Emit an Error impl even in the presence of bad attributes

impl/src/expand.rs

@@ -7,15 +7,39 @@ use quote::{format_ident, quote, quote_spanned, ToTokens};
 use std::collections::BTreeSet as Set;
 use syn::{DeriveInput, GenericArgument, Member, PathArguments, Result, Token, Type};
 
-pub fn derive(node: &DeriveInput) -> Result<TokenStream> {
-    let input = Input::from_syn(node)?;
+pub fn derive(input: &DeriveInput) -> TokenStream {
+    match try_expand(input) {
+        Ok(expanded) => expanded,
+        // If there are invalid attributes in the input, expand to an Error impl
+        // anyway to minimize spurious knock-on errors in other code that uses
+        // this type as an Error.
+        Err(error) => fallback(input, error),
+    }
+}
+
+fn try_expand(input: &DeriveInput) -> Result<TokenStream> {
+    let input = Input::from_syn(input)?;
     input.validate()?;
     Ok(match input {
         Input::Struct(input) => impl_struct(input),
         Input::Enum(input) => impl_enum(input),
     })
 }
 
+fn fallback(input: &DeriveInput, error: syn::Error) -> TokenStream {
+    let ty = &input.ident;
+    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
+
+    let error = error.to_compile_error();
+
+    quote! {
+        #error
+
+        #[allow(unused_qualifications)]
+        impl #impl_generics std::error::Error for #ty #ty_generics #where_clause {}
+    }
+}
+
 fn impl_struct(input: Struct) -> TokenStream {
     let ty = &input.ident;
     let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();

impl/src/lib.rs

@@ -33,7 +33,5 @@ use syn::{parse_macro_input, DeriveInput};
 #[proc_macro_derive(Error, attributes(backtrace, error, from, source))]
 pub fn derive_error(input: TokenStream) -> TokenStream {
     let input = parse_macro_input!(input as DeriveInput);
-    expand::derive(&input)
-        .unwrap_or_else(|err| err.to_compile_error())
-        .into()
+    expand::derive(&input).into()
 }

tests/ui/bad-field-attr.stderr

@@ -3,3 +3,17 @@ 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

@@ -8,3 +8,17 @@ 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

@@ -3,3 +3,17 @@ 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

@@ -3,3 +3,17 @@ 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

@@ -3,3 +3,17 @@ 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

@@ -3,3 +3,17 @@ 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

@@ -3,3 +3,17 @@ 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

@@ -3,3 +3,17 @@ 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.rs

@@ -5,7 +5,7 @@ use thiserror::Error;
 pub struct MyError;
 
 fn main() {
-    // FIXME: there should be no error on the following line. Thiserror should
-    // emit an Error impl regardless of the bad attribute.
+    // No error on the following line. Thiserror emits an Error impl despite the
+    // bad attribute.
     _ = &MyError as &dyn std::error::Error;
 }

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

@@ -4,10 +4,16 @@ error: expected attribute arguments in parentheses: #[error(...)]
 4 | #[error]
   |   ^^^^^
 
-error[E0277]: the trait bound `MyError: std::error::Error` is not satisfied
-  --> tests/ui/invalid-input-impl-anyway.rs:10:9
-   |
-10 |     _ = &MyError as &dyn std::error::Error;
-   |         ^^^^^^^^ the trait `std::error::Error` is not implemented for `MyError`
-   |
-   = note: required for the cast from `&MyError` to `&dyn std::error::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

@@ -9,3 +9,31 @@ 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

@@ -3,3 +3,17 @@ 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

@@ -3,3 +3,17 @@ 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

@@ -4,3 +4,17 @@ 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

@@ -3,3 +3,17 @@ 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

@@ -3,3 +3,17 @@ 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

@@ -3,3 +3,17 @@ 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

@@ -3,3 +3,17 @@ 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`