Fix duplicate field in suggestion

Author: Henri Lunnikivi

clippy_lints/src/field_reassign_with_default.rs

@@ -1,6 +1,7 @@
 use crate::utils::{match_def_path, paths, qpath_res, snippet, span_lint_and_note};
 use if_chain::if_chain;
 use rustc_hir::def::Res;
+use rustc_data_structures::fx::FxHashMap;
 use rustc_hir::{Block, Expr, ExprKind, PatKind, QPath, Stmt, StmtKind};
 use rustc_span::symbol::{Ident, Symbol};
 
@@ -56,7 +57,7 @@ impl LateLintPass<'_> for FieldReassignWithDefault {
             // find all "later statement"'s where the fields of the binding set as
             // Default::default() get reassigned
             let mut first_assign = None;
-            let mut assigned_fields = vec![];
+            let mut assigned_fields = FxHashMap::default();
             for consequtive_statement in &block.stmts[stmt_idx + 1..] {
                 // interrupt if the statement is a let binding (`Local`) that shadows the original
                 // binding
@@ -70,7 +71,9 @@ impl LateLintPass<'_> for FieldReassignWithDefault {
                     if let Some((field_ident, assign_rhs)) = field_reassigned_by_stmt(consequtive_statement, &binding_name) {
                         // extract and store the assigned value for help message
                         let value_snippet = snippet(cx, assign_rhs.span, "..");
-                        assigned_fields.push((field_ident.name, value_snippet));
+                        if !assigned_fields.contains_key(&field_ident.name) {
+                            assigned_fields.insert(field_ident.name, value_snippet);
+                        }
 
                         // also set first instance of error for help message
                         if first_assign.is_none() {

tests/ui/field_reassign_with_default.rs

@@ -56,4 +56,10 @@ fn main() {
     let mut a: A = Default::default();
     a.i = 42;
     a.j = 43;
+
+    // wrong, produces third error in stderr
+    let mut a: A = Default::default();
+    a.i = 42;
+    a.j = 43;
+    a.j = 44;
 }

tests/ui/field_reassign_with_default.stderr

@@ -23,5 +23,17 @@ note: consider initializing the variable immutably with `A { i: 42, j: 43, ..Def
 LL |     let mut a: A = Default::default();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 2 previous errors
+error: field assignment outside of initializer for an instance created with Default::default()
+  --> $DIR/field_reassign_with_default.rs:62:5
+   |
+LL |     a.i = 42;
+   |     ^^^^^^^^^
+   |
+note: consider initializing the variable immutably with `A { i: 42, j: 43, ..Default::default() }`
+  --> $DIR/field_reassign_with_default.rs:61:5
+   |
+LL |     let mut a: A = Default::default();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors