Fix redundant "Error doesn't implement Display" in fallback

impl/src/expand.rs

@@ -37,6 +37,13 @@ fn fallback(input: &DeriveInput, error: syn::Error) -> TokenStream {
 
         #[allow(unused_qualifications)]
         impl #impl_generics std::error::Error for #ty #ty_generics #where_clause {}
+
+        #[allow(unused_qualifications)]
+        impl #impl_generics ::core::fmt::Display for #ty #ty_generics #where_clause {
+            fn fmt(&self, __formatter: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
+                ::core::unreachable!()
+            }
+        }
     }
 }
 

tests/ui/bad-field-attr.stderr

@@ -3,17 +3,3 @@ error: #[error(transparent)] needs to go outside the enum or struct, not on an i
   |
 5 | pub struct Error(#[error(transparent)] std::io::Error);
   |                  ^^^^^^^^^^^^^^^^^^^^^
-
-error[E0277]: `Error` doesn't implement `std::fmt::Display`
- --> tests/ui/bad-field-attr.rs:5:12
-  |
-5 | pub struct Error(#[error(transparent)] std::io::Error);
-  |            ^^^^^ `Error` cannot be formatted with the default formatter
-  |
-  = help: the trait `std::fmt::Display` is not implemented for `Error`
-  = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
-note: required by a bound in `std::error::Error`
- --> $RUST/core/src/error.rs
-  |
-  | pub trait Error: Debug + Display {
-  |                          ^^^^^^^ required by this bound in `Error`

tests/ui/concat-display.stderr

@@ -8,17 +8,3 @@ error: expected string literal
    | ------------------------- in this macro invocation
    |
    = note: this error originates in the macro `error_type` (in Nightly builds, run with -Z macro-backtrace for more info)
-
-error[E0277]: `Error` doesn't implement `std::fmt::Display`
-  --> tests/ui/concat-display.rs:13:13
-   |
-13 | error_type!(Error, "foo");
-   |             ^^^^^ `Error` cannot be formatted with the default formatter
-   |
-   = help: the trait `std::fmt::Display` is not implemented for `Error`
-   = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
-note: required by a bound in `std::error::Error`
-  --> $RUST/core/src/error.rs
-   |
-   | pub trait Error: Debug + Display {
-   |                          ^^^^^^^ required by this bound in `Error`

tests/ui/duplicate-enum-source.stderr

@@ -3,17 +3,3 @@ error: duplicate #[source] attribute
   |
 8 |         #[source]
   |         ^^^^^^^^^
-
-error[E0277]: `ErrorEnum` doesn't implement `std::fmt::Display`
- --> tests/ui/duplicate-enum-source.rs:4:10
-  |
-4 | pub enum ErrorEnum {
-  |          ^^^^^^^^^ `ErrorEnum` cannot be formatted with the default formatter
-  |
-  = help: the trait `std::fmt::Display` is not implemented for `ErrorEnum`
-  = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
-note: required by a bound in `std::error::Error`
- --> $RUST/core/src/error.rs
-  |
-  | pub trait Error: Debug + Display {
-  |                          ^^^^^^^ required by this bound in `Error`

tests/ui/duplicate-fmt.stderr

@@ -3,17 +3,3 @@ error: only one #[error(...)] attribute is allowed
   |
 5 | #[error("...")]
   | ^^^^^^^^^^^^^^^
-
-error[E0277]: `Error` doesn't implement `std::fmt::Display`
- --> tests/ui/duplicate-fmt.rs:6:12
-  |
-6 | pub struct Error;
-  |            ^^^^^ `Error` cannot be formatted with the default formatter
-  |
-  = help: the trait `std::fmt::Display` is not implemented for `Error`
-  = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
-note: required by a bound in `std::error::Error`
- --> $RUST/core/src/error.rs
-  |
-  | pub trait Error: Debug + Display {
-  |                          ^^^^^^^ required by this bound in `Error`

