Auto merge of #86386 - inquisitivecrystal:better-errors-for-display-traits-v3, r=estebank

Better errors for Debug and Display traits

Currently, if someone tries to pass value that does not implement `Debug` or `Display` to a formatting macro, they get a very verbose and confusing error message. This PR changes the error messages for missing `Debug` and `Display` impls to be less overwhelming in this case, as suggested by #85844. I was a little less aggressive in changing the error message than that issue proposed. Still, this implementation would be enough to reduce the number of messages to be much more manageable.

After this PR, information on the cause of an error involving a `Debug` or `Display` implementation would suppressed if the requirement originated within a standard library macro. My reasoning was that errors originating from within a macro are confusing when they mention details that the programmer can't see, and this is particularly problematic for `Debug` and `Display`, which are most often used via macros. It is possible that either a broader or a narrower criterion would be better. I'm quite open to any feedback.

Fixes #85844.

14 files changed, 33 insertions, 44 deletions

diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index 55b6056209d..be4f12c6d1c 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -478,6 +478,7 @@ symbols! {
         discriminant_type,
         discriminant_value,
         dispatch_from_dyn,
+        display_trait,
         div,
         div_assign,
         doc,
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
index 19c3385dd4c..e276d92bf5a 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
@@ -514,6 +514,30 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                             }
                         }
 
+                        // Return early if the trait is Debug or Display and the invocation
+                        // originates within a standard library macro, because the output
+                        // is otherwise overwhelming and unhelpful (see #85844 for an
+                        // example).
+
+                        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 in_std_macro =
+                            match obligation.cause.span.ctxt().outer_expn_data().macro_def_id {
+                                Some(macro_def_id) => {
+                                    let crate_name = tcx.crate_name(macro_def_id.krate);
+                                    crate_name == sym::std || crate_name == sym::core
+                                }
+                                None => false,
+                            };
+
+                        if in_std_macro && (trait_is_debug || trait_is_display) {
+                            err.emit();
+                            return;
+                        }
+
                         err
                     }
 
