unnecessary_sort_by: avoid linting if key borrows

clippy_lints/src/let_and_return.rs

@@ -1,6 +1,5 @@
 use if_chain::if_chain;
 use rustc_errors::Applicability;
-use rustc_hir::def_id::DefId;
 use rustc_hir::intravisit::{walk_expr, NestedVisitorMap, Visitor};
 use rustc_hir::{Block, Expr, ExprKind, PatKind, StmtKind};
 use rustc_lint::{LateContext, LateLintPass, LintContext};
@@ -9,7 +8,7 @@ use rustc_middle::lint::in_external_macro;
 use rustc_middle::ty::subst::GenericArgKind;
 use rustc_session::{declare_lint_pass, declare_tool_lint};
 
-use crate::utils::{in_macro, match_qpath, snippet_opt, span_lint_and_then};
+use crate::utils::{fn_def_id, in_macro, match_qpath, snippet_opt, span_lint_and_then};
 
 declare_clippy_lint! {
     /// **What it does:** Checks for `let`-bindings, which are subsequently
@@ -97,22 +96,6 @@ struct BorrowVisitor<'a, 'tcx> {
     borrows: bool,
 }
 
-impl BorrowVisitor<'_, '_> {
-    fn fn_def_id(&self, expr: &Expr<'_>) -> Option<DefId> {
-        match &expr.kind {
-            ExprKind::MethodCall(..) => self.cx.tables().type_dependent_def_id(expr.hir_id),
-            ExprKind::Call(
-                Expr {
-                    kind: ExprKind::Path(qpath),
-                    ..
-                },
-                ..,
-            ) => self.cx.qpath_res(qpath, expr.hir_id).opt_def_id(),
-            _ => None,
-        }
-    }
-}
-
 impl<'tcx> Visitor<'tcx> for BorrowVisitor<'_, 'tcx> {
     type Map = Map<'tcx>;
 
@@ -121,7 +104,7 @@ impl<'tcx> Visitor<'tcx> for BorrowVisitor<'_, 'tcx> {
             return;
         }
 
-        if let Some(def_id) = self.fn_def_id(expr) {
+        if let Some(def_id) = fn_def_id(self.cx, expr) {
             self.borrows = self
                 .cx
                 .tcx

clippy_lints/src/unnecessary_sort_by.rs

@@ -5,24 +5,23 @@ use if_chain::if_chain;
 use rustc_errors::Applicability;
 use rustc_hir::{Expr, ExprKind, Mutability, Param, Pat, PatKind, Path, PathSegment, QPath};
 use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::ty::{self, subst::GenericArgKind};
 use rustc_session::{declare_lint_pass, declare_tool_lint};
 use rustc_span::symbol::Ident;
 
 declare_clippy_lint! {
     /// **What it does:**
-    /// Detects when people use `Vec::sort_by` and pass in a function
+    /// Detects uses of `Vec::sort_by` passing in a closure
     /// which compares the two arguments, either directly or indirectly.
     ///
     /// **Why is this bad?**
     /// It is more clear to use `Vec::sort_by_key` (or `Vec::sort` if
-    /// possible) than to use `Vec::sort_by` and and a more complicated
+    /// possible) than to use `Vec::sort_by` and a more complicated
     /// closure.
     ///
     /// **Known problems:**
-    /// If the suggested `Vec::sort_by_key` uses Reverse and it isn't
-    /// imported by a use statement in the current frame, then a `use`
-    /// statement that imports it will need to be added (which this lint
-    /// can't do).
+    /// If the suggested `Vec::sort_by_key` uses Reverse and it isn't already
+    /// imported by a use statement, then it will need to be added manually.
     ///
     /// **Example:**
     ///
@@ -201,28 +200,41 @@ fn detect_lint(cx: &LateContext<'_>, expr: &Expr<'_>) -> Option<LintTrigger> {
             };
             let vec_name = Sugg::hir(cx, &args[0], "..").to_string();
             let unstable = name == "sort_unstable_by";
+
             if_chain! {
                 if let ExprKind::Path(QPath::Resolved(_, Path {
                     segments: [PathSegment { ident: left_name, .. }], ..
                 })) = &left_expr.kind;
                 if left_name == left_ident;
                 then {
-                    Some(LintTrigger::Sort(SortDetection { vec_name, unstable }))
-                }
-                else {
-                    Some(LintTrigger::SortByKey(SortByKeyDetection {
-                        vec_name,
-                        unstable,
-                        closure_arg,
-                        closure_body,
-                        reverse
-                    }))
+                    return Some(LintTrigger::Sort(SortDetection { vec_name, unstable }))
+                } else {
+                    if !key_returns_borrow(cx, left_expr) {
+                        return Some(LintTrigger::SortByKey(SortByKeyDetection {
+                            vec_name,
+                            unstable,
+                            closure_arg,
+                            closure_body,
+                            reverse
+                        }))
+                    }
                 }
             }
-        } else {
-            None
         }
     }
+
+    None
+}
+
+fn key_returns_borrow(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
+    if let Some(def_id) = utils::fn_def_id(cx, expr) {
+        let output = cx.tcx.fn_sig(def_id).output();
+        let ty = output.skip_binder();
+        return matches!(ty.kind, ty::Ref(..))
+            || ty.walk().any(|arg| matches!(arg.unpack(), GenericArgKind::Lifetime(_)));
+    }
+
+    false
 }
 
 impl LateLintPass<'_> for UnnecessarySortBy {

clippy_lints/src/utils/mod.rs

@@ -1363,6 +1363,21 @@ pub fn fn_has_unsatisfiable_preds(cx: &LateContext<'_>, did: DefId) -> bool {
     )
 }
 
