Add a new lint: deref coercions

CHANGELOG.md

@@ -5515,6 +5515,7 @@ Released 2018-09-13
 [`deprecated_semver`]: https://rust-lang.github.io/rust-clippy/master/index.html#deprecated_semver
 [`deref_addrof`]: https://rust-lang.github.io/rust-clippy/master/index.html#deref_addrof
 [`deref_by_slicing`]: https://rust-lang.github.io/rust-clippy/master/index.html#deref_by_slicing
+[`deref_coercions`]: https://rust-lang.github.io/rust-clippy/master/index.html#deref_coercions
 [`derivable_impls`]: https://rust-lang.github.io/rust-clippy/master/index.html#derivable_impls
 [`derive_hash_xor_eq`]: https://rust-lang.github.io/rust-clippy/master/index.html#derive_hash_xor_eq
 [`derive_ord_xor_partial_ord`]: https://rust-lang.github.io/rust-clippy/master/index.html#derive_ord_xor_partial_ord

clippy_lints/src/declared_lints.rs

@@ -122,6 +122,7 @@ pub static LINTS: &[&crate::LintInfo] = &[
     crate::default_instead_of_iter_empty::DEFAULT_INSTEAD_OF_ITER_EMPTY_INFO,
     crate::default_numeric_fallback::DEFAULT_NUMERIC_FALLBACK_INFO,
     crate::default_union_representation::DEFAULT_UNION_REPRESENTATION_INFO,
+    crate::deref_coercions::DEREF_COERCIONS_INFO,
     crate::dereference::EXPLICIT_AUTO_DEREF_INFO,
     crate::dereference::EXPLICIT_DEREF_METHODS_INFO,
     crate::dereference::NEEDLESS_BORROW_INFO,

clippy_lints/src/deref_coercions.rs

@@ -0,0 +1,51 @@
+use clippy_utils::diagnostics::span_lint_and_help;
+use rustc_hir::Expr;
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::ty::adjustment::Adjust;
+use rustc_session::declare_lint_pass;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for expressions that use deref coercion.
+    ///
+    /// ### Why is this bad?
+    /// Implicit deref coercion could result in confusing behavior when writing unsafe code.
+    ///
+    /// ### Example
+    /// ```no_run
+    /// let x = &Box::new(true);
+    /// let y: &bool = x;
+    /// ```
+    /// Use instead:
+    /// ```no_run
+    /// use std::ops::Deref;
+    /// let x = &Box::new(true);
+    /// let y: &bool = x.deref();
+    /// ```
+    #[clippy::version = "1.86.0"]
+    pub DEREF_COERCIONS,
+    restriction,
+    "using deref coercion"
+}
+
+declare_lint_pass!(DerefCoercions => [DEREF_COERCIONS]);
+
+impl LateLintPass<'_> for DerefCoercions {
+    fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) {
+        let source = cx.typeck_results().expr_ty(expr).peel_refs();
+        for adjustment in cx.typeck_results().expr_adjustments(expr) {
+            if let Adjust::Deref(_) = adjustment.kind
+                && adjustment.target.peel_refs() != source
+            {
+                span_lint_and_help(
+                    cx,
+                    DEREF_COERCIONS,
+                    expr.span,
+                    "implicit deref coercion",
+                    None,
+                    "consider using `deref() or deref_mut()`",
+                );
+            }
+        }
+    }
+}

clippy_lints/src/lib.rs

@@ -111,6 +111,7 @@ mod default_constructed_unit_structs;
 mod default_instead_of_iter_empty;
 mod default_numeric_fallback;
 mod default_union_representation;
+mod deref_coercions;
 mod dereference;
 mod derivable_impls;
 mod derive;
@@ -976,5 +977,6 @@ pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
     store.register_late_pass(|_| Box::new(unneeded_struct_pattern::UnneededStructPattern));
     store.register_late_pass(|_| Box::<unnecessary_semicolon::UnnecessarySemicolon>::default());
     store.register_late_pass(move |_| Box::new(non_std_lazy_statics::NonStdLazyStatic::new(conf)));
+    store.register_late_pass(|_| Box::new(deref_coercions::DerefCoercions));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }

tests/ui/deref_coercions.rs

@@ -0,0 +1,7 @@
+#![warn(clippy::deref_coercions)]
+use std::ops::Deref;
+
+fn main() {
+    let x = &Box::new(true);
+    let _: &bool = x;
+}

tests/ui/deref_coercions.stderr

@@ -0,0 +1,12 @@
+error: implicit deref coercion
+  --> tests/ui/deref_coercions.rs:6:20
+   |
+LL |     let _: &bool = x;
+   |                    ^
+   |
+   = help: consider using `deref() or deref_mut()`
+   = note: `-D clippy::deref-coercions` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::deref_coercions)]`
+
+error: aborting due to 1 previous error
+