tail expression behind terminating scope

rust-lang · May 26, 2024 · 2f0ad0d · 2f0ad0d
1 parent b092569
commit 2f0ad0d
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Showing 9 changed files with 123 additions and 23 deletions.
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
@@ -590,6 +590,8 @@ 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
+                && tail_expr.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
@@ -1665,6 +1665,7 @@ symbols! {
         shadow_call_stack,
         shl,
         shl_assign,
+        shorter_tail_lifetimes,
         should_panic,
         shr,
         shr_assign,

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/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`.
diff --git a/tests/ui/lifetimes/shorter-tail-expr-lifetime.rs b/tests/ui/lifetimes/shorter-tail-expr-lifetime.rs
@@ -0,0 +1,15 @@
+//@ revisions: edition2021 edition2024
+//@ [edition2024] compile-flags: -Zunstable-options
+//@ [edition2024] edition: 2024
+//@ [edition2024] run-pass
+
+#![cfg_attr(edition2024, feature(shorter_tail_lifetimes))]
+
+fn f() -> usize {
+    let c = std::cell::RefCell::new("..");
+    c.borrow().len() //[edition2021]~ ERROR: `c` does not live long enough
+}
+
+fn main() {
+    let _ = f();
+}
diff --git a/tests/ui/nll/issue-52534-1.rs b/tests/ui/nll/issue-52534-1.rs
@@ -17,14 +17,14 @@ fn foo(x: &u32) -> &u32 {
 fn baz(x: &u32) -> &&u32 {
     let x = 22;
     &&x
-//~^ ERROR cannot return value referencing local variable
+//~^ ERROR cannot return value referencing local variable `x`
 //~| ERROR cannot return reference to temporary value
 }
 
 fn foobazbar<'a>(x: u32, y: &'a u32) -> &'a u32 {
     let x = 22;
     &x
-//~^ ERROR cannot return reference to local variable
+//~^ ERROR cannot return reference to local variable `x`
 }
 
 fn foobar<'a>(x: &'a u32) -> &'a u32 {