Retain order of statements in suggested code

Use Vec instead of HashMap to prevent re-orders. Also flip one
assignment in test to account for this case.

Author: Henri Lunnikivi

clippy_lints/src/field_reassign_with_default.rs

@@ -1,6 +1,5 @@
 use crate::utils::{contains_name, match_def_path, paths, qpath_res, snippet, span_lint_and_note};
 use if_chain::if_chain;
-use rustc_data_structures::fx::FxHashMap;
 use rustc_hir::def::Res;
 use rustc_hir::{Block, Expr, ExprKind, PatKind, QPath, Stmt, StmtKind};
 use rustc_middle::ty::{self, Adt, Ty};
@@ -57,7 +56,7 @@ impl LateLintPass<'_> for FieldReassignWithDefault {
             // find all "later statement"'s where the fields of the binding set as
             // Default::default() get reassigned, unless the reassignment refers to the original binding
             let mut first_assign = None;
-            let mut assigned_fields = FxHashMap::default();
+            let mut assigned_fields = Vec::new();
             let mut cancel_lint = false;
             for consecutive_statement in &block.stmts[stmt_idx + 1..] {
                 // interrupt if the statement is a let binding (`Local`) that shadows the original
@@ -75,8 +74,15 @@ impl LateLintPass<'_> for FieldReassignWithDefault {
                         break;
                     }
 
-                    // always re-insert set value, this way the latest value is stored for output snippet
-                    assigned_fields.insert(field_ident.name, assign_rhs);
+                    // if the field was previously assigned, replace the assignment, otherwise insert the assignment
+                    if let Some(prev) = assigned_fields
+                        .iter_mut()
+                        .find(|(field_name, _)| field_name == &field_ident.name)
+                    {
+                        *prev = (field_ident.name, assign_rhs);
+                    } else {
+                        assigned_fields.push((field_ident.name, assign_rhs));
+                    }
 
                     // also set first instance of error for help message
                     if first_assign.is_none() {
@@ -100,12 +106,12 @@ impl LateLintPass<'_> for FieldReassignWithDefault {
                     let assigned_fields = assigned_fields
                         .into_iter()
                         .filter(|(_, rhs)| !is_expr_default(rhs, cx))
-                        .collect::<FxHashMap<Symbol, &Expr<'_>>>();
+                        .collect::<Vec<(Symbol, &Expr<'_>)>>();
 
                     // if all fields of the struct are not assigned, add `.. Default::default()` to the suggestion.
                     let ext_with_default = !fields_of_type(binding_type)
                         .iter()
-                        .all(|field| assigned_fields.contains_key(&field.name));
+                        .all(|field| assigned_fields.iter().any(|(a, _)| a == &field.name));
 
                     let field_list = assigned_fields
                         .into_iter()

tests/ui/field_reassign_with_default.rs

@@ -54,8 +54,8 @@ fn main() {
 
     // wrong, produces second error in stderr
     let mut a: A = Default::default();
-    a.i = 42;
     a.j = 43;
+    a.i = 42;
 
     // wrong, produces third error in stderr
     let mut a: A = Default::default();

tests/ui/field_reassign_with_default.stderr

@@ -14,10 +14,10 @@ LL |     let mut a: A = Default::default();
 error: field assignment outside of initializer for an instance created with Default::default()
   --> $DIR/field_reassign_with_default.rs:57:5
    |
-LL |     a.i = 42;
+LL |     a.j = 43;
    |     ^^^^^^^^^
    |
-note: consider initializing the variable with `A { i: 42, j: 43 }`
+note: consider initializing the variable with `A { j: 43, i: 42 }`
   --> $DIR/field_reassign_with_default.rs:56:5
    |
 LL |     let mut a: A = Default::default();