Structured suggestion for "missing feature intrinsic"

```
error: `size_of_val` is not yet stable as a const intrinsic
  --> $DIR/const-unstable-intrinsic.rs:17:9
   |
LL |         unstable_intrinsic::size_of_val(&x);
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: add `#![feature(unstable)]` to the crate attributes to enable
help: add `#![feature(unstable)]` to the crate attributes to enable
   |
LL + #![feature("unstable")]
   |
```

Author: estebank

compiler/rustc_const_eval/messages.ftl

@@ -422,6 +422,7 @@ const_eval_unstable_in_stable_exposed =
 
 const_eval_unstable_intrinsic = `{$name}` is not yet stable as a const intrinsic
     .help = add `#![feature({$feature})]` to the crate attributes to enable
+const_eval_unstable_intrinsic_suggestion = add `#![feature({$feature})]` to the crate attributes to enable
 
 const_eval_unterminated_c_string =
     reading a null-terminated string starting at {$pointer} with no null found before end of allocation

compiler/rustc_const_eval/src/check_consts/check.rs

@@ -799,10 +799,17 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                             feature,
                             ..
                         }) => {
+                            let suggestion =
+                                tcx.hir_crate_items(()).definitions().next().and_then(|id| {
+                                    tcx.crate_level_attribute_injection_span(
+                                        tcx.local_def_id_to_hir_id(id),
+                                    )
+                                });
                             self.check_op(ops::IntrinsicUnstable {
                                 name: intrinsic.name,
                                 feature,
                                 const_stable_indirect: is_const_stable,
+                                suggestion,
                             });
                         }
                         Some(ConstStability { level: StabilityLevel::Stable { .. }, .. }) => {

compiler/rustc_const_eval/src/check_consts/ops.rs

@@ -394,6 +394,7 @@ pub(crate) struct IntrinsicUnstable {
     pub name: Symbol,
     pub feature: Symbol,
     pub const_stable_indirect: bool,
+    pub suggestion: Option<Span>,
 }
 
 impl<'tcx> NonConstOp<'tcx> for IntrinsicUnstable {
@@ -413,6 +414,8 @@ impl<'tcx> NonConstOp<'tcx> for IntrinsicUnstable {
             span,
             name: self.name,
             feature: self.feature,
+            suggestion: self.suggestion,
+            help: self.suggestion.is_none(),
         })
     }
 }

compiler/rustc_const_eval/src/errors.rs

@@ -129,6 +129,14 @@ pub(crate) struct UnstableIntrinsic {
     pub span: Span,
     pub name: Symbol,
     pub feature: Symbol,
+    #[suggestion(
+        const_eval_unstable_intrinsic_suggestion,
+        code = "#![feature(\"{feature}\")]\n",
+        applicability = "machine-applicable"
+    )]
+    pub suggestion: Option<Span>,
+    #[help]
+    pub help: bool,
 }
 
 #[derive(Diagnostic)]

tests/ui/consts/const-unstable-intrinsic.stderr

@@ -25,6 +25,10 @@ LL |         unstable_intrinsic::size_of_val(&x);
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: add `#![feature(unstable)]` to the crate attributes to enable
+help: add `#![feature(unstable)]` to the crate attributes to enable
+   |
+LL + #![feature("unstable")]
+   |
 
 error: `min_align_of_val` is not yet stable as a const intrinsic
   --> $DIR/const-unstable-intrinsic.rs:20:9
@@ -33,6 +37,10 @@ LL |         unstable_intrinsic::min_align_of_val(&x);
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: add `#![feature(unstable)]` to the crate attributes to enable
+help: add `#![feature(unstable)]` to the crate attributes to enable
+   |
+LL + #![feature("unstable")]
+   |
 
 error: const function that might be (indirectly) exposed to stable cannot use `#[feature(local)]`
   --> $DIR/const-unstable-intrinsic.rs:24:9