Turn 'useless #[deprecated]' error into a lint.

compiler/rustc_lint_defs/src/builtin.rs

@@ -2705,6 +2705,32 @@ declare_lint! {
     };
 }
 
+declare_lint! {
+    /// The `useless_deprecated` lint detects deprecation attributes with no effect.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// struct X;
+    ///
+    /// #[deprecated = "message"]
+    /// impl Default for X {
+    ///     fn default() -> Self {
+    ///         X
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Deprecation attributes have no effect on trait implementations.
+    pub USELESS_DEPRECATED,
+    Deny,
+    "detects deprecation attributes with no effect",
+}
+
 declare_tool_lint! {
     pub rustc::INEFFECTIVE_UNSTABLE_TRAIT_IMPL,
     Deny,
@@ -2792,6 +2818,7 @@ declare_lint_pass! {
         INEFFECTIVE_UNSTABLE_TRAIT_IMPL,
         UNINHABITED_STATIC,
         FUNCTION_ITEM_REFERENCES,
+        USELESS_DEPRECATED,
     ]
 }
 

compiler/rustc_passes/src/stability.rs

@@ -15,7 +15,7 @@ use rustc_middle::middle::privacy::AccessLevels;
 use rustc_middle::middle::stability::{DeprecationEntry, Index};
 use rustc_middle::ty::{self, query::Providers, TyCtxt};
 use rustc_session::lint;
-use rustc_session::lint::builtin::INEFFECTIVE_UNSTABLE_TRAIT_IMPL;
+use rustc_session::lint::builtin::{INEFFECTIVE_UNSTABLE_TRAIT_IMPL, USELESS_DEPRECATED};
 use rustc_session::parse::feature_err;
 use rustc_session::Session;
 use rustc_span::symbol::{sym, Symbol};
@@ -91,16 +91,16 @@ impl<'a, 'tcx> Annotator<'a, 'tcx> {
             is_deprecated = true;
 
             if kind == AnnotationKind::Prohibited || kind == AnnotationKind::DeprecationProhibited {
-                self.tcx
+                self.tcx.struct_span_lint_hir(USELESS_DEPRECATED, hir_id, *span, |lint| {
-                    .sess
+                    lint.build("this `#[deprecated]' annotation has no effect")
-                    .struct_span_err(*span, "this deprecation annotation is useless")
+                        .span_suggestion(
-                    .span_suggestion(
+                            *span,
-                        *span,
+                            "try removing the deprecation attribute",
-                        "try removing the deprecation attribute",
+                            String::new(),
-                        String::new(),
+                            rustc_errors::Applicability::MachineApplicable,
-                        rustc_errors::Applicability::MachineApplicable,
+                        )
-                    )
+                        .emit()
-                    .emit();
+                });
             }
 
             // `Deprecation` is just two pointers, no need to intern it