tests/ui/duplicate-struct-source.stderr

@@ -3,17 +3,3 @@ error: duplicate #[source] attribute
   |
 7 |     #[source]
   |     ^^^^^^^^^
-
-error[E0277]: `ErrorStruct` doesn't implement `std::fmt::Display`
- --> tests/ui/duplicate-struct-source.rs:4:12
-  |
-4 | pub struct ErrorStruct {
-  |            ^^^^^^^^^^^ `ErrorStruct` cannot be formatted with the default formatter
-  |
-  = help: the trait `std::fmt::Display` is not implemented for `ErrorStruct`
-  = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
-note: required by a bound in `std::error::Error`
- --> $RUST/core/src/error.rs
-  |
-  | pub trait Error: Debug + Display {
-  |                          ^^^^^^^ required by this bound in `Error`

tests/ui/duplicate-transparent.stderr

@@ -3,17 +3,3 @@ error: duplicate #[error(transparent)] attribute
   |
 5 | #[error(transparent)]
   | ^^^^^^^^^^^^^^^^^^^^^
-
-error[E0277]: `Error` doesn't implement `std::fmt::Display`
- --> tests/ui/duplicate-transparent.rs:6:12
-  |
-6 | pub struct Error(anyhow::Error);
-  |            ^^^^^ `Error` cannot be formatted with the default formatter
-  |
-  = help: the trait `std::fmt::Display` is not implemented for `Error`
-  = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
-note: required by a bound in `std::error::Error`
- --> $RUST/core/src/error.rs
-  |
-  | pub trait Error: Debug + Display {
-  |                          ^^^^^^^ required by this bound in `Error`

tests/ui/from-backtrace-backtrace.stderr

@@ -3,17 +3,3 @@ error: deriving From requires no fields other than source and backtrace
   |
 9 |     #[from]
   |     ^^^^^^^
-
-error[E0277]: `Error` doesn't implement `std::fmt::Display`
- --> tests/ui/from-backtrace-backtrace.rs:8:12
-  |
-8 | pub struct Error(
-  |            ^^^^^ `Error` cannot be formatted with the default formatter
-  |
-  = help: the trait `std::fmt::Display` is not implemented for `Error`
-  = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
-note: required by a bound in `std::error::Error`
- --> $RUST/core/src/error.rs
-  |
-  | pub trait Error: Debug + Display {
-  |                          ^^^^^^^ required by this bound in `Error`

tests/ui/from-not-source.stderr

@@ -3,17 +3,3 @@ error: #[from] is only supported on the source field, not any other field
   |
 7 |     #[from]
   |     ^^^^^^^
-
-error[E0277]: `Error` doesn't implement `std::fmt::Display`
- --> tests/ui/from-not-source.rs:4:12
-  |
-4 | pub struct Error {
-  |            ^^^^^ `Error` cannot be formatted with the default formatter
-  |
-  = help: the trait `std::fmt::Display` is not implemented for `Error`
-  = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
-note: required by a bound in `std::error::Error`
- --> $RUST/core/src/error.rs
-  |
-  | pub trait Error: Debug + Display {
-  |                          ^^^^^^^ required by this bound in `Error`

tests/ui/invalid-input-impl-anyway.stderr

@@ -3,17 +3,3 @@ error: expected attribute arguments in parentheses: #[error(...)]
   |
 4 | #[error]
   |   ^^^^^
-
-error[E0277]: `MyError` doesn't implement `std::fmt::Display`
- --> tests/ui/invalid-input-impl-anyway.rs:5:12
-  |
-5 | pub struct MyError;
-  |            ^^^^^^^ `MyError` cannot be formatted with the default formatter
-  |
-  = help: the trait `std::fmt::Display` is not implemented for `MyError`
-  = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
-note: required by a bound in `std::error::Error`
- --> $RUST/core/src/error.rs
-  |
-  | pub trait Error: Debug + Display {
-  |                          ^^^^^^^ required by this bound in `Error`

