Auto merge of #12227 - y21:issue12225, r=Manishearth

[`redundant_locals`]: take by-value closure captures into account

Fixes #12225

The same problem in the linked issue can happen to regular closures too, and conveniently async blocks are closures in the HIR so fixing closures will fix async blocks as well.

changelog: [`redundant_locals`]: avoid linting when redefined variable is captured by-value

3 files changed, 100 insertions, 28 deletions

diff --git a/clippy_lints/src/redundant_locals.rs b/clippy_lints/src/redundant_locals.rs
index 2c511ee0bc0..700a5dd4a85 100644
--- a/clippy_lints/src/redundant_locals.rs
+++ b/clippy_lints/src/redundant_locals.rs
@@ -4,8 +4,10 @@ use clippy_utils::ty::needs_ordered_drop;
 use rustc_ast::Mutability;
 use rustc_hir::def::Res;
 use rustc_hir::{BindingAnnotation, ByRef, ExprKind, HirId, Local, Node, Pat, PatKind, QPath};
+use rustc_hir_typeck::expr_use_visitor::PlaceBase;
 use rustc_lint::{LateContext, LateLintPass, LintContext};
 use rustc_middle::lint::in_external_macro;
+use rustc_middle::ty::UpvarCapture;
 use rustc_session::declare_lint_pass;
 use rustc_span::symbol::Ident;
 use rustc_span::DesugaringKind;
@@ -69,6 +71,7 @@ impl<'tcx> LateLintPass<'tcx> for RedundantLocals {
             // the local is user-controlled
             && !in_external_macro(cx.sess(), local.span)
             && !is_from_proc_macro(cx, expr)
+            && !is_by_value_closure_capture(cx, local.hir_id, binding_id)
         {
             span_lint_and_help(
                 cx,
@@ -82,6 +85,29 @@ impl<'tcx> LateLintPass<'tcx> for RedundantLocals {
     }
 }
 
+/// Checks if the enclosing body is a closure and if the given local is captured by value.
+///
+/// In those cases, the redefinition may be necessary to force a move:
+/// ```
+/// fn assert_static<T: 'static>(_: T) {}
+///
+/// let v = String::new();
+/// let closure = || {
+///   let v = v; // <- removing this redefinition makes `closure` no longer `'static`
+///   dbg!(&v);
+/// };
+/// assert_static(closure);
+/// ```
+fn is_by_value_closure_capture(cx: &LateContext<'_>, redefinition: HirId, root_variable: HirId) -> bool {
+    let closure_def_id = cx.tcx.hir().enclosing_body_owner(redefinition);
+
+    cx.tcx.is_closure_or_coroutine(closure_def_id.to_def_id())
+        && cx.tcx.closure_captures(closure_def_id).iter().any(|c| {
+            matches!(c.info.capture_kind, UpvarCapture::ByValue)
+                && matches!(c.place.base, PlaceBase::Upvar(upvar) if upvar.var_path.hir_id == root_variable)
+        })
+}
+
 /// Find the annotation of a binding introduced by a pattern, or `None` if it's not introduced.
 fn find_binding(pat: &Pat<'_>, name: Ident) -> Option<BindingAnnotation> {
     let mut ret = None;
diff --git a/tests/ui/redundant_locals.rs b/tests/ui/redundant_locals.rs
index 182d067a5e9..f6909828aa9 100644
--- a/tests/ui/redundant_locals.rs
+++ b/tests/ui/redundant_locals.rs
@@ -1,6 +1,7 @@
 //@aux-build:proc_macros.rs
 #![allow(unused, clippy::no_effect, clippy::needless_pass_by_ref_mut)]
 #![warn(clippy::redundant_locals)]
+#![feature(async_closure, coroutines)]
 
