Improve diagnostics for the use of unstable library features

- only emits one error/lint (instead of one per missing feature) per usage of unstable and
  body-unstable items
- only emits one future-name-collision lint (instead of one per missing feature) for unstable trait
  items
- makes diagnostics for unstable, soft-unstable, const-unstable, and body-unstable library features
  translatable, using common subdiagnostic structs.
- adds notes with features, reasons, and issue links to const-unstability errors
- adds notes with issue links to soft_unstable lints
- on nightly, adds `#![feature]` crate attr help to soft_unstable lints
- on nightly, adds compiler-upgrade-suggestion notes to const-unstable and soft_unstable diagnostics

compiler/rustc_const_eval/src/check_consts/check.rs

@@ -716,7 +716,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                         let feature_gate_declared = gate_declared(feature);
                         let implied_gate_declared = implied_by.is_some_and(gate_declared);
                         if !feature_gate_declared && !implied_gate_declared {
-                            bad_gates.push(feature);
+                            bad_gates.push(gate.into());
                             continue;
                         }
 
@@ -735,7 +735,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                             continue;
                         }
 
-                        bad_gates.push(feature);
+                        bad_gates.push(gate.into());
                     }
                     if !bad_gates.is_empty() {
                         self.check_op(ops::FnCallUnstable(callee, bad_gates));

compiler/rustc_const_eval/src/check_consts/ops.rs

@@ -8,6 +8,7 @@ use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_infer::traits::{ImplSource, Obligation, ObligationCause};
+use rustc_middle::middle::stability;
 use rustc_middle::mir::CallSource;
 use rustc_middle::span_bug;
 use rustc_middle::ty::print::{PrintTraitRefExt as _, with_no_trimmed_paths};
@@ -298,28 +299,35 @@ impl<'tcx> NonConstOp<'tcx> for FnCallNonConst<'tcx> {
 ///
 /// Contains the names of the features that would allow the use of this function.
 #[derive(Debug)]
-pub(crate) struct FnCallUnstable(pub DefId, pub Vec<Symbol>);
+pub(crate) struct FnCallUnstable(pub DefId, pub Vec<stability::EvalDenial>);
 
 impl<'tcx> NonConstOp<'tcx> for FnCallUnstable {
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> Diag<'tcx> {
-        let FnCallUnstable(def_id, features) = self;
-
-        let mut err = ccx
-            .dcx()
-            .create_err(errors::UnstableConstFn { span, def_path: ccx.tcx.def_path_str(def_id) });
-
-        // FIXME: make this translatable
-        #[allow(rustc::untranslatable_diagnostic)]
-        if ccx.is_const_stable_const_fn() {
-            err.help(fluent_generated::const_eval_const_stable);
-        } else if ccx.tcx.sess.is_nightly_build() && !features.is_empty() {
-            err.help(format!(
-                "add `#![feature({})]` to the crate attributes to enable",
-                features.iter().map(Symbol::as_str).intersperse(", ").collect::<String>(),
-            ));
-        }
+        let FnCallUnstable(def_id, denials) = self;
 
-        err
+        let in_const_stable_context = ccx.is_const_stable_const_fn();
+        // Only report unstable features when they're present.
+        let (features, info) = if denials.is_empty() {
+            (None, vec![])
+        } else {
+            let (features, info) = stability::unstable_notes(&denials);
+            (Some(features), info)
+        };
+        // Only suggest adding `#![feature]` if it could help.
+        let nightly_subdiags = if in_const_stable_context || denials.is_empty() {
+            vec![]
+        } else {
+            stability::unstable_nightly_subdiags(&ccx.tcx.sess, &denials, None)
+        };
+
+        ccx.dcx().create_err(errors::UnstableConstFn {
+            span,
+            def_path: ccx.tcx.def_path_str(def_id),
+            in_const_stable_context,
+            features,
+            info,
+            nightly_subdiags,
+        })
     }
 }
 

compiler/rustc_const_eval/src/errors.rs

@@ -115,6 +115,14 @@ pub(crate) struct UnstableConstFn {
     #[primary_span]
     pub span: Span,
     pub def_path: String,
+    #[help(const_eval_const_stable)]
+    pub in_const_stable_context: bool,
+    #[subdiagnostic]
+    pub features: Option<rustc_middle::error::UnstableLibraryFeatureNote>,
+    #[subdiagnostic]
+    pub info: Vec<rustc_middle::error::UnstableLibraryFeatureInfo>,
+    #[subdiagnostic]
+    pub nightly_subdiags: Vec<rustc_session::errors::NightlyFeatureDiagnostic>,
 }
 
