New Lint [`impl_hash_with_borrow_str_and_bytes`]

Implements a lint to prevent implementation of Hash, Borrow<str> and
Borrow<[u8]> as it breaks Borrow<T> "semantics". According to the book,
types that implement Borrow<A> and Borrow<B> must ensure equality of
borrow results under Eq,Ord and Hash.

> In particular Eq, Ord and Hash must be equivalent for borrowed and
owned values: x.borrow() == y.borrow() should give the same result as x == y.

In the same way, hash(x) == hash(x as Borrow<[u8]>) != hash(x as Borrow<str>).

changelog: newlint [`impl_hash_with_borrow_str_and_bytes`]

3 files changed, 109 insertions, 0 deletions

diff --git a/clippy_lints/src/declared_lints.rs b/clippy_lints/src/declared_lints.rs
index 85854a0dfb7..55dedc0a658 100644
--- a/clippy_lints/src/declared_lints.rs
+++ b/clippy_lints/src/declared_lints.rs
@@ -204,6 +204,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::if_not_else::IF_NOT_ELSE_INFO,
     crate::if_then_some_else_none::IF_THEN_SOME_ELSE_NONE_INFO,
     crate::ignored_unit_patterns::IGNORED_UNIT_PATTERNS_INFO,
+    crate::impl_hash_with_borrow_str_and_bytes::IMPL_HASH_BORROW_WITH_STR_AND_BYTES_INFO,
     crate::implicit_hasher::IMPLICIT_HASHER_INFO,
     crate::implicit_return::IMPLICIT_RETURN_INFO,
     crate::implicit_saturating_add::IMPLICIT_SATURATING_ADD_INFO,
diff --git a/clippy_lints/src/impl_hash_with_borrow_str_and_bytes.rs b/clippy_lints/src/impl_hash_with_borrow_str_and_bytes.rs
new file mode 100644
index 00000000000..9374582b54b
--- /dev/null
+++ b/clippy_lints/src/impl_hash_with_borrow_str_and_bytes.rs
@@ -0,0 +1,106 @@
+use clippy_utils::diagnostics::span_lint_and_then;
+use clippy_utils::ty::implements_trait;
+use rustc_hir::def::{DefKind, Res};
+use rustc_hir::{Item, ItemKind, Path, TraitRef};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::ty::Ty;
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::symbol::sym;
+
+declare_clippy_lint! {
+    /// ### What it does
+    ///
+    /// This lint is concerned with the semantics of `Borrow` and `Hash` for a
+    /// type that implements all three of `Hash`, `Borrow<str>` and `Borrow<[u8]>`
+    /// as it is impossible to satisfy the semantics of Borrow and `Hash` for
+    /// both `Borrow<str>` and `Borrow<[u8]>`.
+    ///
+    /// ### Why is this bad?
+    ///
+    /// When providing implementations for `Borrow<T>`, one should consider whether the different
+    /// implementations should act as facets or representations of the underlying type. Generic code
+    /// typically uses `Borrow<T>` when it relies on the identical behavior of these additional trait
+    /// implementations. These traits will likely appear as additional trait bounds.
+    ///
+    /// In particular `Eq`, `Ord` and `Hash` must be equivalent for borrowed and owned values:
+    /// `x.borrow() == y.borrow()` should give the same result as `x == y`.
+    /// It follows then that the following equivalence must hold:
+    /// `hash(x) == hash((x as Borrow<[u8]>).borrow()) == hash((x as Borrow<str>).borrow())`
+    ///
+    /// Unfortunately it doesn't hold as `hash("abc") != hash("abc".as_bytes())`.
+    /// This happens because the `Hash` impl for str passes an additional `0xFF` byte to
+    /// the hasher to avoid collisions. For example, given the tuples `("a", "bc")`, and `("ab", "c")`,
+    /// the two tuples would have the same hash value if the `0xFF` byte was not added.
+    ///
+    /// ### Example
+    ///
+    /// ```
+    /// use std::borrow::Borrow;
+    /// use std::hash::{Hash, Hasher};
+    ///
+    /// struct ExampleType {
+    ///     data: String
+    /// }
+    ///
+    /// impl Hash for ExampleType {
+    ///     fn hash<H: Hasher>(&self, state: &mut H) {
