Auto merge of #145838 - dianne:non-extending-super-let, r=<try>

don't apply temporary lifetime extension rules to non-extended `super let`

Author: rust-bors[bot]

compiler/rustc_hir_analysis/src/check/region.rs

@@ -467,6 +467,7 @@ fn resolve_local<'tcx>(
     // A, but the inner rvalues `a()` and `b()` have an extended lifetime
     // due to rule C.
 
+    let mut extend_initializer = true;
     if let_kind == LetKind::Super {
         if let Some(scope) = visitor.extended_super_lets.remove(&pat.unwrap().hir_id.local_id) {
             // This expression was lifetime-extended by a parent let binding. E.g.
@@ -497,10 +498,17 @@ fn resolve_local<'tcx>(
                 }
                 visitor.cx.var_parent = parent;
             }
+            // Don't lifetime-extend child `super let`s or block tail expressions' temporaries in
+            // the initializer when this `super let` is not itself extended by a parent `let`
+            // (#145784). Block tail expressions are temporary drop scopes in Editions 2024 and
+            // later, their temps shouldn't outlive the block in e.g. `f(pin!({ &temp() }))`.
+            extend_initializer = false;
         }
     }
 
-    if let Some(expr) = init {
+    if let Some(expr) = init
+        && extend_initializer
+    {
         record_rvalue_scope_if_borrow_expr(visitor, expr, visitor.cx.var_parent);
 
         if let Some(pat) = pat {

tests/ui/borrowck/format-args-temporary-scopes.e2024.stderr

@@ -0,0 +1,27 @@
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/format-args-temporary-scopes.rs:13:25
+   |
+LL |     println!("{:?}", { &temp() });
+   |                      ---^^^^^---
+   |                      |  |    |
+   |                      |  |    temporary value is freed at the end of this statement
+   |                      |  creates a temporary value which is freed while still in use
+   |                      borrow later used here
+   |
+   = note: consider using a `let` binding to create a longer lived value
+
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/format-args-temporary-scopes.rs:19:29
+   |
+LL |     println!("{:?}{:?}", { &temp() }, ());
+   |                          ---^^^^^---
+   |                          |  |    |
+   |                          |  |    temporary value is freed at the end of this statement
+   |                          |  creates a temporary value which is freed while still in use
+   |                          borrow later used here
+   |
+   = note: consider using a `let` binding to create a longer lived value
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0716`.

tests/ui/borrowck/format-args-temporary-scopes.rs

@@ -0,0 +1,21 @@
+//! Test for #145784 as it relates to format arguments: arguments to macros such as `println!`
+//! should obey normal temporary scoping rules.
+//@ revisions: e2021 e2024
+//@ [e2021] check-pass
+//@ [e2021] edition: 2021
+//@ [e2024] edition: 2024
+
+fn temp() {}
+
+fn main() {
+    // In Rust 2024, block tail expressions are temporary scopes, so the result of `temp()` is
+    // dropped after evaluating `&temp()`.
+    println!("{:?}", { &temp() });
+    //[e2024]~^ ERROR: temporary value dropped while borrowed [E0716]
+
+    // In Rust 1.89, `format_args!` extended the lifetime of all extending expressions in its
+    // arguments when provided with two or more arguments. This caused the result of `temp()` to
+    // outlive the result of the block, making this compile.
+    println!("{:?}{:?}", { &temp() }, ());
+    //[e2024]~^ ERROR: temporary value dropped while borrowed [E0716]
+}

