error[E0433]: failed to resolve: maybe a missing crate `core`?
   --> $DIR/feature-gate-extern_absolute_paths.rs:4:19
@@ -12,7 +13,10 @@ error[E0433]: failed to resolve: maybe a missing crate `core`?
 LL |     let _: u8 = ::core::default::Default();
    |                   ^^^^ maybe a missing crate `core`?
    |
-   = help: consider adding `extern crate core` to use the `core` crate
+help: try using `std` instead of `core`
+   |
+LL |     let _: u8 = ::std::default::Default();
+   |                   ~~~
 help: consider importing this module
    |
 LL + use std::default;

tests/ui/rfcs/rfc-2632-const-trait-impl/issue-102156.stderr

@@ -2,17 +2,20 @@ error[E0433]: failed to resolve: maybe a missing crate `core`?
   --> $DIR/issue-102156.rs:4:5
    |
 LL | use core::convert::{From, TryFrom};
-   |     ^^^^ maybe a missing crate `core`?
-   |
-   = help: consider adding `extern crate core` to use the `core` crate
+   |     ^^^^
+   |     |
+   |     maybe a missing crate `core`?
+   |     help: try using `std` instead of `core`: `std`
 
 error[E0433]: failed to resolve: maybe a missing crate `core`?
   --> $DIR/issue-102156.rs:4:5
    |
 LL | use core::convert::{From, TryFrom};
-   |     ^^^^ maybe a missing crate `core`?
+   |     ^^^^
+   |     |
+   |     maybe a missing crate `core`?
+   |     help: try using `std` instead of `core`: `std`
    |
-   = help: consider adding `extern crate core` to use the `core` crate
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
 error: aborting due to 2 previous errors

tests/ui/simd/portable-intrinsics-arent-exposed.stderr

@@ -2,9 +2,10 @@ error[E0433]: failed to resolve: maybe a missing crate `core`?
   --> $DIR/portable-intrinsics-arent-exposed.rs:4:5
    |
 LL | use core::simd::intrinsics;
-   |     ^^^^ maybe a missing crate `core`?
-   |
-   = help: consider adding `extern crate core` to use the `core` crate
+   |     ^^^^
+   |     |
+   |     maybe a missing crate `core`?
+   |     help: try using `std` instead of `core`: `std`
 
 error[E0432]: unresolved import `std::simd::intrinsics`
   --> $DIR/portable-intrinsics-arent-exposed.rs:5:5