Improve errors for missing Debug and Display impls

compiler/rustc_span/src/symbol.rs

@@ -479,6 +479,7 @@ symbols! {
         discriminant_type,
         discriminant_value,
         dispatch_from_dyn,
+        display_trait,
         div,
         div_assign,
         doc,

compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

@@ -514,6 +514,31 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                             }
                         }
 
+                        // Return early if the trait is Debug or Display and the invocation
+                        // originates within a formatting macro, because the output
+                        // is otherwise overwhelming and unhelpful (see #85844 for an
+                        // example).
+                        if let Some(macro_def_id) =
+                            obligation.cause.span.ctxt().outer_expn_data().macro_def_id
+                        {
+                            let trait_is_debug =
+                                self.tcx.is_diagnostic_item(sym::debug_trait, trait_ref.def_id());
+                            let trait_is_display =
+                                self.tcx.is_diagnostic_item(sym::display_trait, trait_ref.def_id());
+
+                            let macro_is_format_args =
+                                self.tcx.is_diagnostic_item(sym::format_args, macro_def_id);
+                            let macro_is_format_args_nl =
+                                self.tcx.is_diagnostic_item(sym::format_args_nl, macro_def_id);
+
+                            if (macro_is_format_args || macro_is_format_args_nl)
+                                && (trait_is_debug || trait_is_display)
+                            {
+                                err.emit();
+                                return;
+                            }
+                        }
+
                         err
                     }
 

library/core/src/fmt/mod.rs

@@ -564,7 +564,7 @@ impl Display for Arguments<'_> {
     on(
         crate_local,
         label = "`{Self}` cannot be formatted using `{{:?}}`",
-        note = "add `#[derive(Debug)]` or manually implement `{Debug}`"
+        note = "add `#[derive(Debug)]` to `{Self}` or manually implement `{Debug}` for `{Self}`"
     ),
     message = "`{Self}` doesn't implement `{Debug}`",
     label = "`{Self}` cannot be formatted using `{{:?}}` because it doesn't implement `{Debug}`"
@@ -662,6 +662,7 @@ pub use macros::Debug;
     note = "in format strings you may be able to use `{{:?}}` (or {{:#?}} for pretty-print) instead"
 )]
 #[doc(alias = "{}")]
+#[rustc_diagnostic_item = "display_trait"]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait Display {
     /// Formats the value using the given formatter.

library/core/src/macros/mod.rs

@@ -825,6 +825,7 @@ pub(crate) mod builtin {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[allow_internal_unstable(fmt_internals)]
     #[rustc_builtin_macro]
+    #[rustc_diagnostic_item = "format_args"]
     #[macro_export]
     macro_rules! format_args {
         ($fmt:expr) => {{ /* compiler built-in */ }};
@@ -840,6 +841,7 @@ pub(crate) mod builtin {
     )]
     #[allow_internal_unstable(fmt_internals)]
     #[rustc_builtin_macro]
+    #[rustc_diagnostic_item = "format_args_nl"]
     #[macro_export]
     macro_rules! format_args_nl {
         ($fmt:expr) => {{ /* compiler built-in */ }};

src/test/ui/bound-suggestions.stderr

@@ -4,7 +4,6 @@ error[E0277]: `impl Sized` doesn't implement `Debug`
 LL |     println!("{:?}", t);
    |                      ^ `impl Sized` cannot be formatted using `{:?}` because it doesn't implement `Debug`
    |
-   = note: required by `std::fmt::Debug::fmt`
    = note: this error originates in the macro `$crate::format_args_nl` (in Nightly builds, run with -Z macro-backtrace for more info)
 help: consider further restricting this bound
    |
@@ -17,7 +16,6 @@ error[E0277]: `T` doesn't implement `Debug`
 LL |     println!("{:?}", t);
    |                      ^ `T` cannot be formatted using `{:?}` because it doesn't implement `Debug`
    |
-   = note: required by `std::fmt::Debug::fmt`
    = note: this error originates in the macro `$crate::format_args_nl` (in Nightly builds, run with -Z macro-backtrace for more info)
 help: consider restricting type parameter `T`
    |
@@ -30,7 +28,6 @@ error[E0277]: `T` doesn't implement `Debug`
 LL |     println!("{:?}", t);
    |                      ^ `T` cannot be formatted using `{:?}` because it doesn't implement `Debug`
    |
-   = note: required by `std::fmt::Debug::fmt`
    = note: this error originates in the macro `$crate::format_args_nl` (in Nightly builds, run with -Z macro-backtrace for more info)
 help: consider further restricting this bound
    |
@@ -43,7 +40,6 @@ error[E0277]: `Y` doesn't implement `Debug`
 LL |     println!("{:?} {:?}", x, y);
    |                              ^ `Y` cannot be formatted using `{:?}` because it doesn't implement `Debug`
    |
-   = note: required by `std::fmt::Debug::fmt`
    = note: this error originates in the macro `$crate::format_args_nl` (in Nightly builds, run with -Z macro-backtrace for more info)
 help: consider further restricting type parameter `Y`
    |
@@ -56,7 +52,6 @@ error[E0277]: `X` doesn't implement `Debug`
 LL |     println!("{:?}", x);
    |                      ^ `X` cannot be formatted using `{:?}` because it doesn't implement `Debug`
    |
-   = note: required by `std::fmt::Debug::fmt`
    = note: this error originates in the macro `$crate::format_args_nl` (in Nightly builds, run with -Z macro-backtrace for more info)
 help: consider further restricting this bound
    |
@@ -69,7 +64,6 @@ error[E0277]: `X` doesn't implement `Debug`
 LL |     println!("{:?}", x);
    |                      ^ `X` cannot be formatted using `{:?}` because it doesn't implement `Debug`
    |
-   = note: required by `std::fmt::Debug::fmt`
    = note: this error originates in the macro `$crate::format_args_nl` (in Nightly builds, run with -Z macro-backtrace for more info)
 help: consider further restricting type parameter `X`
    |