 extern crate proc_macros;
 use proc_macros::{external, with_span};
@@ -163,3 +164,48 @@ fn drop_compose() {
     let b = ComposeDrop { d: WithDrop(1) };
     let a = a;
 }
+
+fn issue12225() {
+    fn assert_static<T: 'static>(_: T) {}
+
+    let v1 = String::new();
+    let v2 = String::new();
+    let v3 = String::new();
+    let v4 = String::new();
+    let v5 = String::new();
+    let v6 = String::new();
+
+    assert_static(|| {
+        let v1 = v1;
+        dbg!(&v1);
+    });
+    assert_static(async {
+        let v2 = v2;
+        dbg!(&v2);
+    });
+    assert_static(|| async {
+        let v3 = v3;
+        dbg!(&v3);
+    });
+    assert_static(async || {
+        let v4 = v4;
+        dbg!(&v4);
+    });
+    assert_static(static || {
+        let v5 = v5;
+        yield;
+    });
+    assert_static(|| {
+        let v6 = v6;
+        yield;
+    });
+
+    fn foo(a: &str, b: &str) {}
+
+    let do_not_move = String::new();
+    let things_to_move = vec!["a".to_string(), "b".to_string()];
+    let futures = things_to_move.into_iter().map(|move_me| async {
+        let move_me = move_me;
+        foo(&do_not_move, &move_me)
+    });
+}
diff --git a/tests/ui/redundant_locals.stderr b/tests/ui/redundant_locals.stderr
index 30ab4aa2ea9..610d587ddad 100644
--- a/tests/ui/redundant_locals.stderr
+++ b/tests/ui/redundant_locals.stderr
@@ -1,11 +1,11 @@
 error: redundant redefinition of a binding `x`
-  --> $DIR/redundant_locals.rs:12:5
+  --> $DIR/redundant_locals.rs:13:5
    |
 LL |     let x = x;
    |     ^^^^^^^^^^
    |
 help: `x` is initially defined here
-  --> $DIR/redundant_locals.rs:11:9
+  --> $DIR/redundant_locals.rs:12:9
    |
 LL |     let x = 1;
    |         ^
@@ -13,157 +13,157 @@ LL |     let x = 1;
    = help: to override `-D warnings` add `#[allow(clippy::redundant_locals)]`
 
 error: redundant redefinition of a binding `x`
-  --> $DIR/redundant_locals.rs:17:5
+  --> $DIR/redundant_locals.rs:18:5
    |
 LL |     let mut x = x;
    |     ^^^^^^^^^^^^^^
    |
 help: `x` is initially defined here
-  --> $DIR/redundant_locals.rs:16:9
+  --> $DIR/redundant_locals.rs:17:9
    |
 LL |     let mut x = 1;
    |         ^^^^^
 
 error: redundant redefinition of a binding `x`
-  --> $DIR/redundant_locals.rs:47:5
+  --> $DIR/redundant_locals.rs:48:5
    |
 LL |     let x = x;
    |     ^^^^^^^^^^
    |
 help: `x` is initially defined here
-  --> $DIR/redundant_locals.rs:46:14
+  --> $DIR/redundant_locals.rs:47:14
    |
 LL | fn parameter(x: i32) {
    |              ^
 
 error: redundant redefinition of a binding `x`
-  --> $DIR/redundant_locals.rs:52:5
+  --> $DIR/redundant_locals.rs:53:5
    |
 LL |     let x = x;
    |     ^^^^^^^^^^
    |
 help: `x` is initially defined here
-  --> $DIR/redundant_locals.rs:51:9
+  --> $DIR/redundant_locals.rs:52:9
    |
 LL |     let x = 1;
    |         ^
 
 error: redundant redefinition of a binding `x`
-  --> $DIR/redundant_locals.rs:53:5
+  --> $DIR/redundant_locals.rs:54:5
    |
 LL |     let x = x;
    |     ^^^^^^^^^^
    |
 help: `x` is initially defined here
-  --> $DIR/redundant_locals.rs:52:9
+  --> $DIR/redundant_locals.rs:53:9
    |
 LL |     let x = x;
    |         ^
 
