Auto merge of #5993 - taiki-e:default_trait_access, r=phansch

default_trait_access: Fix wrong suggestion

https://github.com/rust-lang/rust-clippy/issues/5975#issuecomment-683751131
> I think the underlying problem is clippy suggests code with complete parameters, not clippy triggers this lint even for complex types. AFAIK, If code compiles with `Default::default`, it doesn't need to specify any parameters, as type inference is working. (So, in this case, `default_trait_access` should suggest `RefCell::default`.)

Fixes #5975 Fixes #5990

changelog: `default_trait_access`: fixed wrong suggestion

clippy_lints/src/default_trait_access.rs

@@ -55,8 +55,8 @@ impl<'tcx> LateLintPass<'tcx> for DefaultTraitAccess {
                         // TODO: Work out a way to put "whatever the imported way of referencing
                         // this type in this file" rather than a fully-qualified type.
                         let expr_ty = cx.typeck_results().expr_ty(expr);
-                        if let ty::Adt(..) = expr_ty.kind {
+                        if let ty::Adt(def, ..) = expr_ty.kind {
-                            let replacement = format!("{}::default()", expr_ty);
+                            let replacement = format!("{}::default()", cx.tcx.def_path_str(def.did));
                             span_lint_and_sugg(
                                 cx,
                                 DEFAULT_TRAIT_ACCESS,

tests/ui/default_trait_access.fixed

@@ -0,0 +1,106 @@
+// run-rustfix
+
+#![allow(unused_imports)]
+#![deny(clippy::default_trait_access)]
+
+use std::default;
+use std::default::Default as D2;
+use std::string;
+
+fn main() {
+    let s1: String = std::string::String::default();
+
+    let s2 = String::default();
+
+    let s3: String = std::string::String::default();
+
+    let s4: String = std::string::String::default();
+
+    let s5 = string::String::default();
+
+    let s6: String = std::string::String::default();
+
+    let s7 = std::string::String::default();
+
+    let s8: String = DefaultFactory::make_t_badly();
+
+    let s9: String = DefaultFactory::make_t_nicely();
+
+    let s10 = DerivedDefault::default();
+
+    let s11: GenericDerivedDefault<String> = GenericDerivedDefault::default();
+
+    let s12 = GenericDerivedDefault::<String>::default();
+
+    let s13 = TupleDerivedDefault::default();
+
+    let s14: TupleDerivedDefault = TupleDerivedDefault::default();
+
+    let s15: ArrayDerivedDefault = ArrayDerivedDefault::default();
+
+    let s16 = ArrayDerivedDefault::default();
+
+    let s17: TupleStructDerivedDefault = TupleStructDerivedDefault::default();
+
+    let s18 = TupleStructDerivedDefault::default();
+
+    let s19 = <DerivedDefault as Default>::default();
+
+    println!(
+        "[{}] [{}] [{}] [{}] [{}] [{}] [{}] [{}] [{}] [{:?}] [{:?}] [{:?}] [{:?}] [{:?}] [{:?}] [{:?}] [{:?}] [{:?}], [{:?}]",
+        s1,
+        s2,
+        s3,
+        s4,
+        s5,
+        s6,
+        s7,
+        s8,
+        s9,
+        s10,
+        s11,
+        s12,
+        s13,
+        s14,
+        s15,
+        s16,
+        s17,
+        s18,
+        s19,
+    );
+}
+
+struct DefaultFactory;
+
+impl DefaultFactory {
+    pub fn make_t_badly<T: Default>() -> T {
+        Default::default()
+    }
+
+    pub fn make_t_nicely<T: Default>() -> T {
+        T::default()
+    }
+}
+
+#[derive(Debug, Default)]
+struct DerivedDefault {
+    pub s: String,
+}
+
+#[derive(Debug, Default)]
+struct GenericDerivedDefault<T: Default + std::fmt::Debug> {
+    pub s: T,
+}
+
+#[derive(Debug, Default)]
+struct TupleDerivedDefault {
+    pub s: (String, String),
+}
+
+#[derive(Debug, Default)]
+struct ArrayDerivedDefault {
+    pub s: [String; 10],
+}
+
+#[derive(Debug, Default)]
+struct TupleStructDerivedDefault(String);

