rust-lang · bors · Jun 19, 2024 · Apr 12, 2024
diff --git a/compiler/rustc_ast_lowering/src/expr.rs b/compiler/rustc_ast_lowering/src/expr.rs
@@ -1716,24 +1716,28 @@ impl<'hir> LoweringContext<'_, 'hir> {
         // `mut iter => { ... }`
         let iter_arm = self.arm(iter_pat, loop_expr);
 
-        let into_iter_expr = match loop_kind {
+        let match_expr = match loop_kind {
             ForLoopKind::For => {
                 // `::std::iter::IntoIterator::into_iter(<head>)`
-                self.expr_call_lang_item_fn(
+                let into_iter_expr = self.expr_call_lang_item_fn(
                     head_span,
                     hir::LangItem::IntoIterIntoIter,
                     arena_vec![self; head],
-                )
+                );
+
+                self.arena.alloc(self.expr_match(
+                    for_span,
+                    into_iter_expr,
+                    arena_vec![self; iter_arm],
+                    hir::MatchSource::ForLoopDesugar,
+                ))
             }
-            // ` unsafe { Pin::new_unchecked(&mut into_async_iter(<head>)) }`
+            // `match into_async_iter(<head>) { ref mut iter => match unsafe { Pin::new_unchecked(iter) } { ... } }`
             ForLoopKind::ForAwait => {
-                // `::core::async_iter::IntoAsyncIterator::into_async_iter(<head>)`
-                let iter = self.expr_call_lang_item_fn(
-                    head_span,
-                    hir::LangItem::IntoAsyncIterIntoIter,
-                    arena_vec![self; head],
-                );
-                let iter = self.expr_mut_addr_of(head_span, iter);
+                let iter_ident = iter;
+                let (async_iter_pat, async_iter_pat_id) =
+                    self.pat_ident_binding_mode(head_span, iter_ident, hir::BindingMode::REF_MUT);
+                let iter = self.expr_ident_mut(head_span, iter_ident, async_iter_pat_id);
                 // `Pin::new_unchecked(...)`
                 let iter = self.arena.alloc(self.expr_call_lang_item_fn_mut(
                     head_span,
@@ -1742,17 +1746,29 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 ));
                 // `unsafe { ... }`
                 let iter = self.arena.alloc(self.expr_unsafe(iter));
-                iter
+                let inner_match_expr = self.arena.alloc(self.expr_match(
+                    for_span,
+                    iter,
+                    arena_vec![self; iter_arm],
+                    hir::MatchSource::ForLoopDesugar,
+                ));
+
+                // `::core::async_iter::IntoAsyncIterator::into_async_iter(<head>)`
+                let iter = self.expr_call_lang_item_fn(
+                    head_span,
+                    hir::LangItem::IntoAsyncIterIntoIter,
+                    arena_vec![self; head],
+                );
+                let iter_arm = self.arm(async_iter_pat, inner_match_expr);
+                self.arena.alloc(self.expr_match(
+                    for_span,
+                    iter,
+                    arena_vec![self; iter_arm],
+                    hir::MatchSource::ForLoopDesugar,
+                ))
             }
         };
 
-        let match_expr = self.arena.alloc(self.expr_match(
-            for_span,
-            into_iter_expr,
-            arena_vec![self; iter_arm],
-            hir::MatchSource::ForLoopDesugar,
-        ));
-
         // This is effectively `{ let _result = ...; _result }`.
         // The construct was introduced in #21984 and is necessary to make sure that
         // temporaries in the `head` expression are dropped and do not leak to the

diff --git a/compiler/rustc_feature/src/unstable.rs b/compiler/rustc_feature/src/unstable.rs
@@ -588,6 +588,8 @@ declare_features! (
     (incomplete, return_type_notation, "1.70.0", Some(109417)),
     /// Allows `extern "rust-cold"`.
     (unstable, rust_cold_cc, "1.63.0", Some(97544)),
+    /// Shortern the tail expression lifetime
+    (unstable, shorter_tail_lifetimes, "1.79.0", Some(123739)),
     /// Allows the use of SIMD types in functions declared in `extern` blocks.
     (unstable, simd_ffi, "1.0.0", Some(27731)),
     /// Allows specialization of implementations (RFC 1210).

