Provide structured suggestion for #![feature(foo)]

```
error: `S2<'_>` is forbidden as the type of a const generic parameter
  --> $DIR/lifetime-in-const-param.rs:5:23
   |
LL | struct S<'a, const N: S2>(&'a ());
   |                       ^^
   |
   = note: the only supported types are integers, `bool` and `char`
help: add `#![feature(adt_const_params)]` to the crate attributes to enable more complex and user defined types
   |
LL + #![feature(adt_const_params)]
   |
```

Fix #55941.

Author: estebank

compiler/rustc_const_eval/src/transform/check_consts/ops.rs

@@ -99,7 +99,7 @@ impl<'tcx> NonConstOp<'tcx> for FnCallNonConst<'tcx> {
     #[allow(rustc::untranslatable_diagnostic)]
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, _: Span) -> Diag<'tcx> {
         let FnCallNonConst { caller, callee, args, span, call_source, feature } = *self;
-        let ConstCx { tcx, param_env, .. } = *ccx;
+        let ConstCx { tcx, param_env, body, .. } = *ccx;
 
         let diag_trait = |err, self_ty: Ty<'_>, trait_id| {
             let trait_ref = TraitRef::from_method(tcx, trait_id, args);
@@ -297,10 +297,12 @@ impl<'tcx> NonConstOp<'tcx> for FnCallNonConst<'tcx> {
             ccx.const_kind(),
         ));
 
-        if let Some(feature) = feature
-            && ccx.tcx.sess.is_nightly_build()
-        {
-            err.help(format!("add `#![feature({feature})]` to the crate attributes to enable",));
+        if let Some(feature) = feature {
+            ccx.tcx.disabled_nightly_feature(
+                &mut err,
+                body.source.def_id().as_local().map(|local| ccx.tcx.local_def_id_to_hir_id(local)),
+                vec![(String::new(), feature)].into_iter(),
+            );
         }
 
         if let ConstContext::Static(_) = ccx.const_kind() {

compiler/rustc_hir_analysis/src/astconv/generics.rs

@@ -16,7 +16,7 @@ use rustc_middle::ty::{
     self, GenericArgsRef, GenericParamDef, GenericParamDefKind, IsSuggestable, Ty, TyCtxt,
 };
 use rustc_session::lint::builtin::LATE_BOUND_LIFETIME_ARGUMENTS;
-use rustc_span::symbol::kw;
+use rustc_span::symbol::{kw, sym};
 use smallvec::SmallVec;
 
 /// Report an error that a generic argument did not match the generic parameter that was
@@ -41,9 +41,11 @@ fn generic_arg_mismatch_err(
     if let GenericParamDefKind::Const { .. } = param.kind {
         if matches!(arg, GenericArg::Type(hir::Ty { kind: hir::TyKind::Infer, .. })) {
             err.help("const arguments cannot yet be inferred with `_`");
-            if sess.is_nightly_build() {
-                err.help("add `#![feature(generic_arg_infer)]` to the crate attributes to enable");
-            }
+            tcx.disabled_nightly_feature(
+                &mut err,
+                param.def_id.as_local().map(|local| tcx.local_def_id_to_hir_id(local)),
+                vec![(String::new(), sym::generic_arg_infer)].into_iter(),
+            );
         }
     }
 

compiler/rustc_hir_analysis/src/check/mod.rs

@@ -291,12 +291,15 @@ fn default_body_is_unstable(
         reason: reason_str,
     });
 
+    let inject_span =
+        item_did.as_local().and_then(|id| tcx.crate_inject_span(tcx.local_def_id_to_hir_id(id)));
     rustc_session::parse::add_feature_diagnostics_for_issue(
         &mut err,
         &tcx.sess,
         feature,
         rustc_feature::GateIssue::Library(issue),
         false,
+        inject_span,
     );
 
     err.emit();

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -983,9 +983,15 @@ fn check_param_wf(tcx: TyCtxt<'_>, param: &hir::GenericParam<'_>) -> Result<(),
                     // Implments `ConstParamTy`, suggest adding the feature to enable.
                     Ok(..) => true,
                 };
-                if may_suggest_feature && tcx.sess.is_nightly_build() {
-                    diag.help(
-                        "add `#![feature(adt_const_params)]` to the crate attributes to enable more complex and user defined types",
+                if may_suggest_feature {
+                    tcx.disabled_nightly_feature(
+                        &mut diag,
+                        Some(param.hir_id),
+                        vec![(
+                            " more complex and user defined types".to_string(),
+                            sym::adt_const_params,
+                        )]
+                        .into_iter(),
                     );
                 }
 

compiler/rustc_hir_typeck/src/method/probe.rs

@@ -1420,15 +1420,13 @@ impl<'tcx> Pick<'tcx> {
                     }
                     _ => {}
                 }
-                if tcx.sess.is_nightly_build() {
-                    for (candidate, feature) in &self.unstable_candidates {
-                        lint.help(format!(
-                            "add `#![feature({})]` to the crate attributes to enable `{}`",
-                            feature,
-                            tcx.def_path_str(candidate.item.def_id),
-                        ));
-                    }
-                }
+                tcx.disabled_nightly_feature(
+                    lint,
+                    Some(scope_expr_id),
+                    self.unstable_candidates.iter().map(|(candidate, feature)| {
+                        (format!(" `{}`", tcx.def_path_str(candidate.item.def_id)), *feature)
+                    }),
+                );
             },
         );
     }

