Rollup merge of rust-lang#154994 - dianne:no-dbg-temp, r=Mark-Simulacrum

don't leak internal temporaries from `dbg!`

Fixes rust-lang#154988

r? @Mark-Simulacrum as the reviewer of rust-lang#154074

Author: JonathanBrouwer

library/std/src/macros.rs

@@ -369,7 +369,8 @@ macro_rules! dbg {
 /// E.g. `dbg_internal!(() () (1, 2))` expands into
 /// ```rust, ignore
 /// match (1, 2) {
-///     (tmp_1, tmp_2) => {
+///     args => {
+///         let (tmp_1, tmp_2) = args;
 ///         eprint!("...", &tmp_1, &tmp_2, /* some other arguments */);
 ///         (tmp_1, tmp_2)
 ///     }
@@ -385,7 +386,9 @@ pub macro dbg_internal {
         // of temporaries - https://stackoverflow.com/a/48732525/1063961
         // Always put the arguments in a tuple to avoid an unused parens lint on the pattern.
         match ($($processed,)+) {
-            ($($bound,)+) => {
+            // Move the entire tuple so it doesn't stick around as a temporary (#154988).
+            args => {
+                let ($($bound,)+) = args;
                 $crate::eprint!(
                     $crate::concat!($($piece),+),
                     $(

library/std/src/macros/tests.rs

@@ -1,5 +1,7 @@
 // ignore-tidy-dbg
 
+use core::fmt::Debug;
+
 /// Test for <https://github.com/rust-lang/rust/issues/153850>:
 /// `dbg!` shouldn't drop arguments' temporaries.
 #[test]
@@ -11,3 +13,25 @@ fn no_dropping_temps() {
     *dbg!(0, &temp()).1;
     *dbg!(0, &temp(), 2).1;
 }
+
+/// Test for <https://github.com/rust-lang/rust/issues/154988>:
+/// `dbg!` shouldn't create a temporary that lives past its invocation.
+#[test]
+fn no_leaking_internal_temps_from_dbg() {
+    #[derive(Debug)]
+    struct Foo;
+
+    #[derive(Debug)]
+    struct Bar<'a>(#[allow(unused)] &'a Foo);
+    impl Drop for Bar<'_> {
+        fn drop(&mut self) {}
+    }
+
+    let foo = Foo;
+    let bar = Bar(&foo);
+    // If `dbg!` creates a `(Bar<'_>,)` temporary that lasts past its expansion, this will fail
+    // to compile, because it will be dropped after `foo`, which it borrows from. The tuple
+    // mimics the drop order of block tail expressions before Rust 2024: first the result of `dbg!`
+    // is dropped, then `foo`, then any temporaries left over from `dbg!` are dropped, if present.
+    (drop(dbg!(bar)), drop(foo));
+}