tests/ui/lifetime.stderr

@@ -9,31 +9,3 @@ error: non-static lifetimes are not allowed in the source of an error, because s
    |
 15 |     Foo(#[from] Generic<&'a str>),
    |                 ^^^^^^^^^^^^^^^^
-
-error[E0277]: `Error<'a>` doesn't implement `std::fmt::Display`
- --> tests/ui/lifetime.rs:6:8
-  |
-6 | struct Error<'a>(#[from] Inner<'a>);
-  |        ^^^^^^^^^ `Error<'a>` cannot be formatted with the default formatter
-  |
-  = help: the trait `std::fmt::Display` is not implemented for `Error<'a>`
-  = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
-note: required by a bound in `std::error::Error`
- --> $RUST/core/src/error.rs
-  |
-  | pub trait Error: Debug + Display {
-  |                          ^^^^^^^ required by this bound in `Error`
-
-error[E0277]: `Enum<'a>` doesn't implement `std::fmt::Display`
-  --> tests/ui/lifetime.rs:13:6
-   |
-13 | enum Enum<'a> {
-   |      ^^^^^^^^ `Enum<'a>` cannot be formatted with the default formatter
-   |
-   = help: the trait `std::fmt::Display` is not implemented for `Enum<'a>`
-   = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
-note: required by a bound in `std::error::Error`
-  --> $RUST/core/src/error.rs
-   |
-   | pub trait Error: Debug + Display {
-   |                          ^^^^^^^ required by this bound in `Error`

tests/ui/missing-fmt.stderr

@@ -3,17 +3,3 @@ error: missing #[error("...")] display attribute
   |
 7 |     B(usize),
   |     ^^^^^^^^
-
-error[E0277]: `Error` doesn't implement `std::fmt::Display`
- --> tests/ui/missing-fmt.rs:4:10
-  |
-4 | pub enum Error {
-  |          ^^^^^ `Error` cannot be formatted with the default formatter
-  |
-  = help: the trait `std::fmt::Display` is not implemented for `Error`
-  = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
-note: required by a bound in `std::error::Error`
- --> $RUST/core/src/error.rs
-  |
-  | pub trait Error: Debug + Display {
-  |                          ^^^^^^^ required by this bound in `Error`

tests/ui/transparent-display.stderr

@@ -3,17 +3,3 @@ error: cannot have both #[error(transparent)] and a display attribute
   |
 5 | #[error("...")]
   | ^^^^^^^^^^^^^^^
-
-error[E0277]: `Error` doesn't implement `std::fmt::Display`
- --> tests/ui/transparent-display.rs:6:12
-  |
-6 | pub struct Error(anyhow::Error);
-  |            ^^^^^ `Error` cannot be formatted with the default formatter
-  |
-  = help: the trait `std::fmt::Display` is not implemented for `Error`
-  = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
-note: required by a bound in `std::error::Error`
- --> $RUST/core/src/error.rs
-  |
-  | pub trait Error: Debug + Display {
-  |                          ^^^^^^^ required by this bound in `Error`

tests/ui/transparent-enum-many.stderr

@@ -4,17 +4,3 @@ error: #[error(transparent)] requires exactly one field
 5 | /     #[error(transparent)]
 6 | |     Other(anyhow::Error, String),
   | |________________________________^
-
-error[E0277]: `Error` doesn't implement `std::fmt::Display`
- --> tests/ui/transparent-enum-many.rs:4:10
-  |
-4 | pub enum Error {
-  |          ^^^^^ `Error` cannot be formatted with the default formatter
-  |
-  = help: the trait `std::fmt::Display` is not implemented for `Error`
-  = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
-note: required by a bound in `std::error::Error`
- --> $RUST/core/src/error.rs
-  |
-  | pub trait Error: Debug + Display {
-  |                          ^^^^^^^ required by this bound in `Error`

tests/ui/transparent-enum-source.stderr

@@ -3,17 +3,3 @@ error: transparent variant can't contain #[source]
   |
 6 |     Other(#[source] anyhow::Error),
   |           ^^^^^^^^^
-
-error[E0277]: `Error` doesn't implement `std::fmt::Display`
- --> tests/ui/transparent-enum-source.rs:4:10
-  |
-4 | pub enum Error {
-  |          ^^^^^ `Error` cannot be formatted with the default formatter
-  |
-  = help: the trait `std::fmt::Display` is not implemented for `Error`
-  = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
-note: required by a bound in `std::error::Error`
- --> $RUST/core/src/error.rs
-  |
-  | pub trait Error: Debug + Display {
-  |                          ^^^^^^^ required by this bound in `Error`

