Implement lint for definition site item shadowing too

1 files changed, 41 insertions, 1 deletions

diff --git a/compiler/rustc_lint_defs/src/builtin.rs b/compiler/rustc_lint_defs/src/builtin.rs
index 55fa7d9d86c..10bf4ec77ed 100644
--- a/compiler/rustc_lint_defs/src/builtin.rs
+++ b/compiler/rustc_lint_defs/src/builtin.rs
@@ -101,6 +101,7 @@ declare_lint_pass! {
         SINGLE_USE_LIFETIMES,
         SOFT_UNSTABLE,
         STABLE_FEATURES,
+        SUPERTRAIT_ITEM_SHADOWING_DEFINITION,
         SUPERTRAIT_ITEM_SHADOWING_USAGE,
         TAIL_EXPR_DROP_ORDER,
         TEST_UNSTABLE_LINT,
@@ -4924,7 +4925,7 @@ declare_lint! {
     ///
     /// ### Example
     ///
-    /// ```rust
+    /// ```rust,compile_fail
     /// #![feature(supertrait_item_shadowing)]
     /// #![deny(supertrait_item_shadowing_usage)]
     ///
@@ -4960,6 +4961,45 @@ declare_lint! {
 }
 
 declare_lint! {
+    /// The `supertrait_item_shadowing_definition` lint detects when the
+    /// definition of an item that is provided by both a subtrait and
+    /// supertrait is shadowed, preferring the subtrait.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// #![feature(supertrait_item_shadowing)]
+    /// #![deny(supertrait_item_shadowing_definition)]
+    ///
+    /// trait Upstream {
+    ///     fn hello(&self) {}
+    /// }
+    /// impl<T> Upstream for T {}
+    ///
+    /// trait Downstream: Upstream {
+    ///     fn hello(&self) {}
+    /// }
+    /// impl<T> Downstream for T {}
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// RFC 3624 specified a heuristic in which a supertrait item would be
+    /// shadowed by a subtrait item when ambiguity occurs during item
+    /// selection. In order to mitigate side-effects of this happening
+    /// silently, this lint detects these cases when users want to deny them
+    /// or fix their trait definitions.
+    pub SUPERTRAIT_ITEM_SHADOWING_DEFINITION,
+    // FIXME(supertrait_item_shadowing): It is not decided if this should
+    // warn by default at the usage site.
+    Allow,
+    "detects when a supertrait item is shadowed by a subtrait item",
+    @feature_gate = supertrait_item_shadowing;
+}
+
+declare_lint! {
     /// The `ptr_to_integer_transmute_in_consts` lint detects pointer to integer
     /// transmute in const functions and associated constants.
     ///