more clear duplicate backtrace field logic

impl/src/valid.rs

@@ -159,6 +159,7 @@ fn check_field_attrs(fields: &[Field]) -> Result<()> {
     let mut source_field = None;
     let mut backtrace_field = None;
     let mut first_implicit_field = None;
+    let mut backtrace_type_field = None;
     for field in fields {
         if let Some(from) = field.attrs.from {
             if from_field.is_some() {
@@ -187,6 +188,15 @@ fn check_field_attrs(fields: &[Field]) -> Result<()> {
             }
             backtrace_field = Some(field);
         }
+        if field.is_backtrace() {
+            if backtrace_type_field.is_some() {
+                return Err(Error::new_spanned(
+                    field.original,
+                    "duplicate field having type: `Backtrace`",
+                ));
+            }
+            backtrace_type_field = Some(field);
+        }
         if let Some(transparent) = field.attrs.transparent {
             return Err(Error::new_spanned(
                 transparent.original,
@@ -205,6 +215,14 @@ fn check_field_attrs(fields: &[Field]) -> Result<()> {
             ));
         }
     }
+    if let (Some(a), Some(b)) = (backtrace_field, backtrace_type_field) {
+        if a.member != b.member {
+            return Err(Error::new_spanned(
+                a.original,
+                "only one backtrace field is allowed; found field with #[backtrace] and field with type `Backtrace`",
+            ));
+        }
+    }
     if let Some(from_field) = from_field {
         let has_unexpected_fields = fields.iter().any(|field| {
             field.attrs.from.is_none()

tests/ui/from-backtrace-backtrace.stderr

@@ -1,5 +1,7 @@
-error: deriving From requires no fields other than source and backtrace
- --> tests/ui/from-backtrace-backtrace.rs:9:5
-  |
-9 |     #[from]
-  |     ^^^^^^^
+error: only one backtrace field is allowed; found field with #[backtrace] and field with type `Backtrace`
+  --> tests/ui/from-backtrace-backtrace.rs:9:5
+   |
+9  | /     #[from]
+10 | |     #[backtrace]
+11 | |     std::io::Error,
+   | |__________________^