Rollup merge of #119978 - compiler-errors:async-closure-captures, r=oli-obk

Move async closure parameters into the resultant closure's future eagerly

Move async closure parameters into the closure's resultant future eagerly.

Before, we used to desugar `async |p1, p2, ..| { body }` as `|p1, p2, ..| { || async { body } }`. Now, we desugar the above like `|p1, p2, ..| { async move { let p1 = p1; let p2 = p2; ... body } }`. This mirrors the same desugaring that `async fn` does with its parameter types, and the compiler literally uses the same code via a shared helper function.

This removes the necessity for E0708, since now expressions like `async |x: i32| { x }` will not give you confusing borrow errors.

This does *not* fix the case where async closures have self-borrows. This will come with a general implementation of async closures, which is still in the works.

r? oli-obk

3 files changed, 82 insertions, 42 deletions

diff --git a/clippy_lints/src/async_yields_async.rs b/clippy_lints/src/async_yields_async.rs
index bb08ac7508b..eeaa3de3725 100644
--- a/clippy_lints/src/async_yields_async.rs
+++ b/clippy_lints/src/async_yields_async.rs
@@ -45,50 +45,72 @@ declare_lint_pass!(AsyncYieldsAsync => [ASYNC_YIELDS_ASYNC]);
 
 impl<'tcx> LateLintPass<'tcx> for AsyncYieldsAsync {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
-        // For functions, with explicitly defined types, don't warn.
-        // XXXkhuey maybe we should?
-        if let ExprKind::Closure(Closure {
-            kind:
-                ClosureKind::Coroutine(CoroutineKind::Desugared(
-                    CoroutineDesugaring::Async,
-                    CoroutineSource::Block | CoroutineSource::Closure,
-                )),
+        let ExprKind::Closure(Closure {
+            kind: ClosureKind::Coroutine(CoroutineKind::Desugared(CoroutineDesugaring::Async, kind)),
             body: body_id,
             ..
         }) = expr.kind
-        {
-            if let Some(future_trait_def_id) = cx.tcx.lang_items().future_trait() {
-                let typeck_results = cx.tcx.typeck_body(*body_id);
-                let body = cx.tcx.hir().body(*body_id);
-                let expr_ty = typeck_results.expr_ty(body.value);
+        else {
+            return;
+        };
 
