don't drop arguments' temporaries in dbg!

dianne · dianne · commit 21dff86d16cb · 2026-03-18T20:46:48.000-07:00
diff --git a/library/std/src/macros.rs b/library/std/src/macros.rs
@@ -359,19 +359,16 @@ macro_rules! dbg {
     };
 }
 
-/// Internal macro that processes a list of expressions and produces a chain of
-/// nested `match`es, one for each expression, before finally calling `eprint!`
-/// with the collected information and returning all the evaluated expressions
-/// in a tuple.
+/// Internal macro that processes a list of expressions, binds their results
+/// with `match`, calls `eprint!` with the collected information, and returns
+/// all the evaluated expressions in a tuple.
 ///
 /// E.g. `dbg_internal!(() () (1, 2))` expands into
 /// ```rust, ignore
-/// match 1 {
-///     tmp_1 => match 2 {
-///         tmp_2 => {
-///             eprint!("...", &tmp_1, &tmp_2, /* some other arguments */);
-///             (tmp_1, tmp_2)
-///         }
+/// match (1, 2) {
+///     (tmp_1, tmp_2) => {
+///         eprint!("...", &tmp_1, &tmp_2, /* some other arguments */);
+///         (tmp_1, tmp_2)
 ///     }
 /// }
 /// ```
@@ -380,37 +377,41 @@ macro_rules! dbg {
 #[doc(hidden)]
 #[rustc_macro_transparency = "semiopaque"]
 pub macro dbg_internal {
-    (($($piece:literal),+) ($($processed:expr => $bound:expr),+) ()) => {{
-        $crate::eprint!(
-            $crate::concat!($($piece),+),
-            $(
-                $crate::stringify!($processed),
-                // The `&T: Debug` check happens here (not in the format literal desugaring)
-                // to avoid format literal related messages and suggestions.
-                &&$bound as &dyn $crate::fmt::Debug
-            ),+,
-            // The location returned here is that of the macro invocation, so
-            // it will be the same for all expressions. Thus, label these
-            // arguments so that they can be reused in every piece of the
-            // formatting template.
-            file=$crate::file!(),
-            line=$crate::line!(),
-            column=$crate::column!()
-        );
-        // Comma separate the variables only when necessary so that this will
-        // not yield a tuple for a single expression, but rather just parenthesize
-        // the expression.
-        ($($bound),+)
-    }},
-    (($($piece:literal),*) ($($processed:expr => $bound:expr),*) ($val:expr $(,$rest:expr)*)) => {
+    (($($piece:literal),+) ($($processed:expr => $bound:ident),+) ()) => {
         // Use of `match` here is intentional because it affects the lifetimes
         // of temporaries - https://stackoverflow.com/a/48732525/1063961
-        match $val {
-            tmp => $crate::macros::dbg_internal!(
-                ($($piece,)* "[{file}:{line}:{column}] {} = {:#?}\n")
-                ($($processed => $bound,)* $val => tmp)
-                ($($rest),*)
-            ),
+        // Always put the arguments in a tuple to avoid an unused parens lint on the pattern.
+        match ($($processed,)+) {
+            ($($bound,)+) => {
+                $crate::eprint!(
+                    $crate::concat!($($piece),+),
+                    $(
+                        $crate::stringify!($processed),
+                        // The `&T: Debug` check happens here (not in the format literal desugaring)
+                        // to avoid format literal related messages and suggestions.
+                        &&$bound as &dyn $crate::fmt::Debug
+                    ),+,
+                    // The location returned here is that of the macro invocation, so
+                    // it will be the same for all expressions. Thus, label these
+                    // arguments so that they can be reused in every piece of the
+                    // formatting template.
+                    file=$crate::file!(),
+                    line=$crate::line!(),
+                    column=$crate::column!()
+                );
+                // Comma separate the variables only when necessary so that this will
+                // not yield a tuple for a single expression, but rather just parenthesize
+                // the expression.
+                ($($bound),+)
+
+            }
         }
     },
+    (($($piece:literal),*) ($($processed:expr => $bound:ident),*) ($val:expr $(,$rest:expr)*)) => {
+        $crate::macros::dbg_internal!(
+            ($($piece,)* "[{file}:{line}:{column}] {} = {:#?}\n")