diff --git a/library/core/src/fmt/mod.rs b/library/core/src/fmt/mod.rs
index 02ac4fb8006..409a84367a7 100644
--- a/library/core/src/fmt/mod.rs
+++ b/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 `impl {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.
diff --git a/src/test/ui/binop/issue-77910-1.stderr b/src/test/ui/binop/issue-77910-1.stderr
index fa967ccb9d0..ed71b99da0f 100644
--- a/src/test/ui/binop/issue-77910-1.stderr
+++ b/src/test/ui/binop/issue-77910-1.stderr
@@ -14,11 +14,6 @@ error[E0277]: `for<'r> fn(&'r i32) -> &'r i32 {foo}` doesn't implement `Debug`
    |
 LL |     assert_eq!(foo, y);
    |     ^^^^^^^^^^^^^^^^^^^ `for<'r> fn(&'r i32) -> &'r i32 {foo}` cannot be formatted using `{:?}` because it doesn't implement `Debug`
-   | 
-  ::: $SRC_DIR/core/src/panicking.rs:LL:COL
-   |
-LL |     T: fmt::Debug + ?Sized,
-   |        ---------- required by this bound in `core::panicking::assert_failed`
    |
    = help: the trait `Debug` is not implemented for `for<'r> fn(&'r i32) -> &'r i32 {foo}`
    = note: this error originates in the macro `assert_eq` (in Nightly builds, run with -Z macro-backtrace for more info)
diff --git a/src/test/ui/bound-suggestions.stderr b/src/test/ui/bound-suggestions.stderr
index a3177903162..78f62592960 100644
--- a/src/test/ui/bound-suggestions.stderr
+++ b/src/test/ui/bound-suggestions.stderr
@@ -4,7 +4,6 @@ error[E0277]: `impl Sized` doesn't implement `Debug`
 LL |     println!("{:?}", t);
    |                      ^ `impl Sized` cannot be formatted using `{:?}` because it doesn't implement `Debug`
    |
-   = note: required by `std::fmt::Debug::fmt`
    = note: this error originates in the macro `$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`
    |
diff --git a/src/test/ui/derives/derives-span-Debug-enum-struct-variant.stderr b/src/test/ui/derives/derives-span-Debug-enum-struct-variant.stderr
index 2151335992f..4ec89fe729f 100644
--- a/src/test/ui/derives/derives-span-Debug-enum-struct-variant.stderr
+++ b/src/test/ui/derives/derives-span-Debug-enum-struct-variant.stderr
@@ -5,9 +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: required because of the requirements on the impl of `Debug` for `&Error`
-   = note: required for the cast to the object type `dyn Debug`
+   = note: add `#[derive(Debug)]` to `Error` or manually `impl Debug for Error`
    = note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to previous error
diff --git a/src/test/ui/derives/derives-span-Debug-enum.stderr b/src/test/ui/derives/derives-span-Debug-enum.stderr
index 99e618b40ec..d564b6ad76b 100644
--- a/src/test/ui/derives/derives-span-Debug-enum.stderr
+++ b/src/test/ui/derives/derives-span-Debug-enum.stderr
@@ -5,9 +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: required because of the requirements on the impl of `Debug` for `&Error`
-   = note: required for the cast to the object type `dyn Debug`
+   = note: add `#[derive(Debug)]` to `Error` or manually `impl Debug for Error`
    = note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to previous error
diff --git a/src/test/ui/derives/derives-span-Debug-struct.stderr b/src/test/ui/derives/derives-span-Debug-struct.stderr
index 0086642c557..352141c7e33 100644
--- a/src/test/ui/derives/derives-span-Debug-struct.stderr
+++ b/src/test/ui/derives/derives-span-Debug-struct.stderr
@@ -5,9 +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: required because of the requirements on the impl of `Debug` for `&Error`
-   = note: required for the cast to the object type `dyn Debug`
+   = note: add `#[derive(Debug)]` to `Error` or manually `impl Debug for Error`
    = note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to previous error
diff --git a/src/test/ui/derives/derives-span-Debug-tuple-struct.stderr b/src/test/ui/derives/derives-span-Debug-tuple-struct.stderr
index 0b74908d1ee..65765490183 100644
--- a/src/test/ui/derives/derives-span-Debug-tuple-struct.stderr
+++ b/src/test/ui/derives/derives-span-Debug-tuple-struct.stderr
@@ -5,9 +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: required because of the requirements on the impl of `Debug` for `&Error`
-   = note: required for the cast to the object type `dyn Debug`
+   = note: add `#[derive(Debug)]` to `Error` or manually `impl Debug for Error`
    = note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to previous error
diff --git a/src/test/ui/issues/issue-59488.stderr b/src/test/ui/issues/issue-59488.stderr
index 93d2f3001f8..46d1f992411 100644
--- a/src/test/ui/issues/issue-59488.stderr
+++ b/src/test/ui/issues/issue-59488.stderr
@@ -84,11 +84,6 @@ error[E0277]: `fn(usize) -> Foo {Foo::Bar}` doesn't implement `Debug`
    |
 LL |     assert_eq!(Foo::Bar, i);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^ `fn(usize) -> Foo {Foo::Bar}` cannot be formatted using `{:?}` because it doesn't implement `Debug`
-   | 
-  ::: $SRC_DIR/core/src/panicking.rs:LL:COL
-   |
-LL |     T: fmt::Debug + ?Sized,
-   |        ---------- required by this bound in `core::panicking::assert_failed`
    |
    = help: the trait `Debug` is not implemented for `fn(usize) -> Foo {Foo::Bar}`
    = note: this error originates in the macro `assert_eq` (in Nightly builds, run with -Z macro-backtrace for more info)
diff --git a/src/test/ui/issues/issue-70724-add_type_neq_err_label-unwrap.stderr b/src/test/ui/issues/issue-70724-add_type_neq_err_label-unwrap.stderr
index 0f095f5a77b..f074a99e5ec 100644
--- a/src/test/ui/issues/issue-70724-add_type_neq_err_label-unwrap.stderr
+++ b/src/test/ui/issues/issue-70724-add_type_neq_err_label-unwrap.stderr
@@ -28,11 +28,6 @@ LL | fn a() -> i32 {
 ...
 LL |     assert_eq!(a, 0);
    |     ^^^^^^^^^^^^^^^^^ `fn() -> i32 {a}` cannot be formatted using `{:?}` because it doesn't implement `Debug`
-   | 
-  ::: $SRC_DIR/core/src/panicking.rs:LL:COL
-   |
-LL |     T: fmt::Debug + ?Sized,
-   |        ---------- required by this bound in `core::panicking::assert_failed`
    |
    = help: the trait `Debug` is not implemented for `fn() -> i32 {a}`
    = help: use parentheses to call the function: `a()`
diff --git a/src/test/ui/on-unimplemented/no-debug.stderr b/src/test/ui/on-unimplemented/no-debug.stderr
index fe4114aeddc..b17c1d4c252 100644
--- a/src/test/ui/on-unimplemented/no-debug.stderr
+++ b/src/test/ui/on-unimplemented/no-debug.stderr
@@ -5,8 +5,7 @@ LL |     println!("{:?} {:?}", Foo, Bar);
    |                           ^^^ `Foo` cannot be formatted using `{:?}`
    |
    = help: the trait `Debug` is not implemented for `Foo`
-   = note: add `#[derive(Debug)]` or manually implement `Debug`
-   = note: required by `std::fmt::Debug::fmt`
+   = note: add `#[derive(Debug)]` to `Foo` or manually `impl Debug for Foo`
    = note: this error originates in the macro `$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
diff --git a/src/test/ui/rfc-2361-dbg-macro/dbg-macro-requires-debug.stderr b/src/test/ui/rfc-2361-dbg-macro/dbg-macro-requires-debug.stderr
index 850ca30405f..3cdab6541e7 100644
--- a/src/test/ui/rfc-2361-dbg-macro/dbg-macro-requires-debug.stderr
+++ b/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 `impl 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
diff --git a/src/test/ui/suggestions/path-display.stderr b/src/test/ui/suggestions/path-display.stderr
index 80ed1cdedf1..25c73c4c874 100644
--- a/src/test/ui/suggestions/path-display.stderr
+++ b/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