compiler/rustc_lint/src/levels.rs

@@ -1082,6 +1082,7 @@ impl<'s, P: LintLevelsProvider> LintLevelsBuilder<'s, P> {
                                 feature,
                                 GateIssue::Language,
                                 lint_from_cli,
+                                None,
                             );
                         },
                     );

compiler/rustc_middle/src/ty/context.rs

@@ -43,7 +43,9 @@ use rustc_data_structures::sync::{self, FreezeReadGuard, Lock, WorkerLocal};
 #[cfg(parallel_compiler)]
 use rustc_data_structures::sync::{DynSend, DynSync};
 use rustc_data_structures::unord::UnordSet;
-use rustc_errors::{DecorateLint, Diag, DiagCtxt, DiagMessage, ErrorGuaranteed, MultiSpan};
+use rustc_errors::{
+    Applicability, DecorateLint, Diag, DiagCtxt, DiagMessage, ErrorGuaranteed, MultiSpan,
+};
 use rustc_hir as hir;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{CrateNum, DefId, LocalDefId, LOCAL_CRATE};
@@ -2104,6 +2106,44 @@ impl<'tcx> TyCtxt<'tcx> {
         lint_level(self.sess, lint, level, src, Some(span.into()), msg, decorate);
     }
 
+    /// Find the crate root and the appropriate span where `use` and outer attributes can be
+    /// inserted at.
+    pub fn crate_inject_span(self, hir_id: HirId) -> Option<Span> {
+        for (_hir_id, node) in self.hir().parent_iter(hir_id) {
+            if let hir::Node::Crate(m) = node {
+                return Some(m.spans.inject_use_span.shrink_to_lo());
+            }
+        }
+        None
+    }
+
+    pub fn disabled_nightly_feature<E: rustc_errors::EmissionGuarantee>(
+        self,
+        diag: &mut Diag<'_, E>,
+        hir_id: Option<HirId>,
+        features: impl Iterator<Item = (String, Symbol)>,
+    ) {
+        if !self.sess.is_nightly_build() {
+            return;
+        }
+
+        let span = hir_id.and_then(|id| self.crate_inject_span(id));
+        for (desc, feature) in features {
+            let msg =
+                format!("add `#![feature({feature})]` to the crate attributes to enable{desc}");
+            if let Some(span) = span {
+                diag.span_suggestion_verbose(
+                    span,
+                    msg,
+                    format!("#![feature({feature})]\n"),
+                    Applicability::MachineApplicable,
+                );
+            } else {
+                diag.help(msg);
+            }
+        }
+    }
+
     /// Emit a lint from a lint struct (some type that implements `DecorateLint`, typically
     /// generated by `#[derive(LintDiagnostic)]`).
     #[track_caller]

compiler/rustc_passes/src/check_const.rs

@@ -155,16 +155,11 @@ impl<'tcx> CheckConstVisitor<'tcx> {
                 //
                 // FIXME(ecstaticmorse): Maybe this could be incorporated into `feature_err`? This
                 // is a pretty narrow case, however.
-                if tcx.sess.is_nightly_build() {
-                    for gate in missing_secondary {
-                        // FIXME: make this translatable
-                        #[allow(rustc::diagnostic_outside_of_impl)]
-                        #[allow(rustc::untranslatable_diagnostic)]
-                        err.help(format!(
-                            "add `#![feature({gate})]` to the crate attributes to enable"
-                        ));
-                    }
-                }
+                tcx.disabled_nightly_feature(
+                    &mut err,
+                    def_id.map(|id| tcx.local_def_id_to_hir_id(id)),
+                    missing_secondary.into_iter().map(|gate| (String::new(), *gate)),
+                );
 
                 err.emit();
             }

compiler/rustc_session/messages.ftl

@@ -24,6 +24,9 @@ session_feature_diagnostic_for_issue =
 session_feature_diagnostic_help =
     add `#![feature({$feature})]` to the crate attributes to enable
 
+session_feature_diagnostic_suggestion =
+    add `#![feature({$feature})]` to the crate attributes to enable
+
 session_feature_suggest_upgrade_compiler =
     this compiler was built on {$date}; consider upgrading it if it is out of date
 

compiler/rustc_session/src/errors.rs

@@ -54,6 +54,18 @@ pub struct FeatureDiagnosticHelp {
     pub feature: Symbol,
 }
 
+#[derive(Subdiagnostic)]
+#[suggestion(
+    session_feature_diagnostic_suggestion,
+    applicability = "maybe-incorrect",
+    code = "#![feature({feature})]\n"
+)]
+pub struct FeatureDiagnosticSuggestion {
+    pub feature: Symbol,
+    #[primary_span]
+    pub span: Span,
+}
+
 #[derive(Subdiagnostic)]
 #[help(session_cli_feature_diagnostic_help)]
 pub struct CliFeatureDiagnosticHelp {