-                if implements_trait(cx, expr_ty, future_trait_def_id, &[]) {
-                    let return_expr_span = match &body.value.kind {
-                        // XXXkhuey there has to be a better way.
-                        ExprKind::Block(block, _) => block.expr.map(|e| e.span),
-                        ExprKind::Path(QPath::Resolved(_, path)) => Some(path.span),
-                        _ => None,
-                    };
-                    if let Some(return_expr_span) = return_expr_span {
-                        span_lint_hir_and_then(
-                            cx,
-                            ASYNC_YIELDS_ASYNC,
-                            body.value.hir_id,
+        let body_expr = match kind {
+            CoroutineSource::Fn => {
+                // For functions, with explicitly defined types, don't warn.
+                // XXXkhuey maybe we should?
+                return;
+            },
+            CoroutineSource::Block => cx.tcx.hir().body(*body_id).value,
+            CoroutineSource::Closure => {
+                // Like `async fn`, async closures are wrapped in an additional block
+                // to move all of the closure's arguments into the future.
+
+                let async_closure_body = cx.tcx.hir().body(*body_id).value;
+                let ExprKind::Block(block, _) = async_closure_body.kind else {
+                    return;
+                };
+                let Some(block_expr) = block.expr else {
+                    return;
+                };
+                let ExprKind::DropTemps(body_expr) = block_expr.kind else {
+                    return;
+                };
+                body_expr
+            },
+        };
+
+        let Some(future_trait_def_id) = cx.tcx.lang_items().future_trait() else {
+            return;
+        };
+
+        let typeck_results = cx.tcx.typeck_body(*body_id);
+        let expr_ty = typeck_results.expr_ty(body_expr);
+
+        if implements_trait(cx, expr_ty, future_trait_def_id, &[]) {
+            let return_expr_span = match &body_expr.kind {
+                // XXXkhuey there has to be a better way.
+                ExprKind::Block(block, _) => block.expr.map(|e| e.span),
+                ExprKind::Path(QPath::Resolved(_, path)) => Some(path.span),
+                _ => None,
+            };
+            if let Some(return_expr_span) = return_expr_span {
+                span_lint_hir_and_then(
+                    cx,
+                    ASYNC_YIELDS_ASYNC,
+                    body_expr.hir_id,
+                    return_expr_span,
+                    "an async construct yields a type which is itself awaitable",
+                    |db| {
+                        db.span_label(body_expr.span, "outer async construct");
+                        db.span_label(return_expr_span, "awaitable value not awaited");
+                        db.span_suggestion(
                             return_expr_span,
-                            "an async construct yields a type which is itself awaitable",
-                            |db| {
-                                db.span_label(body.value.span, "outer async construct");
-                                db.span_label(return_expr_span, "awaitable value not awaited");
-                                db.span_suggestion(
-                                    return_expr_span,
-                                    "consider awaiting this value",
-                                    format!("{}.await", snippet(cx, return_expr_span, "..")),
-                                    Applicability::MaybeIncorrect,
-                                );
-                            },
+                            "consider awaiting this value",
+                            format!("{}.await", snippet(cx, return_expr_span, "..")),
+                            Applicability::MaybeIncorrect,
                         );
-                    }
-                }
+                    },
+                );
             }
         }
     }
diff --git a/clippy_lints/src/redundant_closure_call.rs b/clippy_lints/src/redundant_closure_call.rs
index cde08dfcc74..334e6770ae4 100644
--- a/clippy_lints/src/redundant_closure_call.rs
+++ b/clippy_lints/src/redundant_closure_call.rs
@@ -5,7 +5,9 @@ use clippy_utils::sugg::Sugg;
 use rustc_errors::Applicability;
 use rustc_hir as hir;
 use rustc_hir::intravisit::{Visitor as HirVisitor, Visitor};
-use rustc_hir::{intravisit as hir_visit, ClosureKind, CoroutineDesugaring, CoroutineKind, CoroutineSource, Node};
+use rustc_hir::{
+    intravisit as hir_visit, ClosureKind, CoroutineDesugaring, CoroutineKind, CoroutineSource, ExprKind, Node,
+};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::hir::nested_filter;
 use rustc_middle::lint::in_external_macro;
@@ -166,10 +168,22 @@ impl<'tcx> LateLintPass<'tcx> for RedundantClosureCall {
                         if coroutine_kind.is_async()
                             && let hir::ExprKind::Closure(closure) = body.kind
                         {
-                            let async_closure_body = cx.tcx.hir().body(closure.body);
+                            // Like `async fn`, async closures are wrapped in an additional block
+                            // to move all of the closure's arguments into the future.
+
+                            let async_closure_body = cx.tcx.hir().body(closure.body).value;
+                            let ExprKind::Block(block, _) = async_closure_body.kind else {
+                                return;
+                            };
+                            let Some(block_expr) = block.expr else {
+                                return;
+                            };
+                            let ExprKind::DropTemps(body_expr) = block_expr.kind else {
+                                return;
+                            };
 
                             // `async x` is a syntax error, so it becomes `async { x }`
-                            if !matches!(async_closure_body.value.kind, hir::ExprKind::Block(_, _)) {
+                            if !matches!(body_expr.kind, hir::ExprKind::Block(_, _)) {
                                 hint = hint.blockify();
                             }
 
diff --git a/tests/ui/author/blocks.stdout b/tests/ui/author/blocks.stdout
index 62de661f8ff..8c4d71e68f8 100644
--- a/tests/ui/author/blocks.stdout
+++ b/tests/ui/author/blocks.stdout
@@ -48,7 +48,11 @@ if let ExprKind::Closure { capture_clause: CaptureBy::Value { .. }, fn_decl: fn_
     && expr2 = &cx.tcx.hir().body(body_id1).value
     && let ExprKind::Block(block, None) = expr2.kind
     && block.stmts.is_empty()
-    && block.expr.is_none()
+    && let Some(trailing_expr) = block.expr
+    && let ExprKind::DropTemps(expr3) = trailing_expr.kind
+    && let ExprKind::Block(block1, None) = expr3.kind
+    && block1.stmts.is_empty()
+    && block1.expr.is_none()
 {
     // report your lint here
 }