Suppress redundant listing of unstable library features used on nightly

On nightly, there's a suggestion/help containing the `#![feature(..)]`
attribute required to enable any required unstable features, so the note
listing the features is unnecessary. This only applies to const-unstable
and default-body-unstable errors. Normal "use of unstable library
feature" errors list the features in the error message, so it doesn't
feel redundant.

Author: dianne

compiler/rustc_const_eval/src/check_consts/ops.rs

@@ -328,7 +328,8 @@ impl<'tcx> NonConstOp<'tcx> for FnCallUnstable {
         ccx.dcx().create_err(errors::UnstableConstFn {
             span,
             def_path: ccx.tcx.def_path_str(self.def_id),
-            features: stability::unstable_message(&self.features, self.reason.to_opt_reason()),
+            features: stability::unstable_message(&self.features, self.reason.to_opt_reason())
+                .hide_features_on_nightly(&ccx.tcx.sess),
             issues: stability::unstable_issues(&self.features),
             nightly_subdiags: stability::unstable_nightly_subdiags(
                 &ccx.tcx.sess,
@@ -379,7 +380,8 @@ impl<'tcx> NonConstOp<'tcx> for IntrinsicUnstable {
         ccx.dcx().create_err(errors::UnstableIntrinsic {
             span,
             name: self.name,
-            features: stability::unstable_message(&self.features, self.reason.to_opt_reason()),
+            features: stability::unstable_message(&self.features, self.reason.to_opt_reason())
+                .hide_features_on_nightly(&ccx.tcx.sess),
             issues: stability::unstable_issues(&self.features),
             nightly_subdiags: stability::unstable_nightly_subdiags(
                 &ccx.tcx.sess,

compiler/rustc_const_eval/src/errors.rs

@@ -120,7 +120,7 @@ pub(crate) struct UnstableConstFn {
     pub span: Span,
     pub def_path: String,
     #[subdiagnostic]
-    pub features: rustc_middle::error::UnstableLibraryFeatureNote,
+    pub features: Option<rustc_middle::error::UnstableLibraryFeatureNote>,
     #[subdiagnostic]
     pub issues: Vec<rustc_middle::error::UnstableLibraryFeatureIssue>,
     #[subdiagnostic]
@@ -134,7 +134,7 @@ pub(crate) struct UnstableIntrinsic {
     pub span: Span,
     pub name: Symbol,
     #[subdiagnostic]
-    pub features: rustc_middle::error::UnstableLibraryFeatureNote,
+    pub features: Option<rustc_middle::error::UnstableLibraryFeatureNote>,
     #[subdiagnostic]
     pub issues: Vec<rustc_middle::error::UnstableLibraryFeatureIssue>,
     #[subdiagnostic]

compiler/rustc_hir_analysis/src/check/mod.rs

@@ -300,11 +300,10 @@ fn default_body_is_unstable(
     let inject_span = item_did
         .as_local()
         .and_then(|id| tcx.crate_level_attribute_injection_span(tcx.local_def_id_to_hir_id(id)));
-
     tcx.dcx().emit_err(errors::MissingTraitItemUnstable {
         span: impl_span,
         missing_item_name,
-        features: stability::unstable_message(denials, reason),
+        features: stability::unstable_message(denials, reason).hide_features_on_nightly(&tcx.sess),
         issues: stability::unstable_issues(denials),
         nightly_subdiags: stability::unstable_nightly_subdiags(&tcx.sess, denials, inject_span),
     });

compiler/rustc_hir_analysis/src/errors.rs

@@ -979,7 +979,7 @@ pub(crate) struct MissingTraitItemUnstable {
     pub span: Span,
     pub missing_item_name: Symbol,
     #[subdiagnostic]
-    pub features: rustc_middle::error::UnstableLibraryFeatureNote,
+    pub features: Option<rustc_middle::error::UnstableLibraryFeatureNote>,
     #[subdiagnostic]
     pub issues: Vec<rustc_middle::error::UnstableLibraryFeatureIssue>,
     #[subdiagnostic]

compiler/rustc_middle/messages.ftl

@@ -117,6 +117,19 @@ middle_unstable_library_feature_issue =
         *[false] {""}
     }
 
