Only apply inferred bounds if interpolation refers to field

impl/src/fmt.rs

@@ -69,6 +69,20 @@ impl Display<'_> {
                 }
                 _ => continue,
             };
+            let formatvar = match &member {
+                MemberUnraw::Unnamed(index) => IdentUnraw::new(format_ident!("_{}", index)),
+                MemberUnraw::Named(ident) => ident.clone(),
+            };
+            out += &formatvar.to_string();
+            if !named_args.insert(formatvar.clone()) {
+                // Already specified in the format argument list.
+                continue;
+            }
+            if !has_trailing_comma {
+                args.extend(quote_spanned!(span=> ,));
+            }
+            let local = formatvar.to_local();
+            args.extend(quote_spanned!(span=> #formatvar = #local));
             if let Some(&field) = member_index.get(&member) {
                 let end_spec = match read.find('}') {
                     Some(end_spec) => end_spec,
@@ -83,28 +97,15 @@ impl Display<'_> {
                     Some('b') => Trait::Binary,
                     Some('e') => Trait::LowerExp,
                     Some('E') => Trait::UpperExp,
-                    Some(_) | None => Trait::Display,
+                    Some(_) => Trait::Display,
+                    None => {
+                        has_bonus_display = true;
+                        args.extend(quote_spanned!(span=> .as_display()));
+                        Trait::Display
+                    }
                 };
                 implied_bounds.insert((field, bound));
             }
-            let formatvar = match &member {
-                MemberUnraw::Unnamed(index) => IdentUnraw::new(format_ident!("_{}", index)),
-                MemberUnraw::Named(ident) => ident.clone(),
-            };
-            out += &formatvar.to_string();
-            if !named_args.insert(formatvar.clone()) {
-                // Already specified in the format argument list.
-                continue;
-            }
-            if !has_trailing_comma {
-                args.extend(quote_spanned!(span=> ,));
-            }
-            let local = formatvar.to_local();
-            args.extend(quote_spanned!(span=> #formatvar = #local));
-            if read.starts_with('}') && member_index.contains_key(&member) {
-                has_bonus_display = true;
-                args.extend(quote_spanned!(span=> .as_display()));
-            }
             has_trailing_comma = false;
         }
 

tests/test_generics.rs

@@ -159,3 +159,16 @@ pub struct StructFromGeneric<E> {
 #[derive(Error, Debug)]
 #[error(transparent)]
 pub struct StructTransparentGeneric<E>(pub E);
+
+// Regression test for https://github.com/dtolnay/thiserror/issues/345
+#[test]
+fn test_no_bound_on_named_fmt() {
+    #[derive(Error, Debug)]
+    #[error("{thing}", thing = "...")]
+    struct Error<T> {
+        thing: T,
+    }
+
+    let error = Error { thing: DebugOnly };
+    assert_eq!(error.to_string(), "...");
+}