diff --git a/compiler/rustc_hir_analysis/src/check/region.rs b/compiler/rustc_hir_analysis/src/check/region.rs
@@ -6,7 +6,6 @@
 //!
 //! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/borrow_check.html
 
-use rustc_ast::visit::visit_opt;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
@@ -168,7 +167,14 @@ fn resolve_block<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, blk: &'tcx h
                 hir::StmtKind::Expr(..) | hir::StmtKind::Semi(..) => visitor.visit_stmt(statement),
             }
         }
-        visit_opt!(visitor, visit_expr, &blk.expr);
+        if let Some(tail_expr) = blk.expr {
+            if visitor.tcx.features().shorter_tail_lifetimes
+                && blk.span.edition().at_least_rust_2024()
+            {
+                visitor.terminating_scopes.insert(tail_expr.hir_id.local_id);
+            }
+            visitor.visit_expr(tail_expr);
+        }
     }
 
     visitor.cx = prev_cx;

diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
@@ -1678,6 +1678,7 @@ symbols! {
         shadow_call_stack,
         shl,
         shl_assign,
+        shorter_tail_lifetimes,
         should_panic,
         shr,
         shr_assign,

diff --git a/tests/ui/drop/auxiliary/edition-2021-macros.rs b/tests/ui/drop/auxiliary/edition-2021-macros.rs
@@ -0,0 +1,8 @@
+//@ edition:2021
+
+#[macro_export]
+macro_rules! edition_2021_block {
+    ($($c:tt)*) => {
+        { $($c)* }
+    }
+}
diff --git a/tests/ui/drop/auxiliary/edition-2024-macros.rs b/tests/ui/drop/auxiliary/edition-2024-macros.rs
@@ -0,0 +1,9 @@
+//@ edition:2024
+//@ compile-flags: -Zunstable-options
+
+#[macro_export]
+macro_rules! edition_2024_block {
+    ($($c:tt)*) => {
+        { $($c)* }
+    }
+}
diff --git a/tests/ui/drop/tail-expr-drop-order-negative.edition2024.stderr b/tests/ui/drop/tail-expr-drop-order-negative.edition2024.stderr
@@ -0,0 +1,16 @@
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/tail-expr-drop-order-negative.rs:11:15
+   |
+LL |     x.replace(std::cell::RefCell::new(123).borrow()).is_some()
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^                   - temporary value is freed at the end of this statement
+   |               |
+   |               creates a temporary value which is freed while still in use
+LL |
+LL | }
+   | - borrow might be used here, when `x` is dropped and runs the destructor for type `Option<Ref<'_, i32>>`
+   |
+   = note: consider using a `let` binding to create a longer lived value
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0716`.
diff --git a/tests/ui/drop/tail-expr-drop-order-negative.rs b/tests/ui/drop/tail-expr-drop-order-negative.rs
@@ -0,0 +1,17 @@
+//@ revisions: edition2021 edition2024
+//@ [edition2024] compile-flags: -Zunstable-options
+//@ [edition2024] edition: 2024
+//@ [edition2021] check-pass
+
+#![feature(shorter_tail_lifetimes)]
+
+fn why_would_you_do_this() -> bool {
+    let mut x = None;
+    // Make a temporary `RefCell` and put a `Ref` that borrows it in `x`.
+    x.replace(std::cell::RefCell::new(123).borrow()).is_some()
+    //[edition2024]~^ ERROR: temporary value dropped while borrowed
+}
+
+fn main() {
+    why_would_you_do_this();
+}
diff --git a/tests/ui/drop/tail-expr-drop-order.rs b/tests/ui/drop/tail-expr-drop-order.rs
@@ -0,0 +1,108 @@
+//@ aux-build:edition-2021-macros.rs
+//@ aux-build:edition-2024-macros.rs
+//@ compile-flags: -Z validate-mir -Zunstable-options
+//@ edition: 2024
+//@ run-pass
