std: avoid tearing dbg! prints

library/std/src/lib.rs

@@ -469,7 +469,9 @@ extern crate std as realstd;
 
 // The standard macros that are not built-in to the compiler.
 #[macro_use]
-mod macros;
+#[doc(hidden)]
+#[unstable(feature = "std_internals", issue = "none")]
+pub mod macros;
 
 // The runtime entry point and a few unstable public functions used by the
 // compiler

library/std/src/macros.rs

@@ -347,35 +347,70 @@ macro_rules! eprintln {
 /// [`debug!`]: https://docs.rs/log/*/log/macro.debug.html
 /// [`log`]: https://crates.io/crates/log
 #[macro_export]
+#[allow_internal_unstable(std_internals)]
 #[cfg_attr(not(test), rustc_diagnostic_item = "dbg_macro")]
 #[stable(feature = "dbg_macro", since = "1.32.0")]
 macro_rules! dbg {
-    // NOTE: We cannot use `concat!` to make a static string as a format argument
-    // of `eprintln!` because `file!` could contain a `{` or
-    // `$val` expression could be a block (`{ .. }`), in which case the `eprintln!`
-    // will be malformed.
     () => {
         $crate::eprintln!("[{}:{}:{}]", $crate::file!(), $crate::line!(), $crate::column!())
     };
-    ($val:expr $(,)?) => {
+    ($($val:expr),+ $(,)?) => {
+        $crate::macros::dbg_internal!(() () ($($val),+))
+    };
+}
+
+/// Internal macro that processes a list of expressions and produces a chain of
+/// nested `match`es, one for each expression, before finally calling `eprint!`
+/// with the collected information and returning all the evaluated expressions
+/// in a tuple.
+///
+/// E.g. `dbg_internal!(() () (1, 2))` expands into
+/// ```rust, ignore
+/// match 1 {
+///     tmp_1 => match 2 {
+///         tmp_2 => {
+///             eprint!("...", &tmp_1, &tmp_2, /* some other arguments */);
+///             (tmp_1, tmp_2)
+///         }
+///     }
+/// }
+/// ```
+///
+/// This is necessary so that `dbg!` outputs don't get torn, see #136703.
+#[doc(hidden)]
+#[rustc_macro_transparency = "semitransparent"]
+pub macro dbg_internal {
+    (($($piece:literal),+) ($($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 $crate::fmt::Debug
+            ),+,
+            // The location returned here is that of the macro invokation, so
+            // it will be the same for all expressions. Thus, label these
+            // arguments so that they can be reused in every piece of the
+            // formatting template.
+            file=$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)*)) => {{
         // Use of `match` here is intentional because it affects the lifetimes
         // of temporaries - https://stackoverflow.com/a/48732525/1063961
         match $val {
-            tmp => {
-                $crate::eprintln!("[{}:{}:{}] {} = {:#?}",
-                    $crate::file!(),
-                    $crate::line!(),
-                    $crate::column!(),
-                    $crate::stringify!($val),
-                    // The `&T: Debug` check happens here (not in the format literal desugaring)
-                    // to avoid format literal related messages and suggestions.
-                    &&tmp as &dyn $crate::fmt::Debug,
-                );
-                tmp
-            }
+            tmp => $crate::macros::dbg_internal!(
+                ($($piece,)* "[{file}:{line}:{column}] {} = {:#?}]\n")
+                ($($processed => $bound,)* $val => tmp)
+                ($($rest),*)
+            ),
         }
-    };
-    ($($val:expr),+ $(,)?) => {
-        ($($crate::dbg!($val)),+,)
-    };
+    }},
 }