src/test/ui/derives/derives-span-Debug-enum-struct-variant.stderr

@@ -5,7 +5,7 @@ LL |      x: Error
    |      ^^^^^^^^ `Error` cannot be formatted using `{:?}`
    |
    = help: the trait `Debug` is not implemented for `Error`
-   = note: add `#[derive(Debug)]` or manually implement `Debug`
+   = note: add `#[derive(Debug)]` to `Error` or manually implement `Debug` for `Error`
    = note: required because of the requirements on the impl of `Debug` for `&Error`
    = note: required for the cast to the object type `dyn Debug`
    = note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)

src/test/ui/derives/derives-span-Debug-enum.stderr

@@ -5,7 +5,7 @@ LL |      Error
    |      ^^^^^ `Error` cannot be formatted using `{:?}`
    |
    = help: the trait `Debug` is not implemented for `Error`
-   = note: add `#[derive(Debug)]` or manually implement `Debug`
+   = note: add `#[derive(Debug)]` to `Error` or manually implement `Debug` for `Error`
    = note: required because of the requirements on the impl of `Debug` for `&Error`
    = note: required for the cast to the object type `dyn Debug`
    = note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)

src/test/ui/derives/derives-span-Debug-struct.stderr

@@ -5,7 +5,7 @@ LL |     x: Error
    |     ^^^^^^^^ `Error` cannot be formatted using `{:?}`
    |
    = help: the trait `Debug` is not implemented for `Error`
-   = note: add `#[derive(Debug)]` or manually implement `Debug`
+   = note: add `#[derive(Debug)]` to `Error` or manually implement `Debug` for `Error`
    = note: required because of the requirements on the impl of `Debug` for `&Error`
    = note: required for the cast to the object type `dyn Debug`
    = note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)

src/test/ui/derives/derives-span-Debug-tuple-struct.stderr

@@ -5,7 +5,7 @@ LL |     Error
    |     ^^^^^ `Error` cannot be formatted using `{:?}`
    |
    = help: the trait `Debug` is not implemented for `Error`
-   = note: add `#[derive(Debug)]` or manually implement `Debug`
+   = note: add `#[derive(Debug)]` to `Error` or manually implement `Debug` for `Error`
    = note: required because of the requirements on the impl of `Debug` for `&Error`
    = note: required for the cast to the object type `dyn Debug`
    = note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)

src/test/ui/on-unimplemented/no-debug.stderr

@@ -5,8 +5,7 @@ LL |     println!("{:?} {:?}", Foo, Bar);
    |                           ^^^ `Foo` cannot be formatted using `{:?}`
    |
    = help: the trait `Debug` is not implemented for `Foo`
-   = note: add `#[derive(Debug)]` or manually implement `Debug`
-   = note: required by `std::fmt::Debug::fmt`
+   = note: add `#[derive(Debug)]` to `Foo` or manually implement `Debug` for `Foo`
    = note: this error originates in the macro `$crate::format_args_nl` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0277]: `Bar` doesn't implement `Debug`
@@ -16,7 +15,6 @@ LL |     println!("{:?} {:?}", Foo, Bar);
    |                                ^^^ `Bar` cannot be formatted using `{:?}` because it doesn't implement `Debug`
    |
    = help: the trait `Debug` is not implemented for `Bar`
-   = note: required by `std::fmt::Debug::fmt`
    = note: this error originates in the macro `$crate::format_args_nl` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0277]: `Foo` doesn't implement `std::fmt::Display`
@@ -27,7 +25,6 @@ LL |     println!("{} {}", Foo, Bar);
    |
    = help: the trait `std::fmt::Display` is not implemented for `Foo`
    = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
-   = note: required by `std::fmt::Display::fmt`
    = note: this error originates in the macro `$crate::format_args_nl` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0277]: `Bar` doesn't implement `std::fmt::Display`
@@ -38,7 +35,6 @@ LL |     println!("{} {}", Foo, Bar);
    |
    = help: the trait `std::fmt::Display` is not implemented for `Bar`
    = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
-   = note: required by `std::fmt::Display::fmt`
    = note: this error originates in the macro `$crate::format_args_nl` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to 4 previous errors

src/test/ui/rfc-2361-dbg-macro/dbg-macro-requires-debug.stderr

@@ -5,9 +5,7 @@ LL |     let _: NotDebug = dbg!(NotDebug);
    |                       ^^^^^^^^^^^^^^ `NotDebug` cannot be formatted using `{:?}`
    |
    = help: the trait `Debug` is not implemented for `NotDebug`
-   = note: add `#[derive(Debug)]` or manually implement `Debug`
-   = note: required because of the requirements on the impl of `Debug` for `&NotDebug`
-   = note: required by `std::fmt::Debug::fmt`
+   = note: add `#[derive(Debug)]` to `NotDebug` or manually implement `Debug` for `NotDebug`
    = note: this error originates in the macro `$crate::format_args_nl` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to previous error

src/test/ui/suggestions/path-display.stderr

@@ -6,8 +6,6 @@ LL |     println!("{}", path);
    |
    = help: the trait `std::fmt::Display` is not implemented for `Path`
    = note: call `.display()` or `.to_string_lossy()` to safely print paths, as they may contain non-Unicode data
-   = note: required because of the requirements on the impl of `std::fmt::Display` for `&Path`
-   = note: required by `std::fmt::Display::fmt`
    = note: this error originates in the macro `$crate::format_args_nl` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to previous error