Auto merge of rust-lang#11061 - Alexendoo:let-and-return-closures, r=llogiq

`let_and_return`: lint 'static lifetimes, don't lint borrows in closures

Fixes rust-lang#11056

Now also ignores functions returning `'static` lifetimes, since I noticed the `stdin.lock()` example was still being linted but doesn't need to be since rust-lang#93965

changelog: none

Author: bors

clippy_lints/src/returns.rs

@@ -1,6 +1,6 @@
 use clippy_utils::diagnostics::{span_lint_and_then, span_lint_hir_and_then};
 use clippy_utils::source::{snippet_opt, snippet_with_context};
-use clippy_utils::visitors::{for_each_expr, Descend};
+use clippy_utils::visitors::{for_each_expr_with_closures, Descend};
 use clippy_utils::{fn_def_id, path_to_local_id, span_find_starting_semi};
 use core::ops::ControlFlow;
 use if_chain::if_chain;
@@ -328,7 +328,7 @@ fn emit_return_lint(cx: &LateContext<'_>, ret_span: Span, semi_spans: Vec<Span>,
 }
 
 fn last_statement_borrows<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) -> bool {
-    for_each_expr(expr, |e| {
+    for_each_expr_with_closures(cx, expr, |e| {
         if let Some(def_id) = fn_def_id(cx, e)
             && cx
                 .tcx
@@ -337,7 +337,7 @@ fn last_statement_borrows<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>)
                 .skip_binder()
                 .output()
                 .walk()
-                .any(|arg| matches!(arg.unpack(), GenericArgKind::Lifetime(_)))
+                .any(|arg| matches!(arg.unpack(), GenericArgKind::Lifetime(re) if !re.is_static()))
         {
             ControlFlow::Break(())
         } else {

tests/ui/let_and_return.rs

@@ -1,6 +1,8 @@
 #![allow(unused)]
 #![warn(clippy::let_and_return)]
 
+use std::cell::RefCell;
+
 fn test() -> i32 {
     let _y = 0; // no warning
     let x = 5;
@@ -65,45 +67,46 @@ macro_rules! tuple_encode {
     );
 }
 
+fn issue_3792() -> String {
+    use std::io::{self, BufRead, Stdin};
+
+    let stdin = io::stdin();
+    // `Stdin::lock` returns `StdinLock<'static>` so `line` doesn't borrow from `stdin`
+    // https://github.com/rust-lang/rust/pull/93965
+    let line = stdin.lock().lines().next().unwrap().unwrap();
+    line
+}
+
 tuple_encode!(T0, T1, T2, T3, T4, T5, T6, T7);
 
 mod no_lint_if_stmt_borrows {
-    mod issue_3792 {
-        use std::io::{self, BufRead, Stdin};
+    use std::cell::RefCell;
+    use std::rc::{Rc, Weak};
+    struct Bar;
 
-        fn read_line() -> String {
-            let stdin = io::stdin();
-            let line = stdin.lock().lines().next().unwrap().unwrap();
-            line
+    impl Bar {
+        fn new() -> Self {
+            Bar {}
         }
-    }
-
-    mod issue_3324 {
-        use std::cell::RefCell;
-        use std::rc::{Rc, Weak};
-
-        fn test(value: Weak<RefCell<Bar>>) -> u32 {
-            let value = value.upgrade().unwrap();
-            let ret = value.borrow().baz();
-            ret
+        fn baz(&self) -> u32 {
+            0
         }
+    }
 
-        struct Bar;
+    fn issue_3324(value: Weak<RefCell<Bar>>) -> u32 {
+        let value = value.upgrade().unwrap();
+        let ret = value.borrow().baz();
+        ret
+    }
 
-        impl Bar {
-            fn new() -> Self {
-                Bar {}
-            }
-            fn baz(&self) -> u32 {
-                0
-            }
+    fn borrows_in_closure(value: Weak<RefCell<Bar>>) -> u32 {
+        fn f(mut x: impl FnMut() -> u32) -> impl FnMut() -> u32 {
+            x
         }
 
-        fn main() {
-            let a = Rc::new(RefCell::new(Bar::new()));
-            let b = Rc::downgrade(&a);
-            test(b);
-        }
+        let value = value.upgrade().unwrap();
+        let ret = f(|| value.borrow().baz())();
+        ret
     }
 
     mod free_function {

tests/ui/let_and_return.stderr

@@ -1,5 +1,5 @@
 error: returning the result of a `let` binding from a block
-  --> $DIR/let_and_return.rs:7:5
+  --> $DIR/let_and_return.rs:9:5
    |
 LL |     let x = 5;
    |     ---------- unnecessary `let` binding
@@ -14,7 +14,7 @@ LL ~     5
    |
 
 error: returning the result of a `let` binding from a block
-  --> $DIR/let_and_return.rs:13:9
+  --> $DIR/let_and_return.rs:15:9
    |
 LL |         let x = 5;
    |         ---------- unnecessary `let` binding
@@ -28,7 +28,21 @@ LL ~         5
    |
 
 error: returning the result of a `let` binding from a block
-  --> $DIR/let_and_return.rs:164:13
+  --> $DIR/let_and_return.rs:77:5
+   |
+LL |     let line = stdin.lock().lines().next().unwrap().unwrap();
+   |     --------------------------------------------------------- unnecessary `let` binding
+LL |     line
+   |     ^^^^
+   |
+help: return the expression directly
+   |
+LL ~     
+LL ~     stdin.lock().lines().next().unwrap().unwrap()
+   |
+
+error: returning the result of a `let` binding from a block
+  --> $DIR/let_and_return.rs:167:13
    |
 LL |             let clone = Arc::clone(&self.foo);
    |             ---------------------------------- unnecessary `let` binding
@@ -41,5 +55,5 @@ LL ~
 LL ~             Arc::clone(&self.foo) as _
    |
 
-error: aborting due to 3 previous errors
+error: aborting due to 4 previous errors