Rollup merge of #78626 - fusion-engineering-forks:deprecated-trait-impl, r=estebank

Improve errors about #[deprecated] attribute

This change:

1. Turns `#[deprecated]` on a trait impl block into an error, which fixes #78625;
2. Changes these and other errors about `#[deprecated]` to use the span of the attribute instead of the item; and
3. Turns this error into a lint, to make sure it can be capped with `--cap-lints` and doesn't break any existing dependencies.

Can be reviewed per commit.

---
Example:
```rust
struct X;

#[deprecated = "a"]
impl Default for X {
    #[deprecated = "b"]
    fn default() -> Self {
        X
    }
}
```

Before:
```
error: This deprecation annotation is useless
 --> src/main.rs:6:5
  |
6 | /     fn default() -> Self {
7 | |         X
8 | |     }
  | |_____^
```

After:
```
error: this `#[deprecated]' annotation has no effect
 --> src/main.rs:3:1
  |
3 | #[deprecated = "a"]
  | ^^^^^^^^^^^^^^^^^^^ help: try removing the deprecation attribute
  |
  = note: `#[deny(useless_deprecated)]` on by default

error: this `#[deprecated]' annotation has no effect
 --> src/main.rs:5:5
  |
5 |     #[deprecated = "b"]
  |     ^^^^^^^^^^^^^^^^^^^ help: try removing the deprecation attribute
```

8 files changed, 96 insertions, 38 deletions

diff --git a/compiler/rustc_attr/src/builtin.rs b/compiler/rustc_attr/src/builtin.rs
index 48238c8bbf5..2fd625c2a6c 100644
--- a/compiler/rustc_attr/src/builtin.rs
+++ b/compiler/rustc_attr/src/builtin.rs
@@ -637,19 +637,15 @@ pub struct Deprecation {
 }
 
 /// Finds the deprecation attribute. `None` if none exists.
-pub fn find_deprecation(sess: &Session, attrs: &[Attribute], item_sp: Span) -> Option<Deprecation> {
-    find_deprecation_generic(sess, attrs.iter(), item_sp)
+pub fn find_deprecation(sess: &Session, attrs: &[Attribute]) -> Option<(Deprecation, Span)> {
+    find_deprecation_generic(sess, attrs.iter())
 }
 
-fn find_deprecation_generic<'a, I>(
-    sess: &Session,
-    attrs_iter: I,
-    item_sp: Span,
-) -> Option<Deprecation>
+fn find_deprecation_generic<'a, I>(sess: &Session, attrs_iter: I) -> Option<(Deprecation, Span)>
 where
     I: Iterator<Item = &'a Attribute>,
 {
-    let mut depr: Option<Deprecation> = None;
+    let mut depr: Option<(Deprecation, Span)> = None;
     let diagnostic = &sess.parse_sess.span_diagnostic;
 
     'outer: for attr in attrs_iter {
@@ -658,8 +654,11 @@ where
             continue;
         }
 
-        if depr.is_some() {
-            struct_span_err!(diagnostic, item_sp, E0550, "multiple deprecated attributes").emit();
+        if let Some((_, span)) = &depr {
+            struct_span_err!(diagnostic, attr.span, E0550, "multiple deprecated attributes")
+                .span_label(attr.span, "repeated deprecation attribute")
+                .span_label(*span, "first deprecation attribute")
+                .emit();
             break;
         }
 
@@ -780,7 +779,7 @@ where
         sess.mark_attr_used(&attr);
 
         let is_since_rustc_version = sess.check_name(attr, sym::rustc_deprecated);
-        depr = Some(Deprecation { since, note, suggestion, is_since_rustc_version });
+        depr = Some((Deprecation { since, note, suggestion, is_since_rustc_version }, attr.span));
     }
 
     depr
diff --git a/compiler/rustc_expand/src/base.rs b/compiler/rustc_expand/src/base.rs
index b0e43a260e9..b435def87ac 100644
--- a/compiler/rustc_expand/src/base.rs
+++ b/compiler/rustc_expand/src/base.rs
@@ -793,7 +793,7 @@ impl SyntaxExtension {
             allow_internal_unsafe: sess.contains_name(attrs, sym::allow_internal_unsafe),
             local_inner_macros,
             stability,
-            deprecation: attr::find_deprecation(&sess, attrs, span),
+            deprecation: attr::find_deprecation(&sess, attrs).map(|(d, _)| d),
             helper_attrs,
             edition,
             is_builtin,
diff --git a/compiler/rustc_lint_defs/src/builtin.rs b/compiler/rustc_lint_defs/src/builtin.rs
index 048f096aabe..a1b7c13e4c0 100644
--- a/compiler/rustc_lint_defs/src/builtin.rs
+++ b/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,
     ]
 }
 
diff --git a/compiler/rustc_passes/src/stability.rs b/compiler/rustc_passes/src/stability.rs
index c9497f2a5b2..04b5c65e464 100644
--- a/compiler/rustc_passes/src/stability.rs
+++ b/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};
@@ -31,6 +31,8 @@ enum AnnotationKind {
     Required,
     // Annotation is useless, reject it
     Prohibited,
+    // Deprecation annotation is useless, reject it. (Stability attribute is still required.)
+    DeprecationProhibited,
     // Annotation itself is useless, but it can be propagated to children
     Container,
 }
@@ -83,14 +85,22 @@ impl<'a, 'tcx> Annotator<'a, 'tcx> {
             did_error = self.forbid_staged_api_attrs(hir_id, attrs, inherit_deprecation.clone());
         }
 