 #[derive(Diagnostic)]

compiler/rustc_hir_analysis/messages.ftl

@@ -309,8 +309,6 @@ hir_analysis_missing_trait_item_suggestion = implement the missing item: `{$snip
 
 hir_analysis_missing_trait_item_unstable = not all trait items implemented, missing: `{$missing_item_name}`
     .note = default implementation of `{$missing_item_name}` is unstable
-    .some_note = use of unstable library feature `{$feature}`: {$reason}
-    .none_note = use of unstable library feature `{$feature}`
 
 hir_analysis_missing_type_params =
     the type {$parameterCount ->

compiler/rustc_hir_analysis/src/check/check.rs

@@ -933,14 +933,7 @@ fn check_impl_items_against_trait<'tcx>(
                 let full_impl_span = tcx.hir().span_with_body(tcx.local_def_id_to_hir_id(impl_id));
                 match tcx.eval_default_body_stability(trait_item_id, full_impl_span) {
                     EvalResult::Deny { denials, .. } => {
-                        for denial in denials {
-                            default_body_is_unstable(
-                                tcx,
-                                full_impl_span,
-                                trait_item_id,
-                                denial.unstability,
-                            );
-                        }
+                        default_body_is_unstable(tcx, full_impl_span, trait_item_id, &denials);
                     }
 
                     // Unmarked default bodies are considered stable (at least for now).

compiler/rustc_hir_analysis/src/check/mod.rs

@@ -80,6 +80,7 @@ use rustc_index::bit_set::BitSet;
 use rustc_infer::infer::outlives::env::OutlivesEnvironment;
 use rustc_infer::infer::{self, TyCtxtInferExt as _};
 use rustc_infer::traits::ObligationCause;
+use rustc_middle::middle::stability;
 use rustc_middle::query::Providers;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
 use rustc_middle::ty::{self, GenericArgs, GenericArgsRef, Ty, TyCtxt};
@@ -293,41 +294,22 @@ fn default_body_is_unstable(
     tcx: TyCtxt<'_>,
     impl_span: Span,
     item_did: DefId,
-    unstability: rustc_attr::Unstability,
+    denials: &[stability::EvalDenial],
 ) {
-    let rustc_attr::Unstability { feature, reason, issue, .. } = unstability;
     let missing_item_name = tcx.associated_item(item_did).name;
-    let (mut some_note, mut none_note, mut reason_str) = (false, false, String::new());
-    match reason.to_opt_reason() {
-        Some(r) => {
-            some_note = true;
-            reason_str = r.to_string();
-        }
-        None => none_note = true,
-    };
-
-    let mut err = tcx.dcx().create_err(errors::MissingTraitItemUnstable {
-        span: impl_span,
-        some_note,
-        none_note,
-        missing_item_name,
-        feature,
-        reason: reason_str,
-    });
-
     let inject_span = item_did
         .as_local()
         .and_then(|id| tcx.crate_level_attribute_injection_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,
-    );
+    let (features, info) = stability::unstable_notes(denials);
+    let nightly_subdiags = stability::unstable_nightly_subdiags(&tcx.sess, denials, inject_span);
 
-    err.emit();
+    tcx.dcx().emit_err(errors::MissingTraitItemUnstable {
+        span: impl_span,
+        missing_item_name,
+        features,
+        info,
+        nightly_subdiags,
+    });
 }
 
 /// Re-sugar `ty::GenericPredicates` in a way suitable to be used in structured suggestions.

compiler/rustc_hir_analysis/src/errors.rs

@@ -977,13 +977,13 @@ pub(crate) struct MissingOneOfTraitItem {
 pub(crate) struct MissingTraitItemUnstable {
     #[primary_span]
     pub span: Span,
-    #[note(hir_analysis_some_note)]
-    pub some_note: bool,
-    #[note(hir_analysis_none_note)]
-    pub none_note: bool,
     pub missing_item_name: Symbol,
-    pub feature: Symbol,
-    pub reason: String,
+    #[subdiagnostic]
+    pub features: rustc_middle::error::UnstableLibraryFeatureNote,
+    #[subdiagnostic]
+    pub info: Vec<rustc_middle::error::UnstableLibraryFeatureInfo>,
+    #[subdiagnostic]
+    pub nightly_subdiags: Vec<rustc_session::errors::NightlyFeatureDiagnostic>,
 }
 
 #[derive(Diagnostic)]

compiler/rustc_hir_typeck/src/lib.rs

@@ -4,6 +4,7 @@
 #![feature(array_windows)]
 #![feature(box_patterns)]
 #![feature(if_let_guard)]
+#![feature(iter_intersperse)]
 #![feature(let_chains)]
 #![feature(never_type)]
 #![feature(try_blocks)]