+            ($($processed => $bound,)* $val => tmp)
+            ($($rest),*)
+        )
+    },
 }
diff --git a/library/std/tests/dbg-macro.rs b/library/std/tests/dbg-macro.rs
@@ -0,0 +1,13 @@
+// ignore-tidy-dbg
+
+/// Test for <https://github.com/rust-lang/rust/issues/153850>:
+/// `dbg!` shouldn't drop arguments' temporaries.
+#[test]
+fn no_dropping_temps() {
+    fn temp() {}
+
+    *dbg!(&temp());
+    *dbg!(&temp(), 1).0;
+    *dbg!(0, &temp()).1;
+    *dbg!(0, &temp(), 2).1;
+}
diff --git a/src/tools/clippy/clippy_lints/src/dbg_macro.rs b/src/tools/clippy/clippy_lints/src/dbg_macro.rs
@@ -5,7 +5,7 @@ use clippy_utils::source::snippet_with_applicability;
 use clippy_utils::{is_in_test, sym};
 use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::Applicability;
-use rustc_hir::{Arm, Closure, ClosureKind, CoroutineKind, Expr, ExprKind, LetStmt, LocalSource, Node, Stmt, StmtKind};
+use rustc_hir::{Closure, ClosureKind, CoroutineKind, Expr, ExprKind, LetStmt, LocalSource, Node, Stmt, StmtKind};
 use rustc_lint::{LateContext, LateLintPass, LintContext};
 use rustc_session::impl_lint_pass;
 use rustc_span::{Span, SyntaxContext};
@@ -92,16 +92,15 @@ impl LateLintPass<'_> for DbgMacro {
                                 (macro_call.span, String::from("()"))
                             }
                         },
-                        ExprKind::Match(first, arms, _) => {
-                            let vals = collect_vals(first, arms);
-                            let suggestion = match *vals.as_slice() {
+                        ExprKind::Match(args, _, _) => {
+                            let suggestion = match args.kind {
                                 // dbg!(1) => 1
-                                [val] => {
+                                ExprKind::Tup([val]) => {
                                     snippet_with_applicability(cx, val.span.source_callsite(), "..", &mut applicability)
                                         .to_string()
                                 },
                                 // dbg!(2, 3) => (2, 3)
-                                [first, .., last] => {
+                                ExprKind::Tup([first, .., last]) => {
                                     let snippet = snippet_with_applicability(
                                         cx,
                                         first.span.source_callsite().to(last.span.source_callsite()),
@@ -165,39 +164,3 @@ fn is_async_move_desugar<'tcx>(expr: &'tcx Expr<'tcx>) -> Option<&'tcx Expr<'tcx
 fn first_dbg_macro_in_expansion(cx: &LateContext<'_>, span: Span) -> Option<MacroCall> {
     macro_backtrace(span).find(|mc| cx.tcx.is_diagnostic_item(sym::dbg_macro, mc.def_id))
 }
-
-/// Extracts all value expressions from the `match`-tree generated by `dbg!`.
-///
-/// E.g. from
-/// ```rust, ignore
-/// match 1 {
-///     tmp_1 => match 2 {
-///         tmp_2 => {
-///             /* printing */
-///             (tmp_1, tmp_2)
-///         }
-///     }
-/// }
-/// ```
-/// this extracts `1` and `2`.
-fn collect_vals<'hir>(first: &'hir Expr<'hir>, mut arms: &'hir [Arm<'hir>]) -> Vec<&'hir Expr<'hir>> {
-    let mut vals = vec![first];
-    loop {
-        let [arm] = arms else {
-            unreachable!("dbg! macro expansion only has single-arm matches")
-        };
-
-        match is_async_move_desugar(arm.body)
-            .unwrap_or(arm.body)
-            .peel_drop_temps()
-            .kind
-        {
-            ExprKind::Block(..) => return vals,
-            ExprKind::Match(val, a, _) => {
-                vals.push(val);
-                arms = a;
-            },
-            _ => unreachable!("dbg! macro expansion only results in block or match expressions"),
-        }
-    }
-}
diff --git a/tests/ui/borrowck/dbg-issue-120327.stderr b/tests/ui/borrowck/dbg-issue-120327.stderr
@@ -4,44 +4,44 @@ error[E0382]: use of moved value: `a`
 LL |     let a = String::new();
    |         - move occurs because `a` has type `String`, which does not implement the `Copy` trait
 LL |     dbg!(a);
-   |     ------- value moved here
+   |          - value moved here
 LL |     return a;
    |            ^ value used here after move
    |
-help: consider borrowing instead of transferring ownership
+help: consider cloning the value if the performance cost is acceptable
    |
-LL |     dbg!(&a);
-   |          +
+LL |     dbg!(a.clone());
+   |           ++++++++
 
