New lint: copy_then_borrow_mut

CHANGELOG.md

@@ -5497,6 +5497,7 @@ Released 2018-09-13
 [`const_is_empty`]: https://rust-lang.github.io/rust-clippy/master/index.html#const_is_empty
 [`const_static_lifetime`]: https://rust-lang.github.io/rust-clippy/master/index.html#const_static_lifetime
 [`copy_iterator`]: https://rust-lang.github.io/rust-clippy/master/index.html#copy_iterator
+[`copy_then_borrow_mut`]: https://rust-lang.github.io/rust-clippy/master/index.html#copy_then_borrow_mut
 [`crate_in_macro_def`]: https://rust-lang.github.io/rust-clippy/master/index.html#crate_in_macro_def
 [`create_dir`]: https://rust-lang.github.io/rust-clippy/master/index.html#create_dir
 [`crosspointer_transmute`]: https://rust-lang.github.io/rust-clippy/master/index.html#crosspointer_transmute

clippy_lints/src/copy_then_borrow_mut.rs

@@ -0,0 +1,57 @@
+use clippy_utils::diagnostics::span_lint_and_note;
+use clippy_utils::ty::is_copy;
+use rustc_hir::{BorrowKind, Expr, ExprKind, Mutability};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::declare_lint_pass;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for mutable reference on a freshly copied data due to
+    /// the use of a block to return an value implementing `Copy`.
+    ///
+    /// ### Why is this bad?
+    /// Using a block will make a copy of the block result if its type
+    /// implements `Copy`. This might be an indication of a failed attempt
+    /// to borrow the original data instead.
+    ///
+    /// ### Example
+    /// ```no_run
+    /// # fn f(_: &mut [i32]) {}
+    /// let arr = &mut [10, 20, 30];
+    /// f(&mut { *arr });
+    /// ```
+    /// If you intend to modify `arr` in `f`, use instead:
+    /// ```no_run
+    /// # fn f(_: &mut [i32]) {}
+    /// let arr = &mut [10, 20, 30];
+    /// f(arr);
+    /// ```
+    #[clippy::version = "1.86.0"]
+    pub COPY_THEN_BORROW_MUT,
+    suspicious,
+    "mutable borrow of a data which was just copied"
+}
+
+declare_lint_pass!(CopyThenBorrowMut => [COPY_THEN_BORROW_MUT]);
+
+impl LateLintPass<'_> for CopyThenBorrowMut {
+    fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) {
+        if !expr.span.from_expansion()
+            && let ExprKind::AddrOf(BorrowKind::Ref, Mutability::Mut, sub_expr) = expr.kind
+            && let ExprKind::Block(block, _) = sub_expr.kind
+            && block.span.eq_ctxt(expr.span)
+            && let Some(block_expr) = block.expr
+            && let block_ty = cx.typeck_results().expr_ty_adjusted(block_expr)
+            && is_copy(cx, block_ty)
+        {
+            span_lint_and_note(
+                cx,
+                COPY_THEN_BORROW_MUT,
+                expr.span,
+                "mutable borrow of a value which was just copied",
+                (!block.targeted_by_break).then_some(block_expr.span),
+                "the return value of the block implements `Copy` and will be copied",
+            );
+        }
+    }
+}

clippy_lints/src/declared_lints.rs

@@ -113,6 +113,7 @@ pub static LINTS: &[&crate::LintInfo] = &[
     crate::copies::IF_SAME_THEN_ELSE_INFO,
     crate::copies::SAME_FUNCTIONS_IN_IF_CONDITION_INFO,
     crate::copy_iterator::COPY_ITERATOR_INFO,
+    crate::copy_then_borrow_mut::COPY_THEN_BORROW_MUT_INFO,
     crate::crate_in_macro_def::CRATE_IN_MACRO_DEF_INFO,
     crate::create_dir::CREATE_DIR_INFO,
     crate::dbg_macro::DBG_MACRO_INFO,

clippy_lints/src/lib.rs

@@ -103,6 +103,7 @@ mod collection_is_never_read;
 mod comparison_chain;
 mod copies;
 mod copy_iterator;
+mod copy_then_borrow_mut;
 mod crate_in_macro_def;
 mod create_dir;
 mod dbg_macro;
@@ -974,5 +975,6 @@ pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
     store.register_late_pass(move |_| Box::new(arbitrary_source_item_ordering::ArbitrarySourceItemOrdering::new(conf)));
     store.register_late_pass(|_| Box::new(unneeded_struct_pattern::UnneededStructPattern));
     store.register_late_pass(|_| Box::<unnecessary_semicolon::UnnecessarySemicolon>::default());
+    store.register_late_pass(|_| Box::new(copy_then_borrow_mut::CopyThenBorrowMut));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }

tests/ui-toml/excessive_nesting/excessive_nesting.rs

@@ -13,6 +13,7 @@
     clippy::collapsible_if,
     clippy::blocks_in_conditions,
     clippy::single_match,
+    clippy::copy_then_borrow_mut
 )]
 
 #[macro_use]