Reject display attribute on a field

impl/src/attr.rs

@@ -8,12 +8,13 @@ use syn::{
 };
 
 pub struct Attrs<'a> {
-    pub display: Option<Display>,
+    pub display: Option<Display<'a>>,
     pub source: Option<Source<'a>>,
 }
 
 #[derive(Clone)]
-pub struct Display {
+pub struct Display<'a> {
+    pub original: &'a Attribute,
     pub fmt: LitStr,
     pub args: TokenStream,
     pub was_shorthand: bool,
@@ -54,6 +55,7 @@ pub fn get(input: &[Attribute]) -> Result<Attrs> {
 fn parse_display(attr: &Attribute) -> Result<Display> {
     attr.parse_args_with(|input: ParseStream| {
         let mut display = Display {
+            original: attr,
             fmt: input.parse()?,
             args: parse_token_expr(input, false)?,
             was_shorthand: false,
@@ -116,7 +118,7 @@ fn parse_source(attr: &Attribute) -> Result<Source> {
     Ok(Source { original: attr })
 }
 
-impl ToTokens for Display {
+impl ToTokens for Display<'_> {
     fn to_tokens(&self, tokens: &mut TokenStream) {
         let fmt = &self.fmt;
         let args = &self.args;

impl/src/fmt.rs

@@ -3,7 +3,7 @@ use proc_macro2::TokenStream;
 use quote::quote_spanned;
 use syn::{Ident, LitStr};
 
-impl Display {
+impl Display<'_> {
     // Transform `"error {var}"` to `"error {}", var`.
     pub fn expand_shorthand(&mut self) {
         if !self.args.is_empty() {

impl/src/valid.rs

@@ -17,6 +17,9 @@ impl Struct<'_> {
     fn validate(&self) -> Result<()> {
         check_no_source(&self.attrs)?;
         find_duplicate_source(&self.fields)?;
+        for field in &self.fields {
+            field.validate()?;
+        }
         Ok(())
     }
 }
@@ -42,6 +45,21 @@ impl Variant<'_> {
     fn validate(&self) -> Result<()> {
         check_no_source(&self.attrs)?;
         find_duplicate_source(&self.fields)?;
+        for field in &self.fields {
+            field.validate()?;
+        }
+        Ok(())
+    }
+}
+
+impl Field<'_> {
+    fn validate(&self) -> Result<()> {
+        if let Some(display) = &self.attrs.display {
+            return Err(Error::new_spanned(
+                display.original,
+                "not expected here; the #[error(...)] attribute belongs on top of a struct or an enum variant",
+            ));
+        }
         Ok(())
     }
 }

tests/ui/unexpected-field-fmt.rs

@@ -0,0 +1,11 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum Error {
+    What {
+        #[error("...")]
+        io: std::io::Error,
+    },
+}
+
+fn main() {}

tests/ui/unexpected-field-fmt.stderr

@@ -0,0 +1,5 @@
+error: not expected here; the #[error(...)] attribute belongs on top of a struct or an enum variant
+ --> $DIR/unexpected-field-fmt.rs:6:9
+  |
+6 |         #[error("...")]
+  |         ^^^^^^^^^^^^^^^