 error[E0382]: use of moved value: `a`
   --> $DIR/dbg-issue-120327.rs:10:12
    |
 LL |     let a = String::new();
    |         - move occurs because `a` has type `String`, which does not implement the `Copy` trait
 LL |     dbg!(1, 2, a, 1, 2);
-   |     ------------------- value moved here
+   |                - value moved here
 LL |     return a;
    |            ^ value used here after move
    |
-help: consider borrowing instead of transferring ownership
+help: consider cloning the value if the performance cost is acceptable
    |
-LL |     dbg!(1, 2, &a, 1, 2);
-   |                +
+LL |     dbg!(1, 2, a.clone(), 1, 2);
+   |                 ++++++++
 
 error[E0382]: use of moved value: `b`
   --> $DIR/dbg-issue-120327.rs:16:12
    |
 LL |     let b: String = "".to_string();
    |         - move occurs because `b` has type `String`, which does not implement the `Copy` trait
 LL |     dbg!(a, b);
-   |     ---------- value moved here
+   |             - value moved here
 LL |     return b;
    |            ^ value used here after move
    |
-help: consider borrowing instead of transferring ownership
+help: consider cloning the value if the performance cost is acceptable
    |
-LL |     dbg!(a, &b);
-   |             +
+LL |     dbg!(a, b.clone());
+   |              ++++++++
 
 error[E0382]: use of moved value: `a`
   --> $DIR/dbg-issue-120327.rs:22:12
@@ -50,14 +50,14 @@ LL | fn x(a: String) -> String {
    |      - move occurs because `a` has type `String`, which does not implement the `Copy` trait
 LL |     let b: String = "".to_string();
 LL |     dbg!(a, b);
-   |     ---------- value moved here
+   |          - value moved here
 LL |     return a;
    |            ^ value used here after move
    |
-help: consider borrowing instead of transferring ownership
+help: consider cloning the value if the performance cost is acceptable
    |
-LL |     dbg!(&a, b);
-   |          +
+LL |     dbg!(a.clone(), b);
+   |           ++++++++
 
 error[E0382]: use of moved value: `b`
   --> $DIR/dbg-issue-120327.rs:46:12
@@ -103,14 +103,14 @@ error[E0382]: borrow of moved value: `a`
 LL |     let a: String = "".to_string();
    |         - move occurs because `a` has type `String`, which does not implement the `Copy` trait
 LL |     let _res = get_expr(dbg!(a));
-   |                         ------- value moved here
+   |                              - value moved here
 LL |     let _l = a.len();
    |              ^ value borrowed here after move
    |
-help: consider borrowing instead of transferring ownership
+help: consider cloning the value if the performance cost is acceptable
    |
-LL |     let _res = get_expr(dbg!(&a));
-   |                              +
+LL |     let _res = get_expr(dbg!(a.clone()));
+   |                               ++++++++
 
 error: aborting due to 7 previous errors
 
diff --git a/tests/ui/liveness/liveness-upvars.rs b/tests/ui/liveness/liveness-upvars.rs
@@ -98,7 +98,7 @@ pub fn g<T: Default>(mut v: T) {
 }
 
 pub fn h<T: Copy + Default + std::fmt::Debug>() {
-    let mut z = T::default();
+    let mut z = T::default(); //~ WARN unused variable: `z`
     let _ = move |b| {
         loop {
             if b {
diff --git a/tests/ui/liveness/liveness-upvars.stderr b/tests/ui/liveness/liveness-upvars.stderr
@@ -156,6 +156,14 @@ LL |                 z = T::default();
    |
    = help: maybe it is overwritten before being read?
 
