Migration lint

Rustfix remains TODO

1 files changed, 36 insertions, 0 deletions

diff --git a/compiler/rustc_lint_defs/src/builtin.rs b/compiler/rustc_lint_defs/src/builtin.rs
index 2713690f812..bc36a587a9e 100644
--- a/compiler/rustc_lint_defs/src/builtin.rs
+++ b/compiler/rustc_lint_defs/src/builtin.rs
@@ -38,6 +38,7 @@ declare_lint_pass! {
         DEPRECATED_CFG_ATTR_CRATE_TYPE_NAME,
         DEPRECATED_IN_FUTURE,
         DEPRECATED_WHERE_CLAUSE_LOCATION,
+        DEREFERENCING_MUT_BINDING,
         DUPLICATE_MACRO_ATTRIBUTES,
         ELIDED_LIFETIMES_IN_ASSOCIATED_CONSTANT,
         ELIDED_LIFETIMES_IN_PATHS,
@@ -1628,6 +1629,41 @@ declare_lint! {
 }
 
 declare_lint! {
+    /// The `dereferencing_mut_binding` lint detects a `mut x` pattern that resets the binding mode,
+    /// as this behavior will change in rust 2024.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// # #![warn(dereferencing_mut_binding)]
+    /// let x = Some(123u32);
+    /// let _y = match &x {
+    ///     Some(mut x) => {
+    ///         x += 1;
+    ///         x
+    ///     }
+    ///     None => 0,
+    /// };
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Without the `mut`, `x` would have type `&u32`. Pre-2024, adding `mut` makes `x` have type
+    /// `u32`, which was deeped surprising. After edition 2024, adding `mut` will not change the
+    /// type of `x`. This lint warns users of editions before 2024 to update their code.
+    pub DEREFERENCING_MUT_BINDING,
+    Allow,
+    "detects `mut x` bindings that change the type of `x`",
+    @feature_gate = sym::mut_dont_reset_binding_mode_2024;
+    @future_incompatible = FutureIncompatibleInfo {
+        reason: FutureIncompatibilityReason::EditionSemanticsChange(Edition::Edition2024),
+        reference: "123076",
+    };
+}
+
+declare_lint! {
     /// The `unconditional_recursion` lint detects functions that cannot
     /// return without calling themselves.
     ///