+/// Returns the `DefId` of the callee if the given expression is a function or method call.
+pub fn fn_def_id(cx: &LateContext<'_>, expr: &Expr<'_>) -> Option<DefId> {
+    match &expr.kind {
+        ExprKind::MethodCall(..) => cx.tables().type_dependent_def_id(expr.hir_id),
+        ExprKind::Call(
+            Expr {
+                kind: ExprKind::Path(qpath),
+                ..
+            },
+            ..,
+        ) => cx.tables().qpath_res(qpath, expr.hir_id).opt_def_id(),
+        _ => None,
+    }
+}
+
 pub fn run_lints(cx: &LateContext<'_>, lints: &[&'static Lint], id: HirId) -> bool {
     lints.iter().any(|lint| {
         matches!(

tests/ui/unnecessary_sort_by.fixed

@@ -2,11 +2,11 @@
 
 use std::cmp::Reverse;
 
-fn id(x: isize) -> isize {
-    x
-}
+fn unnecessary_sort_by() {
+    fn id(x: isize) -> isize {
+        x
+    }
 
-fn main() {
     let mut vec: Vec<isize> = vec![3, 6, 1, 2, 5];
     // Forward examples
     vec.sort();
@@ -24,3 +24,41 @@ fn main() {
     vec.sort_by(|_, b| b.cmp(c));
     vec.sort_unstable_by(|a, _| a.cmp(c));
 }
+
+// Should not be linted to avoid hitting https://github.com/rust-lang/rust/issues/34162
+mod issue_5754 {
+    struct Test(String);
+
+    #[derive(PartialOrd, Ord, PartialEq, Eq)]
+    struct Wrapper<'a>(&'a str);
+
+    impl Test {
+        fn name(&self) -> &str {
+            &self.0
+        }
+
+        fn wrapped(&self) -> Wrapper<'_> {
+            Wrapper(&self.0)
+        }
+    }
+
+    pub fn test() {
+        let mut args: Vec<Test> = vec![];
+
+        // Forward
+        args.sort_by(|a, b| a.name().cmp(b.name()));
+        args.sort_by(|a, b| a.wrapped().cmp(&b.wrapped()));
+        args.sort_unstable_by(|a, b| a.name().cmp(b.name()));
+        args.sort_unstable_by(|a, b| a.wrapped().cmp(&b.wrapped()));
+        // Reverse
+        args.sort_by(|a, b| b.name().cmp(a.name()));
+        args.sort_by(|a, b| b.wrapped().cmp(&a.wrapped()));
+        args.sort_unstable_by(|a, b| b.name().cmp(a.name()));
+        args.sort_unstable_by(|a, b| b.wrapped().cmp(&a.wrapped()));
+    }
+}
+
+fn main() {
+    unnecessary_sort_by();
+    issue_5754::test();
+}

tests/ui/unnecessary_sort_by.rs

@@ -2,11 +2,11 @@
 
 use std::cmp::Reverse;
 
-fn id(x: isize) -> isize {
-    x
-}
+fn unnecessary_sort_by() {
+    fn id(x: isize) -> isize {
+        x
+    }
 
-fn main() {
     let mut vec: Vec<isize> = vec![3, 6, 1, 2, 5];
     // Forward examples
     vec.sort_by(|a, b| a.cmp(b));
@@ -24,3 +24,41 @@ fn main() {
     vec.sort_by(|_, b| b.cmp(c));
     vec.sort_unstable_by(|a, _| a.cmp(c));
 }
+
+// Should not be linted to avoid hitting https://github.com/rust-lang/rust/issues/34162
+mod issue_5754 {
+    struct Test(String);
+
+    #[derive(PartialOrd, Ord, PartialEq, Eq)]
+    struct Wrapper<'a>(&'a str);
+
+    impl Test {
+        fn name(&self) -> &str {
+            &self.0
+        }
+
+        fn wrapped(&self) -> Wrapper<'_> {
+            Wrapper(&self.0)
+        }
+    }
+
+    pub fn test() {
+        let mut args: Vec<Test> = vec![];
+
+        // Forward
+        args.sort_by(|a, b| a.name().cmp(b.name()));
+        args.sort_by(|a, b| a.wrapped().cmp(&b.wrapped()));
+        args.sort_unstable_by(|a, b| a.name().cmp(b.name()));
+        args.sort_unstable_by(|a, b| a.wrapped().cmp(&b.wrapped()));
+        // Reverse
+        args.sort_by(|a, b| b.name().cmp(a.name()));
+        args.sort_by(|a, b| b.wrapped().cmp(&a.wrapped()));
+        args.sort_unstable_by(|a, b| b.name().cmp(a.name()));
+        args.sort_unstable_by(|a, b| b.wrapped().cmp(&a.wrapped()));
+    }
+}
+
+fn main() {
+    unnecessary_sort_by();
+    issue_5754::test();
+}