+
+#![feature(shorter_tail_lifetimes)]
+#![allow(unused_imports)]
+#![allow(dead_code)]
+#![allow(unused_variables)]
+
+#[macro_use]
+extern crate edition_2021_macros;
+#[macro_use]
+extern crate edition_2024_macros;
+use std::cell::RefCell;
+use std::convert::TryInto;
+
+#[derive(Default)]
+struct DropOrderCollector(RefCell<Vec<u32>>);
+
+struct LoudDrop<'a>(&'a DropOrderCollector, u32);
+
+impl Drop for LoudDrop<'_> {
+    fn drop(&mut self) {
+        println!("{}", self.1);
+        self.0.0.borrow_mut().push(self.1);
+    }
+}
+
+impl DropOrderCollector {
+    fn option_loud_drop(&self, n: u32) -> Option<LoudDrop> {
+        Some(LoudDrop(self, n))
+    }
+
+    fn loud_drop(&self, n: u32) -> LoudDrop {
+        LoudDrop(self, n)
+    }
+
+    fn assert_sorted(&self, expected: usize) {
+        let result = self.0.borrow();
+        assert_eq!(result.len(), expected);
+        for i in 1..result.len() {
+            assert!(
+                result[i - 1] < result[i],
+                "inversion at {} ({} followed by {})",
+                i - 1,
+                result[i - 1],
+                result[i]
+            );
+        }
+    }
+}
+
+fn edition_2021_around_2021() {
+    let c = DropOrderCollector::default();
+    let _ = edition_2021_block! {
+        let a = c.loud_drop(1);
+        edition_2021_block! {
+            let b = c.loud_drop(0);
+            c.loud_drop(2).1
+        }
+    };
+    c.assert_sorted(3);
+}
+
+fn edition_2021_around_2024() {
+    let c = DropOrderCollector::default();
+    let _ = edition_2021_block! {
+        let a = c.loud_drop(2);
+        edition_2024_block! {
+            let b = c.loud_drop(1);
+            c.loud_drop(0).1
+        }
+    };
+    c.assert_sorted(3);
+}
+
+fn edition_2024_around_2021() {
+    let c = DropOrderCollector::default();
+    let _ = edition_2024_block! {
+        let a = c.loud_drop(2);
+        edition_2021_block! {
+            let b = c.loud_drop(0);
+            c.loud_drop(1).1
+        }
+    };
+    c.assert_sorted(3);
+}
+
+fn edition_2024_around_2024() {
+    let c = DropOrderCollector::default();
+    let _ = edition_2024_block! {
+        let a = c.loud_drop(2);
+        edition_2024_block! {
+            let b = c.loud_drop(1);
+            c.loud_drop(0).1
+        }
+    };
+    c.assert_sorted(3);
+}
+
+fn main() {
+    edition_2021_around_2021();
+    edition_2021_around_2024();
+    edition_2024_around_2021();
+    edition_2024_around_2024();
+}
diff --git a/tests/ui/feature-gates/feature-gate-shorter_tail_lifetimes.rs b/tests/ui/feature-gates/feature-gate-shorter_tail_lifetimes.rs
@@ -0,0 +1,8 @@
+fn f() -> usize {
+    let c = std::cell::RefCell::new("..");
+    c.borrow().len() //~ ERROR: `c` does not live long enough
+}
+
+fn main() {
+    let _ = f();
+}
diff --git a/tests/ui/feature-gates/feature-gate-shorter_tail_lifetimes.stderr b/tests/ui/feature-gates/feature-gate-shorter_tail_lifetimes.stderr
@@ -0,0 +1,26 @@
+error[E0597]: `c` does not live long enough
+  --> $DIR/feature-gate-shorter_tail_lifetimes.rs:3:5
+   |
+LL |     let c = std::cell::RefCell::new("..");
+   |         - binding `c` declared here
+LL |     c.borrow().len()
+   |     ^---------
+   |     |
+   |     borrowed value does not live long enough
+   |     a temporary with access to the borrow is created here ...
+LL | }
+   | -
+   | |
+   | `c` dropped here while still borrowed