-        let depr =
-            if did_error { None } else { attr::find_deprecation(&self.tcx.sess, attrs, item_sp) };
+        let depr = if did_error { None } else { attr::find_deprecation(&self.tcx.sess, attrs) };
         let mut is_deprecated = false;
-        if let Some(depr) = &depr {
+        if let Some((depr, span)) = &depr {
             is_deprecated = true;
 
-            if kind == AnnotationKind::Prohibited {
-                self.tcx.sess.span_err(item_sp, "This deprecation annotation is useless");
+            if kind == AnnotationKind::Prohibited || kind == AnnotationKind::DeprecationProhibited {
+                self.tcx.struct_span_lint_hir(USELESS_DEPRECATED, hir_id, *span, |lint| {
+                    lint.build("this `#[deprecated]` annotation has no effect")
+                        .span_suggestion_short(
+                            *span,
+                            "remove the unnecessary deprecation attribute",
+                            String::new(),
+                            rustc_errors::Applicability::MachineApplicable,
+                        )
+                        .emit()
+                });
             }
 
             // `Deprecation` is just two pointers, no need to intern it
@@ -114,7 +124,7 @@ impl<'a, 'tcx> Annotator<'a, 'tcx> {
             }
         } else {
             self.recurse_with_stability_attrs(
-                depr.map(|d| DeprecationEntry::local(d, hir_id)),
+                depr.map(|(d, _)| DeprecationEntry::local(d, hir_id)),
                 None,
                 None,
                 visit_children,
@@ -139,11 +149,11 @@ impl<'a, 'tcx> Annotator<'a, 'tcx> {
             }
         }
 
-        if depr.as_ref().map_or(false, |d| d.is_since_rustc_version) {
+        if let Some((rustc_attr::Deprecation { is_since_rustc_version: true, .. }, span)) = &depr {
             if stab.is_none() {
                 struct_span_err!(
                     self.tcx.sess,
-                    item_sp,
+                    *span,
                     E0549,
                     "rustc_deprecated attribute must be paired with \
                     either stable or unstable attribute"
@@ -166,7 +176,7 @@ impl<'a, 'tcx> Annotator<'a, 'tcx> {
             // Check if deprecated_since < stable_since. If it is,
             // this is *almost surely* an accident.
             if let (&Some(dep_since), &attr::Stable { since: stab_since }) =
-                (&depr.as_ref().and_then(|d| d.since), &stab.level)
+                (&depr.as_ref().and_then(|(d, _)| d.since), &stab.level)
             {
                 // Explicit version of iter::order::lt to handle parse errors properly
                 for (dep_v, stab_v) in
@@ -212,7 +222,7 @@ impl<'a, 'tcx> Annotator<'a, 'tcx> {
         }
 
         self.recurse_with_stability_attrs(
-            depr.map(|d| DeprecationEntry::local(d, hir_id)),
+            depr.map(|(d, _)| DeprecationEntry::local(d, hir_id)),
             stab,
             const_stab,
             visit_children,
@@ -322,6 +332,7 @@ impl<'a, 'tcx> Visitor<'tcx> for Annotator<'a, 'tcx> {
             }
             hir::ItemKind::Impl { of_trait: Some(_), .. } => {
                 self.in_trait_impl = true;
+                kind = AnnotationKind::DeprecationProhibited;
             }
             hir::ItemKind::Struct(ref sd, _) => {
                 if let Some(ctor_hir_id) = sd.ctor_hir_id() {
diff --git a/src/test/ui/deprecation/deprecation-sanity.rs b/src/test/ui/deprecation/deprecation-sanity.rs
index a559908b792..4fc3fddadb9 100644
--- a/src/test/ui/deprecation/deprecation-sanity.rs
+++ b/src/test/ui/deprecation/deprecation-sanity.rs
@@ -24,10 +24,19 @@ mod bogus_attribute_types_1 {
 }
 
 #[deprecated(since = "a", note = "b")]
-#[deprecated(since = "a", note = "b")]
-fn multiple1() { } //~ ERROR multiple deprecated attributes
+#[deprecated(since = "a", note = "b")] //~ ERROR multiple deprecated attributes
+fn multiple1() { }
 
 #[deprecated(since = "a", since = "b", note = "c")] //~ ERROR multiple 'since' items
 fn f1() { }
 