compiler/rustc_hir_typeck/src/method/probe.rs

@@ -1329,10 +1329,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                 if let stability::EvalResult::Deny { denials, .. } =
                     self.tcx.eval_stability(candidate.item.def_id, None, self.span, None)
                 {
-                    uc.push((
-                        candidate.clone(),
-                        denials.iter().map(|d| d.unstability.feature).collect(),
-                    ));
+                    uc.push((candidate.clone(), denials.iter().map(|d| d.feature).collect()));
                     return false;
                 }
                 true
@@ -1431,10 +1428,11 @@ impl<'tcx> Pick<'tcx> {
             tcx.disabled_nightly_features(
                 lint,
                 Some(scope_expr_id),
-                self.unstable_candidates.iter().flat_map(|(candidate, features)| {
-                    features.iter().map(|feature| {
-                        (format!(" `{}`", tcx.def_path_str(candidate.item.def_id)), *feature)
-                    })
+                self.unstable_candidates.iter().map(|(candidate, features)| {
+                    (
+                        format!(" `{}`", tcx.def_path_str(candidate.item.def_id)),
+                        features.iter().map(Symbol::as_str).intersperse(", ").collect::<String>(),
+                    )
                 }),
             );
         });

compiler/rustc_lint/src/context/diagnostics.rs

@@ -382,8 +382,12 @@ pub(super) fn decorate_lint(sess: &Session, diagnostic: BuiltinLintDiag, diag: &
         BuiltinLintDiag::MacroRuleNeverUsed(n, name) => {
             lints::MacroRuleNeverUsed { n: n + 1, name }.decorate_lint(diag);
         }
-        BuiltinLintDiag::UnstableFeature(msg) => {
-            lints::UnstableFeature { msg }.decorate_lint(diag);
+        BuiltinLintDiag::SoftUnstableMacro { features } => {
+            let denials = features
+                .iter()
+                .map(|&(feature, reason, issue)| stability::EvalDenial { feature, reason, issue })
+                .collect::<Vec<_>>();
+            stability::soft_unstable(sess, &denials, vec![]).decorate_lint(diag);
         }
         BuiltinLintDiag::AvoidUsingIntelSyntax => {
             lints::AvoidIntelSyntax.decorate_lint(diag);

compiler/rustc_lint/src/lints.rs

@@ -2404,16 +2404,6 @@ pub(crate) struct MacroRuleNeverUsed {
     pub name: Symbol,
 }
 
-pub(crate) struct UnstableFeature {
-    pub msg: DiagMessage,
-}
-
-impl<'a> LintDiagnostic<'a, ()> for UnstableFeature {
-    fn decorate_lint<'b>(self, diag: &'b mut Diag<'a, ()>) {
-        diag.primary_message(self.msg);
-    }
-}
-
 #[derive(LintDiagnostic)]
 #[diag(lint_avoid_intel_syntax)]
 pub(crate) struct AvoidIntelSyntax;

compiler/rustc_lint_defs/src/lib.rs

@@ -2,13 +2,15 @@
 #![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
+use std::num::NonZero;
+
 use rustc_ast::node_id::NodeId;
 use rustc_ast::{AttrId, Attribute};
 use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
 use rustc_data_structures::stable_hasher::{
     HashStable, StableCompare, StableHasher, ToStableHashKey,
 };
-use rustc_error_messages::{DiagMessage, MultiSpan};
+use rustc_error_messages::MultiSpan;
 use rustc_hir::def::Namespace;
 use rustc_hir::{HashStableContext, HirId, MissingLifetimeKind};
 use rustc_macros::{Decodable, Encodable, HashStable_Generic};
@@ -715,7 +717,10 @@ pub enum BuiltinLintDiag {
     MacroIsPrivate(Ident),
     UnusedMacroDefinition(Symbol),
     MacroRuleNeverUsed(usize, Symbol),
-    UnstableFeature(DiagMessage),
+    SoftUnstableMacro {
+        /// The name, optional reason, and issue number for each disabled unstable feature used.
+        features: Vec<(Symbol, Option<Symbol>, Option<NonZero<u32>>)>,
+    },
     AvoidUsingIntelSyntax,
     AvoidUsingAttSyntax,
     IncompleteInclude,

compiler/rustc_middle/messages.ftl

@@ -102,6 +102,25 @@ middle_type_length_limit = reached the type-length limit while instantiating `{$
 middle_unknown_layout =
     the type `{$ty}` has an unknown layout
 
+middle_unstable_library_feature = use of unstable library {$count ->
+    [one] feature {$features}{$single_feature_has_reason ->
+        [true] : {$reason_for_single_feature}
+        *[false] {""}
+    }
+    *[other] features {$features}
+}
+
+middle_unstable_library_feature_issue =
+    see issue #{$issue} <https://github.com/rust-lang/rust/issues/{$issue}> for more information{$show_feature ->
+        [true] {" "}about `{$feature}`
+        *[false] {""}
+    }
+
+middle_unstable_library_feature_reason = reason for `{$feature}`: {$reason}
+
+middle_unstable_library_feature_suggestion_for_allocator_api =
+    consider wrapping the inner types in tuple
+
 middle_values_too_big =
     values of the type `{$ty}` are too big for the target architecture
 middle_written_to_path = the full type name has been written to '{$path}'