tests/ui/default_trait_access.rs

@@ -1,4 +1,7 @@
-#![warn(clippy::default_trait_access)]
+// run-rustfix
+
+#![allow(unused_imports)]
+#![deny(clippy::default_trait_access)]
 
 use std::default;
 use std::default::Default as D2;

tests/ui/default_trait_access.stderr

@@ -1,49 +1,53 @@
 error: calling `std::string::String::default()` is more clear than this expression
-  --> $DIR/default_trait_access.rs:8:22
+  --> $DIR/default_trait_access.rs:11:22
    |
 LL |     let s1: String = Default::default();
    |                      ^^^^^^^^^^^^^^^^^^ help: try: `std::string::String::default()`
    |
-   = note: `-D clippy::default-trait-access` implied by `-D warnings`
+note: the lint level is defined here
+  --> $DIR/default_trait_access.rs:4:9
+   |
+LL | #![deny(clippy::default_trait_access)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: calling `std::string::String::default()` is more clear than this expression
-  --> $DIR/default_trait_access.rs:12:22
+  --> $DIR/default_trait_access.rs:15:22
    |
 LL |     let s3: String = D2::default();
    |                      ^^^^^^^^^^^^^ help: try: `std::string::String::default()`
 
 error: calling `std::string::String::default()` is more clear than this expression
-  --> $DIR/default_trait_access.rs:14:22
+  --> $DIR/default_trait_access.rs:17:22
    |
 LL |     let s4: String = std::default::Default::default();
    |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `std::string::String::default()`
 
 error: calling `std::string::String::default()` is more clear than this expression
-  --> $DIR/default_trait_access.rs:18:22
+  --> $DIR/default_trait_access.rs:21:22
    |
 LL |     let s6: String = default::Default::default();
    |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `std::string::String::default()`
 
-error: calling `GenericDerivedDefault<std::string::String>::default()` is more clear than this expression
+error: calling `GenericDerivedDefault::default()` is more clear than this expression
-  --> $DIR/default_trait_access.rs:28:46
+  --> $DIR/default_trait_access.rs:31:46
    |
 LL |     let s11: GenericDerivedDefault<String> = Default::default();
-   |                                              ^^^^^^^^^^^^^^^^^^ help: try: `GenericDerivedDefault<std::string::String>::default()`
+   |                                              ^^^^^^^^^^^^^^^^^^ help: try: `GenericDerivedDefault::default()`
 
 error: calling `TupleDerivedDefault::default()` is more clear than this expression
-  --> $DIR/default_trait_access.rs:34:36
+  --> $DIR/default_trait_access.rs:37:36
    |
 LL |     let s14: TupleDerivedDefault = Default::default();
    |                                    ^^^^^^^^^^^^^^^^^^ help: try: `TupleDerivedDefault::default()`
 
 error: calling `ArrayDerivedDefault::default()` is more clear than this expression
-  --> $DIR/default_trait_access.rs:36:36
+  --> $DIR/default_trait_access.rs:39:36
    |
 LL |     let s15: ArrayDerivedDefault = Default::default();
    |                                    ^^^^^^^^^^^^^^^^^^ help: try: `ArrayDerivedDefault::default()`
 
 error: calling `TupleStructDerivedDefault::default()` is more clear than this expression
-  --> $DIR/default_trait_access.rs:40:42
+  --> $DIR/default_trait_access.rs:43:42
    |
 LL |     let s17: TupleStructDerivedDefault = Default::default();
    |                                          ^^^^^^^^^^^^^^^^^^ help: try: `TupleStructDerivedDefault::default()`