Stop dbg! macro yapping about format modifiers

Author: mejrs

library/std/src/macros.rs

@@ -363,7 +363,14 @@ macro_rules! dbg {
         match $val {
             tmp => {
                 $crate::eprintln!("[{}:{}:{}] {} = {:#?}",
-                    $crate::file!(), $crate::line!(), $crate::column!(), $crate::stringify!($val), &tmp);
+                    $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
             }
         }

tests/ui/modules/issue-107649.stderr

@@ -2,12 +2,10 @@ error[E0277]: `Dummy` doesn't implement `Debug`
    --> $DIR/issue-107649.rs:105:5
     |
 105 |     dbg!(lib::Dummy);
-    |     ^^^^^^^^^^^^^^^^ `Dummy` cannot be formatted using `{:?}` because it doesn't implement `Debug`
+    |     ^^^^^^^^^^^^^^^^ the trait `Debug` is not implemented for `Dummy`
     |
-    = help: the trait `Debug` is not implemented for `Dummy`
     = note: add `#[derive(Debug)]` to `Dummy` or manually `impl Debug for Dummy`
-    = note: required for `&Dummy` to implement `Debug`
-    = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `dbg` (in Nightly builds, run with -Z macro-backtrace for more info)
+    = note: this error originates in the macro `dbg` (in Nightly builds, run with -Z macro-backtrace for more info)
 help: consider annotating `Dummy` with `#[derive(Debug)]`
    --> $DIR/auxiliary/dummy_lib.rs:2:1
     |

tests/ui/rfcs/rfc-2361-dbg-macro/dbg-macro-requires-debug.rs

@@ -1,4 +1,7 @@
 // Test ensuring that `dbg!(expr)` requires the passed type to implement `Debug`.
+//
+// `dbg!` shouldn't tell the user about format literal syntax; the user didn't write one.
+//@ forbid-output: cannot be formatted using
 
 struct NotDebug;
 

tests/ui/rfcs/rfc-2361-dbg-macro/dbg-macro-requires-debug.stderr

@@ -1,13 +1,11 @@
 error[E0277]: `NotDebug` doesn't implement `Debug`
-  --> $DIR/dbg-macro-requires-debug.rs:6:23
+  --> $DIR/dbg-macro-requires-debug.rs:9:23
    |
 LL |     let _: NotDebug = dbg!(NotDebug);
-   |                       ^^^^^^^^^^^^^^ `NotDebug` cannot be formatted using `{:?}` because it doesn't implement `Debug`
+   |                       ^^^^^^^^^^^^^^ the trait `Debug` is not implemented for `NotDebug`
    |
-   = help: the trait `Debug` is not implemented for `NotDebug`
    = note: add `#[derive(Debug)]` to `NotDebug` or manually `impl Debug for NotDebug`
-   = note: required for `&NotDebug` to implement `Debug`
-   = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `dbg` (in Nightly builds, run with -Z macro-backtrace for more info)
+   = note: this error originates in the macro `dbg` (in Nightly builds, run with -Z macro-backtrace for more info)
 help: consider annotating `NotDebug` with `#[derive(Debug)]`
    |
 LL + #[derive(Debug)]