Auto merge of #12832 - lowr:fix/impl-default-members-no-codegen, r=Veykril

fix: don't replace default members' body

cc #12779, #12821
addresses https://github.com/rust-lang/rust-analyzer/pull/12821#issuecomment-1190157506

`gen_trait_fn_body()` only attempts to implement required trait member functions, so we shouldn't call it for `Implement default members` assist.

This patch also documents the precondition of `gen_trait_fn_body()` and inserts `debug_assert!`, but I'm not entirely sure if the assertions are appropriate.

2 files changed, 16 insertions, 12 deletions

diff --git a/crates/ide-assists/src/handlers/add_missing_impl_members.rs b/crates/ide-assists/src/handlers/add_missing_impl_members.rs
index 7f2a26ad067..c808c010c67 100644
--- a/crates/ide-assists/src/handlers/add_missing_impl_members.rs
+++ b/crates/ide-assists/src/handlers/add_missing_impl_members.rs
@@ -145,13 +145,16 @@ fn add_missing_impl_members_inner(
             Some(cap) => {
                 let mut cursor = Cursor::Before(first_new_item.syntax());
                 let placeholder;
-                if let ast::AssocItem::Fn(func) = &first_new_item {
-                    if try_gen_trait_body(ctx, func, &trait_, &impl_def).is_none() {
-                        if let Some(m) = func.syntax().descendants().find_map(ast::MacroCall::cast)
-                        {
-                            if m.syntax().text() == "todo!()" {
-                                placeholder = m;
-                                cursor = Cursor::Replace(placeholder.syntax());
+                if let DefaultMethods::No = mode {
+                    if let ast::AssocItem::Fn(func) = &first_new_item {
+                        if try_gen_trait_body(ctx, func, &trait_, &impl_def).is_none() {
+                            if let Some(m) =
+                                func.syntax().descendants().find_map(ast::MacroCall::cast)
+                            {
+                                if m.syntax().text() == "todo!()" {
+                                    placeholder = m;
+                                    cursor = Cursor::Replace(placeholder.syntax());
+                                }
                             }
                         }
                     }
diff --git a/crates/ide-assists/src/utils/gen_trait_fn_body.rs b/crates/ide-assists/src/utils/gen_trait_fn_body.rs
index ec4835969f8..43aaad3150c 100644
--- a/crates/ide-assists/src/utils/gen_trait_fn_body.rs
+++ b/crates/ide-assists/src/utils/gen_trait_fn_body.rs
@@ -5,7 +5,7 @@ use syntax::{
     ted,
 };
 
-/// Generate custom trait bodies where possible.
+/// Generate custom trait bodies without default implementation where possible.
 ///
 /// Returns `Option` so that we can use `?` rather than `if let Some`. Returning
 /// `None` means that generating a custom trait body failed, and the body will remain
@@ -28,6 +28,7 @@ pub(crate) fn gen_trait_fn_body(
 
 /// Generate a `Clone` impl based on the fields and members of the target type.
 fn gen_clone_impl(adt: &ast::Adt, func: &ast::Fn) -> Option<()> {
+    debug_assert!(func.name().map_or(false, |name| name.text() == "clone"));
     fn gen_clone_call(target: ast::Expr) -> ast::Expr {
         let method = make::name_ref("clone");
         make::expr_method_call(target, method, make::arg_list(None))
@@ -339,6 +340,7 @@ fn gen_default_impl(adt: &ast::Adt, func: &ast::Fn) -> Option<()> {
 
 /// Generate a `Hash` impl based on the fields and members of the target type.
 fn gen_hash_impl(adt: &ast::Adt, func: &ast::Fn) -> Option<()> {
+    debug_assert!(func.name().map_or(false, |name| name.text() == "hash"));
     fn gen_hash_call(target: ast::Expr) -> ast::Stmt {
         let method = make::name_ref("hash");
         let arg = make::expr_path(make::ext::ident_path("state"));
@@ -394,9 +396,7 @@ fn gen_hash_impl(adt: &ast::Adt, func: &ast::Fn) -> Option<()> {
 
 /// Generate a `PartialEq` impl based on the fields and members of the target type.
 fn gen_partial_eq(adt: &ast::Adt, func: &ast::Fn) -> Option<()> {
-    if func.name().map_or(false, |name| name.text() == "ne") {
-        return None;
-    }
+    debug_assert!(func.name().map_or(false, |name| name.text() == "eq"));
     fn gen_eq_chain(expr: Option<ast::Expr>, cmp: ast::Expr) -> Option<ast::Expr> {
         match expr {
             Some(expr) => Some(make::expr_bin_op(expr, BinaryOp::LogicOp(LogicOp::And), cmp)),
@@ -573,6 +573,7 @@ fn gen_partial_eq(adt: &ast::Adt, func: &ast::Fn) -> Option<()> {
 }
 
 fn gen_partial_ord(adt: &ast::Adt, func: &ast::Fn) -> Option<()> {
+    debug_assert!(func.name().map_or(false, |name| name.text() == "partial_cmp"));
     fn gen_partial_eq_match(match_target: ast::Expr) -> Option<ast::Stmt> {
         let mut arms = vec![];
 
@@ -643,7 +644,7 @@ fn gen_partial_ord(adt: &ast::Adt, func: &ast::Fn) -> Option<()> {
                 make::block_expr(stmts.into_iter(), tail).indent(ast::edit::IndentLevel(1))
             }
 
-            // No fields in the body means there's nothing to hash.
+            // No fields in the body means there's nothing to compare.
             None => {
                 let expr = make::expr_literal("true").into();
                 make::block_expr(None, Some(expr)).indent(ast::edit::IndentLevel(1))