+   | ... and the borrow might be used here, when that temporary is dropped and runs the destructor for type `Ref<'_, &str>`
+   |
+   = note: the temporary is part of an expression at the end of a block;
+           consider forcing this temporary to be dropped sooner, before the block's local variables are dropped
+help: for example, you could save the expression's value in a new local variable `x` and then make `x` be the expression at the end of the block
+   |
+LL |     let x = c.borrow().len(); x
+   |     +++++++                 +++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0597`.
diff --git a/tests/ui/lifetimes/refcell-in-tail-expr.edition2021.stderr b/tests/ui/lifetimes/refcell-in-tail-expr.edition2021.stderr
@@ -0,0 +1,26 @@
+error[E0597]: `cell` does not live long enough
+  --> $DIR/refcell-in-tail-expr.rs:12:27
+   |
+LL |     let cell = std::cell::RefCell::new(0u8);
+   |         ---- binding `cell` declared here
+LL |
+LL |     if let Ok(mut byte) = cell.try_borrow_mut() {
+   |                           ^^^^-----------------
+   |                           |
+   |                           borrowed value does not live long enough
+   |                           a temporary with access to the borrow is created here ...
+...
+LL | }
+   | -
+   | |
+   | `cell` dropped here while still borrowed
+   | ... and the borrow might be used here, when that temporary is dropped and runs the destructor for type `Result<RefMut<'_, u8>, BorrowMutError>`
+   |
+help: consider adding semicolon after the expression so its temporaries are dropped sooner, before the local variables declared by the block are dropped
+   |
+LL |     };
+   |      +
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0597`.
diff --git a/tests/ui/lifetimes/refcell-in-tail-expr.rs b/tests/ui/lifetimes/refcell-in-tail-expr.rs
@@ -0,0 +1,16 @@
+//@ revisions: edition2021 edition2024
+//@ [edition2021] edition: 2021
+//@ [edition2024] edition: 2024
+//@ [edition2024] compile-flags: -Zunstable-options
+//@ [edition2024] check-pass
+
+#![cfg_attr(edition2024, feature(shorter_tail_lifetimes))]
+
+fn main() {
+    let cell = std::cell::RefCell::new(0u8);
+
+    if let Ok(mut byte) = cell.try_borrow_mut() {
+        //[edition2021]~^ ERROR: `cell` does not live long enough
+        *byte = 1;
+    }
+}
diff --git a/tests/ui/lifetimes/shorter-tail-expr-lifetime.edition2021.stderr b/tests/ui/lifetimes/shorter-tail-expr-lifetime.edition2021.stderr
@@ -0,0 +1,26 @@
+error[E0597]: `c` does not live long enough
+  --> $DIR/shorter-tail-expr-lifetime.rs:10:5
+   |
+LL |     let c = std::cell::RefCell::new("..");
+   |         - binding `c` declared here
+LL |     c.borrow().len()
+   |     ^---------
+   |     |
+   |     borrowed value does not live long enough
+   |     a temporary with access to the borrow is created here ...
+LL | }
+   | -
+   | |
+   | `c` dropped here while still borrowed
+   | ... and the borrow might be used here, when that temporary is dropped and runs the destructor for type `Ref<'_, &str>`
+   |
+   = note: the temporary is part of an expression at the end of a block;
+           consider forcing this temporary to be dropped sooner, before the block's local variables are dropped
+help: for example, you could save the expression's value in a new local variable `x` and then make `x` be the expression at the end of the block
+   |
+LL |     let x = c.borrow().len(); x
+   |     +++++++                 +++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0597`.