 error: redundant redefinition of a binding `x`
-  --> $DIR/redundant_locals.rs:54:5
+  --> $DIR/redundant_locals.rs:55:5
    |
 LL |     let x = x;
    |     ^^^^^^^^^^
    |
 help: `x` is initially defined here
-  --> $DIR/redundant_locals.rs:53:9
+  --> $DIR/redundant_locals.rs:54:9
    |
 LL |     let x = x;
    |         ^
 
 error: redundant redefinition of a binding `x`
-  --> $DIR/redundant_locals.rs:55:5
+  --> $DIR/redundant_locals.rs:56:5
    |
 LL |     let x = x;
    |     ^^^^^^^^^^
    |
 help: `x` is initially defined here
-  --> $DIR/redundant_locals.rs:54:9
+  --> $DIR/redundant_locals.rs:55:9
    |
 LL |     let x = x;
    |         ^
 
 error: redundant redefinition of a binding `a`
-  --> $DIR/redundant_locals.rs:61:5
+  --> $DIR/redundant_locals.rs:62:5
    |
 LL |     let a = a;
    |     ^^^^^^^^^^
    |
 help: `a` is initially defined here
-  --> $DIR/redundant_locals.rs:59:9
+  --> $DIR/redundant_locals.rs:60:9
    |
 LL |     let a = 1;
    |         ^
 
 error: redundant redefinition of a binding `b`
-  --> $DIR/redundant_locals.rs:62:5
+  --> $DIR/redundant_locals.rs:63:5
    |
 LL |     let b = b;
    |     ^^^^^^^^^^
    |
 help: `b` is initially defined here
-  --> $DIR/redundant_locals.rs:60:9
+  --> $DIR/redundant_locals.rs:61:9
    |
 LL |     let b = 2;
    |         ^
 
 error: redundant redefinition of a binding `x`
-  --> $DIR/redundant_locals.rs:68:9
+  --> $DIR/redundant_locals.rs:69:9
    |
 LL |         let x = x;
    |         ^^^^^^^^^^
    |
 help: `x` is initially defined here
-  --> $DIR/redundant_locals.rs:67:13
+  --> $DIR/redundant_locals.rs:68:13
    |
 LL |         let x = 1;
    |             ^
 
 error: redundant redefinition of a binding `x`
-  --> $DIR/redundant_locals.rs:75:9
+  --> $DIR/redundant_locals.rs:76:9
    |
 LL |         let x = x;
    |         ^^^^^^^^^^
    |
 help: `x` is initially defined here
-  --> $DIR/redundant_locals.rs:74:13
+  --> $DIR/redundant_locals.rs:75:13
    |
 LL |         let x = 1;
    |             ^
 
 error: redundant redefinition of a binding `x`
-  --> $DIR/redundant_locals.rs:78:9
+  --> $DIR/redundant_locals.rs:79:9
    |
 LL |         let x = x;
    |         ^^^^^^^^^^
    |
 help: `x` is initially defined here
-  --> $DIR/redundant_locals.rs:77:6
+  --> $DIR/redundant_locals.rs:78:6
    |
 LL |     |x: i32| {
    |      ^
 
 error: redundant redefinition of a binding `x`
-  --> $DIR/redundant_locals.rs:97:9
+  --> $DIR/redundant_locals.rs:98:9
    |
 LL |         let x = x;
    |         ^^^^^^^^^^
    |
 help: `x` is initially defined here
-  --> $DIR/redundant_locals.rs:94:9
+  --> $DIR/redundant_locals.rs:95:9
    |
 LL |     let x = 1;
    |         ^
 
 error: redundant redefinition of a binding `a`
-  --> $DIR/redundant_locals.rs:152:5
+  --> $DIR/redundant_locals.rs:153:5
    |
 LL |     let a = a;
    |     ^^^^^^^^^^
    |
 help: `a` is initially defined here
-  --> $DIR/redundant_locals.rs:150:9
+  --> $DIR/redundant_locals.rs:151:9
    |
 LL |     let a = WithoutDrop(1);
    |         ^