+    ///         self.data.hash(state);
+    ///     }
+    /// }
+    ///
+    /// impl Borrow<str> for ExampleType {
+    ///     fn borrow(&self) -> &str {
+    ///         &self.data
+    ///     }
+    /// }
+    ///
+    /// impl Borrow<[u8]> for ExampleType {
+    ///     fn borrow(&self) -> &[u8] {
+    ///         self.data.as_bytes()
+    ///     }
+    /// }
+    /// ```
+    /// As a consequence, hashing a `&ExampleType` and hashing the result of the two
+    /// borrows will result in different values.
+    ///
+    #[clippy::version = "1.76.0"]
+    pub IMPL_HASH_BORROW_WITH_STR_AND_BYTES,
+    correctness,
+    "ensures that the semantics of `Borrow` for `Hash` are satisfied when `Borrow<str>` and `Borrow<[u8]>` are implemented"
+}
+
+declare_lint_pass!(ImplHashWithBorrowStrBytes => [IMPL_HASH_BORROW_WITH_STR_AND_BYTES]);
+
+impl LateLintPass<'_> for ImplHashWithBorrowStrBytes {
+    /// We are emitting this lint at the Hash impl of a type that implements all
+    /// three of `Hash`, `Borrow<str>` and `Borrow<[u8]>`.
+    fn check_item(&mut self, cx: &LateContext<'_>, item: &Item<'_>) {
+        if let ItemKind::Impl(imp) = item.kind
+            && let Some(TraitRef {path: Path {span, res, ..}, ..}) = imp.of_trait
+            && let ty = cx.tcx.type_of(item.owner_id).instantiate_identity()
+            && let Some(hash_id) = cx.tcx.get_diagnostic_item(sym::Hash)
+            && Res::Def(DefKind::Trait, hash_id) == *res
+            && let Some(borrow_id) = cx.tcx.get_diagnostic_item(sym::Borrow)
+            // since we are in the `Hash` impl, we don't need to check for that.
+            // we need only to check for `Borrow<str>` and `Borrow<[u8]>`
+            && implements_trait(cx, ty, borrow_id, &[cx.tcx.types.str_.into()])
+            && implements_trait(cx, ty, borrow_id, &[Ty::new_slice(cx.tcx, cx.tcx.types.u8).into()])
+        {
+            span_lint_and_then(
+                cx,
+                IMPL_HASH_BORROW_WITH_STR_AND_BYTES,
+                *span,
+                "the semantics of `Borrow<T>` around `Hash` can't be satisfied when both `Borrow<str>` and `Borrow<[u8]>` are implemented",
+                |diag| {
+                    diag.note("the `Borrow` semantics require that `Hash` must behave the same for all implementations of Borrow<T>");
+                    diag.note(
+          "however, the hash implementations of a string (`str`) and the bytes of a string `[u8]` do not behave the same ..."
+      );
+                    diag.note("... as (`hash(\"abc\") != hash(\"abc\".as_bytes())`");
+                    diag.help("consider either removing one of the  `Borrow` implementations (`Borrow<str>` or `Borrow<[u8]>`) ...");
+                    diag.help("... or not implementing `Hash` for this type");
+                },
+            );
+        }
+    }
+}
diff --git a/clippy_lints/src/lib.rs b/clippy_lints/src/lib.rs
index c462c933082..952625c78be 100644
--- a/clippy_lints/src/lib.rs
+++ b/clippy_lints/src/lib.rs
@@ -144,6 +144,7 @@ mod if_let_mutex;
 mod if_not_else;
 mod if_then_some_else_none;
 mod ignored_unit_patterns;
+mod impl_hash_with_borrow_str_and_bytes;
 mod implicit_hasher;
 mod implicit_return;
 mod implicit_saturating_add;
@@ -1066,6 +1067,7 @@ pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
     store.register_late_pass(move |_| Box::new(manual_hash_one::ManualHashOne::new(msrv())));
     store.register_late_pass(|_| Box::new(iter_without_into_iter::IterWithoutIntoIter));
     store.register_late_pass(|_| Box::new(iter_over_hash_type::IterOverHashType));
+    store.register_late_pass(|_| Box::new(impl_hash_with_borrow_str_and_bytes::ImplHashWithBorrowStrBytes));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }