Fix miscounting fields when from and backtrace are same field

Author: dtolnay

impl/src/valid.rs

@@ -180,7 +180,11 @@ fn check_field_attrs(fields: &[Field]) -> Result<()> {
         }
     }
     if let Some(from_field) = from_field {
-        if fields.len() > 1 + has_backtrace as usize {
+        let max_expected_fields = match backtrace_field {
+            Some(backtrace_field) => 1 + !same_member(from_field, backtrace_field) as usize,
+            None => 1 + has_backtrace as usize,
+        };
+        if fields.len() > max_expected_fields {
             return Err(Error::new_spanned(
                 from_field.attrs.from,
                 "deriving From requires no fields other than source and backtrace",

tests/ui/from-backtrace-backtrace.stderr

@@ -1,5 +1,5 @@
-error[E0063]: missing field `1` in initializer of `Error`
-  --> tests/ui/from-backtrace-backtrace.rs:10:12
+error: deriving From requires no fields other than source and backtrace
+  --> tests/ui/from-backtrace-backtrace.rs:10:18
    |
 10 | pub struct Error(#[from] #[backtrace] std::io::Error, Backtrace);
-   |            ^^^^^ missing `1`
+   |                  ^^^^^^^