+warning: unused variable: `z`
+  --> $DIR/liveness-upvars.rs:101:9
+   |
+LL |     let mut z = T::default();
+   |         ^^^^^ help: if this is intentional, prefix it with an underscore: `_z`
+   |
+   = note: `#[warn(unused_variables)]` implied by `#[warn(unused)]`
+
 warning: value captured by `state` is never read
   --> $DIR/liveness-upvars.rs:131:9
    |
@@ -196,5 +204,5 @@ LL |         s = yield ();
    |
    = help: maybe it is overwritten before being read?
 
-warning: 24 warnings emitted
+warning: 25 warnings emitted
 
diff --git a/tests/ui/rfcs/rfc-2361-dbg-macro/dbg-macro-move-semantics.stderr b/tests/ui/rfcs/rfc-2361-dbg-macro/dbg-macro-move-semantics.stderr
@@ -1,17 +1,21 @@
 error[E0382]: use of moved value: `a`
-  --> $DIR/dbg-macro-move-semantics.rs:9:13
+  --> $DIR/dbg-macro-move-semantics.rs:9:18
    |
 LL |     let a = NoCopy(0);
    |         - move occurs because `a` has type `NoCopy`, which does not implement the `Copy` trait
 LL |     let _ = dbg!(a);
-   |             ------- value moved here
+   |                  - value moved here
 LL |     let _ = dbg!(a);
-   |             ^^^^^^^ value used here after move
+   |                  ^ value used here after move
    |
-help: consider borrowing instead of transferring ownership
+note: if `NoCopy` implemented `Clone`, you could clone the value
+  --> $DIR/dbg-macro-move-semantics.rs:4:1
    |
-LL |     let _ = dbg!(&a);
-   |                  +
+LL | struct NoCopy(usize);
+   | ^^^^^^^^^^^^^ consider implementing `Clone` for this type
+...
+LL |     let _ = dbg!(a);
+   |                  - you could clone this value
 
 error: aborting due to 1 previous error
 

Original file line number	Diff line number	Diff line change
`@@ -98,7 +98,7 @@ pub fn g<T: Default>(mut v: T) {`
`98`	`98`	`}`
`99`	`99`
`100`	`100`	`pub fn h<T: Copy + Default + std::fmt::Debug>() {`
`101`		`- let mut z = T::default();`
	`101`	+ let mut z = T::default(); //~ WARN unused variable: `z`
`102`	`102`	`let _ = move \|b\| {`
`103`	`103`	`loop {`
`104`	`104`	`if b {`
Original file line number	Diff line number	Diff line change
`@@ -156,6 +156,14 @@ LL \| z = T::default();`
`156`	`156`	`\|`
`157`	`157`	`= help: maybe it is overwritten before being read?`
`158`	`158`
	`159`	+warning: unused variable: `z`
	`160`	`+ --> $DIR/liveness-upvars.rs:101:9`
	`161`	`+ \|`
	`162`	`+LL \| let mut z = T::default();`
	`163`	+ \| ^^^^^ help: if this is intentional, prefix it with an underscore: `_z`
	`164`	`+ \|`
	`165`	+ = note: `#[warn(unused_variables)]` implied by `#[warn(unused)]`
	`166`	`+`
`159`	`167`	warning: value captured by `state` is never read
`160`	`168`	`--> $DIR/liveness-upvars.rs:131:9`
`161`	`169`	`\|`
`@@ -196,5 +204,5 @@ LL \| s = yield ();`
`196`	`204`	`\|`
`197`	`205`	`= help: maybe it is overwritten before being read?`
`198`	`206`
`199`		`-warning: 24 warnings emitted`
	`207`	`+warning: 25 warnings emitted`
`200`	`208`