+struct X;
+
+#[deprecated = "hello"] //~ ERROR this `#[deprecated]` annotation has no effect
+impl Default for X {
+    fn default() -> Self {
+        X
+    }
+}
+
 fn main() { }
diff --git a/src/test/ui/deprecation/deprecation-sanity.stderr b/src/test/ui/deprecation/deprecation-sanity.stderr
index 57143d68105..7e70c35eeab 100644
--- a/src/test/ui/deprecation/deprecation-sanity.stderr
+++ b/src/test/ui/deprecation/deprecation-sanity.stderr
@@ -41,10 +41,12 @@ LL |     #[deprecated("test")]
    |                  ^^^^^^
 
 error[E0550]: multiple deprecated attributes
-  --> $DIR/deprecation-sanity.rs:28:1
+  --> $DIR/deprecation-sanity.rs:27:1
    |
-LL | fn multiple1() { }
-   | ^^^^^^^^^^^^^^^^^^
+LL | #[deprecated(since = "a", note = "b")]
+   | -------------------------------------- first deprecation attribute
+LL | #[deprecated(since = "a", note = "b")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ repeated deprecation attribute
 
 error[E0538]: multiple 'since' items
   --> $DIR/deprecation-sanity.rs:30:27
@@ -52,7 +54,15 @@ error[E0538]: multiple 'since' items
 LL | #[deprecated(since = "a", since = "b", note = "c")]
    |                           ^^^^^^^^^^^
 
-error: aborting due to 9 previous errors
+error: this `#[deprecated]` annotation has no effect
+  --> $DIR/deprecation-sanity.rs:35:1
+   |
+LL | #[deprecated = "hello"]
+   | ^^^^^^^^^^^^^^^^^^^^^^^ help: remove the unnecessary deprecation attribute
+   |
+   = note: `#[deny(useless_deprecated)]` on by default
+
+error: aborting due to 10 previous errors
 
 Some errors have detailed explanations: E0538, E0541, E0550, E0551, E0565.
 For more information about an error, try `rustc --explain E0538`.
diff --git a/src/test/ui/stability-attribute/stability-attribute-sanity.rs b/src/test/ui/stability-attribute/stability-attribute-sanity.rs
index 80d7ae6dc63..abd603b356e 100644
--- a/src/test/ui/stability-attribute/stability-attribute-sanity.rs
+++ b/src/test/ui/stability-attribute/stability-attribute-sanity.rs
@@ -59,14 +59,14 @@ fn multiple3() { }
 
 #[stable(feature = "a", since = "b")]
 #[rustc_deprecated(since = "b", reason = "text")]
-#[rustc_deprecated(since = "b", reason = "text")]
+#[rustc_deprecated(since = "b", reason = "text")] //~ ERROR multiple deprecated attributes
 #[rustc_const_unstable(feature = "c", issue = "none")]
 #[rustc_const_unstable(feature = "d", issue = "none")] //~ ERROR multiple stability levels
-pub const fn multiple4() { } //~ ERROR multiple deprecated attributes
+pub const fn multiple4() { }
 //~^ ERROR Invalid stability or deprecation version found
 
 #[rustc_deprecated(since = "a", reason = "text")]
 fn deprecated_without_unstable_or_stable() { }
-//~^ ERROR rustc_deprecated attribute must be paired with either stable or unstable attribute
+//~^^ ERROR rustc_deprecated attribute must be paired with either stable or unstable attribute
 
 fn main() { }
diff --git a/src/test/ui/stability-attribute/stability-attribute-sanity.stderr b/src/test/ui/stability-attribute/stability-attribute-sanity.stderr
index 134c657c620..bf2436a535f 100644
--- a/src/test/ui/stability-attribute/stability-attribute-sanity.stderr
+++ b/src/test/ui/stability-attribute/stability-attribute-sanity.stderr
@@ -83,10 +83,12 @@ LL | #[stable(feature = "a", since = "b")]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0550]: multiple deprecated attributes
-  --> $DIR/stability-attribute-sanity.rs:65:1
+  --> $DIR/stability-attribute-sanity.rs:62:1
    |
-LL | pub const fn multiple4() { }
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | #[rustc_deprecated(since = "b", reason = "text")]
+   | ------------------------------------------------- first deprecation attribute
+LL | #[rustc_deprecated(since = "b", reason = "text")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ repeated deprecation attribute
 
 error[E0544]: multiple stability levels
   --> $DIR/stability-attribute-sanity.rs:64:1
@@ -101,10 +103,10 @@ LL | pub const fn multiple4() { }
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0549]: rustc_deprecated attribute must be paired with either stable or unstable attribute
-  --> $DIR/stability-attribute-sanity.rs:69:1
+  --> $DIR/stability-attribute-sanity.rs:68:1
    |
-LL | fn deprecated_without_unstable_or_stable() { }
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | #[rustc_deprecated(since = "a", reason = "text")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 18 previous errors