+middle_unstable_library_feature_note =
+    {$only_show_reason ->
+        [true] reason for unstability: {$reason}
+        *[false] -> use of unstable library {$count ->
+            [one] feature
+            *[other] features
+        } {$features}{STREQ($reason, "") ->
+            [true] {""}
+            *[false] : {$reason}
+        }
+    }
+
+
 middle_unstable_library_feature_suggestion_for_allocator_api =
     consider wrapping the inner types in tuple
 

compiler/rustc_middle/src/error.rs

@@ -199,11 +199,28 @@ pub struct SoftUnstableLibraryFeature {
 
 /// A note for other diagnostics which report unstable library features
 #[derive(Subdiagnostic)]
-#[note(middle_unstable_library_feature)]
+#[note(middle_unstable_library_feature_note)]
 pub struct UnstableLibraryFeatureNote {
     pub features: DiagSymbolList,
     pub count: usize,
     pub reason: String,
+    pub only_show_reason: bool,
+}
+
+impl UnstableLibraryFeatureNote {
+    /// On nightly, in errors for const-unstable and default-body-unstable items, suppress the
+    /// feature list, since it's redundant with the `#![feature(...)]` suggestion.
+    pub fn hide_features_on_nightly(self, sess: &rustc_session::Session) -> Option<Self> {
+        if sess.is_nightly_build() {
+            if self.reason.is_empty() {
+                None
+            } else {
+                Some(UnstableLibraryFeatureNote { only_show_reason: true, ..self })
+            }
+        } else {
+            Some(self)
+        }
+    }
 }
 
 #[derive(Subdiagnostic)]

compiler/rustc_middle/src/middle/stability.rs

@@ -118,6 +118,7 @@ pub fn unstable_message(
         features: denials.iter().map(|d| d.feature).collect(),
         count: denials.len(),
         reason: reason.map(|r| r.to_string()).unwrap_or_default(),
+        only_show_reason: false,
     }
 }
 
@@ -164,7 +165,8 @@ pub fn report_unstable(
     suggestions: Vec<UnstableLibraryFeatureSugg>,
     span: Span,
 ) {
-    let UnstableLibraryFeatureNote { features, count, reason } = unstable_message(denials, reason);
+    let UnstableLibraryFeatureNote { features, count, reason, .. } =
+        unstable_message(denials, reason);
     let issues = unstable_issues(denials);
     let nightly_subdiags = unstable_nightly_subdiags(sess, denials, None);
 
@@ -186,7 +188,8 @@ pub fn soft_unstable(
     reason: Option<Symbol>,
     suggestions: Vec<UnstableLibraryFeatureSugg>,
 ) -> SoftUnstableLibraryFeature {
-    let UnstableLibraryFeatureNote { features, count, reason } = unstable_message(denials, reason);
+    let UnstableLibraryFeatureNote { features, count, reason, .. } =
+        unstable_message(denials, reason);
     let mut issues = unstable_issues(denials);
     let nightly_subdiags = unstable_nightly_subdiags(sess, denials, None);
 

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

@@ -32,7 +32,6 @@ error: `min_align_of_val` is not yet stable as a const intrinsic
 LL |         unstable_intrinsic::min_align_of_val(&x);
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: use of unstable library feature `unstable`
    = note: see issue #42 <https://github.com/rust-lang/rust/issues/42> for more information
    = help: add `#![feature(unstable)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date

tests/ui/stability-attribute/const-stability-attribute-implies-no-feature.stderr

@@ -4,7 +4,6 @@ error: `foobar` is not yet stable as a const fn
 LL |     foobar();
    |     ^^^^^^^^
    |
-   = note: use of unstable library feature `const_foobar`
    = note: see issue #1 <https://github.com/rust-lang/rust/issues/1> for more information
    = help: add `#![feature(const_foobar)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date

tests/ui/stability-attribute/default-body-stability-err.stderr

@@ -5,7 +5,6 @@ LL | impl JustTrait for Type {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: default implementation of `CONSTANT` is unstable
-   = note: use of unstable library feature `constant_default_body`
    = help: add `#![feature(constant_default_body)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
@@ -16,7 +15,6 @@ LL | impl JustTrait for Type {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: default implementation of `fun` is unstable
-   = note: use of unstable library feature `fun_default_body`
    = help: add `#![feature(fun_default_body)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
@@ -27,7 +25,7 @@ LL | impl JustTrait for Type {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: default implementation of `fun2` is unstable
-   = note: use of unstable library feature `fun_default_body`: reason
+   = note: reason for unstability: reason
    = help: add `#![feature(fun_default_body)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
@@ -43,7 +41,6 @@ LL | | }
    | |_^
    |
    = note: default implementation of `eq` is unstable
-   = note: use of unstable library feature `eq_default_body`
    = help: add `#![feature(eq_default_body)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date