tests/ui/drop/super-let-tail-expr-drop-order.rs

@@ -0,0 +1,192 @@
+//! Test for #145784: the argument to `pin!` should be treated as an extending expression if and
+//! only if the whole `pin!` invocation is an extending expression. Likewise, since `pin!` is
+//! implemented in terms of `super let`, test the same for `super let` initializers. Since the
+//! argument to `pin!` and the initializer of `super let` are not temporary drop scopes, this only
+//! affects lifetimes in two cases:
+//!
+//! - Block tail expressions in Rust 2024, which are both extending expressions and temporary drop
+//! scopes; treating them as extending expressions within a non-extending `pin!` resulted in borrow
+//! expression operands living past the end of the block.
+//!
+//! - Nested `super let` statements, which can have their binding and temporary lifetimes extended
+//! when the block they're in is an extending expression.
+//!
+//! For more information on extending expressions, see
+//! https://doc.rust-lang.org/reference/destructors.html#extending-based-on-expressions
+//!
+//! For tests that `super let` initializers aren't temporary drop scopes, and tests for
+//! lifetime-extended `super let`, see tests/ui/borrowck/super-let-lifetime-and-drop.rs
+//@ run-pass
+//@ revisions: e2021 e2024
+//@ [e2021] edition: 2021
+//@ [e2024] edition: 2024
+
+#![feature(super_let)]
+#![allow(unused_braces)]
+
+use std::cell::RefCell;
+use std::pin::pin;
+
+fn f<T>(_: LogDrop<'_>, x: T) -> T { x }
+
+fn main() {
+    // Test block arguments to `pin!` in non-extending expressions.
+    // In Rust 2021, block tail expressions aren't temporary drop scopes, so their temporaries
+    // should outlive the `pin!` invocation.
+    // In Rust 2024, block tail expressions are temporary drop scopes, so their temporaries should
+    // be dropped after evaluating the tail expression within the `pin!` invocation.
+    // By nesting two `pin!` calls, this ensures non-extended `pin!` doesn't extend an inner `pin!`.
+    assert_drop_order(1..=3, |o| {
+        #[cfg(e2021)]
+        (
+            pin!((
+                pin!({ &o.log(3) as *const LogDrop<'_> }),
+                drop(o.log(1)),
+            )),
+            drop(o.log(2)),
+        );
+        #[cfg(e2024)]
+        (
+            pin!((
+                pin!({ &o.log(1) as *const LogDrop<'_> }),
+                drop(o.log(2)),
+            )),
+            drop(o.log(3)),
+        );
+    });
+
+    // The same holds for `super let` initializers in non-extending expressions.
+    assert_drop_order(1..=4, |o| {
+        #[cfg(e2021)]
+        (
+            {
+                super let _ = {
+                    super let _ = { &o.log(4) as *const LogDrop<'_> };
+                    drop(o.log(1))
+                };
+                drop(o.log(2))
+            },
+            drop(o.log(3)),
+        );
+        #[cfg(e2024)]
+        (
+            {
+                super let _ = {
+                    super let _ = { &o.log(1) as *const LogDrop<'_> };
+                    drop(o.log(2))
+                };
+                drop(o.log(3))
+            },
+            drop(o.log(4)),
+        );
+    });
+
+    // Within an extending expression, the argument to `pin!` is also an extending expression,
+    // allowing borrow operands in block tail expressions to have extended lifetimes.
+    assert_drop_order(1..=2, |o| {
+        let _ = pin!({ &o.log(2) as *const LogDrop<'_> });
+        drop(o.log(1));
+    });
+
+    // The same holds for `super let` initializers in extending expressions.
+    assert_drop_order(1..=2, |o| {
+        let _ =  { super let _ = { &o.log(2) as *const LogDrop<'_> }; };
+        drop(o.log(1));
+    });
+
+    // We have extending borrow expressions within an extending block
+    // expression (within an extending borrow expression) within a
+    // non-extending expresion within the initializer expression.
+    #[cfg(e2021)]
+    {
+        // These two should be the same.
+        assert_drop_order(1..=3, |e| {
+            let _v = f(e.log(1), &{ &raw const *&e.log(2) });
+            drop(e.log(3));
+        });
+        assert_drop_order(1..=3, |e| {
+            let _v = f(e.log(1), {
+                super let v = &{ &raw const *&e.log(2) };
+                v
+            });
+            drop(e.log(3));
+        });
+    }
+    #[cfg(e2024)]
+    {
+        // These two should be the same.
+        assert_drop_order(1..=3, |e| {
+            let _v = f(e.log(2), &{ &raw const *&e.log(1) });
+            drop(e.log(3));
+        });
+        assert_drop_order(1..=3, |e| {
+            let _v = f(e.log(2), {
+                super let v = &{ &raw const *&e.log(1) };
+                v
+            });
+            drop(e.log(3));
+        });
+    }
+
+    // We have extending borrow expressions within a non-extending
+    // expression within the initializer expression.
+    //
+    // These two should be the same.
+    assert_drop_order(1..=3, |e| {
+        let _v = f(e.log(1), &&raw const *&e.log(2));
+        drop(e.log(3));
+    });
+    assert_drop_order(1..=3, |e| {
+        let _v = f(e.log(1), {
+            super let v = &&raw const *&e.log(2);
+            v
+        });
+        drop(e.log(3));
+    });
+
+    // We have extending borrow expressions within an extending block
+    // expression (within an extending borrow expression) within the
+    // initializer expression.
+    //
+    // These two should be the same.
+    assert_drop_order(1..=2, |e| {
+        let _v = &{ &raw const *&e.log(2) };
+        drop(e.log(1));
+    });
+    assert_drop_order(1..=2, |e| {
+        let _v = {
+            super let v = &{ &raw const *&e.log(2) };
+            v
+        };
+        drop(e.log(1));
+    });
+}
+
+// # Test scaffolding...
+
+struct DropOrder(RefCell<Vec<u64>>);
+struct LogDrop<'o>(&'o DropOrder, u64);
+
+impl DropOrder {
+    fn log(&self, n: u64) -> LogDrop<'_> {
+        LogDrop(self, n)
+    }
+}
+
+impl<'o> Drop for LogDrop<'o> {
+    fn drop(&mut self) {
+        self.0.0.borrow_mut().push(self.1);
+    }
+}
+
+#[track_caller]
+fn assert_drop_order(
+    ex: impl IntoIterator<Item = u64>,
+    f: impl Fn(&DropOrder),
+) {
+    let order = DropOrder(RefCell::new(Vec::new()));
+    f(&order);
+    let order = order.0.into_inner();
+    let expected: Vec<u64> = ex.into_iter().collect();
+    assert_eq!(order, expected);
+}