tests/ui/transparent-struct-many.stderr

@@ -3,17 +3,3 @@ error: #[error(transparent)] requires exactly one field
   |
 4 | #[error(transparent)]
   | ^^^^^^^^^^^^^^^^^^^^^
-
-error[E0277]: `Error` doesn't implement `std::fmt::Display`
- --> tests/ui/transparent-struct-many.rs:5:12
-  |
-5 | pub struct Error {
-  |            ^^^^^ `Error` cannot be formatted with the default formatter
-  |
-  = help: the trait `std::fmt::Display` is not implemented for `Error`
-  = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
-note: required by a bound in `std::error::Error`
- --> $RUST/core/src/error.rs
-  |
-  | pub trait Error: Debug + Display {
-  |                          ^^^^^^^ required by this bound in `Error`

tests/ui/transparent-struct-source.stderr

@@ -3,17 +3,3 @@ error: transparent error struct can't contain #[source]
   |
 5 | pub struct Error(#[source] anyhow::Error);
   |                  ^^^^^^^^^
-
-error[E0277]: `Error` doesn't implement `std::fmt::Display`
- --> tests/ui/transparent-struct-source.rs:5:12
-  |
-5 | pub struct Error(#[source] anyhow::Error);
-  |            ^^^^^ `Error` cannot be formatted with the default formatter
-  |
-  = help: the trait `std::fmt::Display` is not implemented for `Error`
-  = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
-note: required by a bound in `std::error::Error`
- --> $RUST/core/src/error.rs
-  |
-  | pub trait Error: Debug + Display {
-  |                          ^^^^^^^ required by this bound in `Error`

tests/ui/unexpected-field-fmt.stderr

@@ -3,17 +3,3 @@ error: not expected here; the #[error(...)] attribute belongs on top of a struct
   |
 6 |         #[error("...")]
   |         ^^^^^^^^^^^^^^^
-
-error[E0277]: `Error` doesn't implement `std::fmt::Display`
- --> tests/ui/unexpected-field-fmt.rs:4:10
-  |
-4 | pub enum Error {
-  |          ^^^^^ `Error` cannot be formatted with the default formatter
-  |
-  = help: the trait `std::fmt::Display` is not implemented for `Error`
-  = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
-note: required by a bound in `std::error::Error`
- --> $RUST/core/src/error.rs
-  |
-  | pub trait Error: Debug + Display {
-  |                          ^^^^^^^ required by this bound in `Error`

tests/ui/unexpected-struct-source.stderr

@@ -3,17 +3,3 @@ error: not expected here; the #[source] attribute belongs on a specific field
   |
 4 | #[source]
   | ^^^^^^^^^
-
-error[E0277]: `Error` doesn't implement `std::fmt::Display`
- --> tests/ui/unexpected-struct-source.rs:5:12
-  |
-5 | pub struct Error;
-  |            ^^^^^ `Error` cannot be formatted with the default formatter
-  |
-  = help: the trait `std::fmt::Display` is not implemented for `Error`
-  = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
-note: required by a bound in `std::error::Error`
- --> $RUST/core/src/error.rs
-  |
-  | pub trait Error: Debug + Display {
-  |                          ^^^^^^^ required by this bound in `Error`

tests/ui/union.stderr

@@ -7,20 +7,6 @@ error: union as errors are not supported
 7 | | }
   | |_^
 
-error[E0277]: `U` doesn't implement `std::fmt::Display`
- --> tests/ui/union.rs:4:11
-  |
-4 | pub union U {
-  |           ^ `U` cannot be formatted with the default formatter
-  |
-  = help: the trait `std::fmt::Display` is not implemented for `U`
-  = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
-note: required by a bound in `std::error::Error`
- --> $RUST/core/src/error.rs
-  |
-  | pub trait Error: Debug + Display {
-  |                          ^^^^^^^ required by this bound in `Error`
-
 error[E0277]: `U` doesn't implement `Debug`
  --> tests/ui/union.rs:4:11
   |