Auto merge of rust-lang#8433 - hellow554:update_default_trait, r=flip1995

Don't lint Default::default if it is the udpate syntax base

changelog: Don't lint `Default::default` it is part of the update syntax

Current clippy warns about this:

```
warning: calling `Foo::default()` is more clear than this expression
  --> src/main.rs:12:11
   |
12 |         ..Default::default()
   |           ^^^^^^^^^^^^^^^^^^ help: try: `Foo::default()`
   |
```

With these changes, it will not lint that particular expression anymore.

Author: bors

clippy_lints/src/default.rs

@@ -1,7 +1,7 @@
 use clippy_utils::diagnostics::{span_lint_and_note, span_lint_and_sugg};
 use clippy_utils::source::snippet_with_macro_callsite;
 use clippy_utils::ty::{has_drop, is_copy};
-use clippy_utils::{any_parent_is_automatically_derived, contains_name, match_def_path, paths};
+use clippy_utils::{any_parent_is_automatically_derived, contains_name, get_parent_expr, match_def_path, paths};
 use if_chain::if_chain;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::Applicability;
@@ -88,6 +88,7 @@ impl<'tcx> LateLintPass<'tcx> for Default {
             if let ExprKind::Path(ref qpath) = path.kind;
             if let Some(def_id) = cx.qpath_res(qpath, path.hir_id).opt_def_id();
             if match_def_path(cx, def_id, &paths::DEFAULT_TRAIT_METHOD);
+            if !is_update_syntax_base(cx, expr);
             // Detect and ignore <Foo as Default>::default() because these calls do explicitly name the type.
             if let QPath::Resolved(None, _path) = qpath;
             let expr_ty = cx.typeck_results().expr_ty(expr);
@@ -290,3 +291,16 @@ fn field_reassigned_by_stmt<'tcx>(this: &Stmt<'tcx>, binding_name: Symbol) -> Op
         }
     }
 }
+
+/// Returns whether `expr` is the update syntax base: `Foo { a: 1, .. base }`
+fn is_update_syntax_base<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) -> bool {
+    if_chain! {
+        if let Some(parent) = get_parent_expr(cx, expr);
+        if let ExprKind::Struct(_, _, Some(base)) = parent.kind;
+        then {
+            base.hir_id == expr.hir_id
+        } else {
+            false
+        }
+    }
+}

tests/ui/default_trait_access.fixed

@@ -46,9 +46,14 @@ fn main() {
 
     let s19 = <DerivedDefault as Default>::default();
 
+    let s20 = UpdateSyntax {
+        s: "foo",
+        ..Default::default()
+    };
+
     println!(
-        "[{}] [{}] [{}] [{}] [{}] [{}] [{}] [{}] [{}] [{:?}] [{:?}] [{:?}] [{:?}] [{:?}] [{:?}] [{:?}] [{:?}] [{:?}], [{:?}]",
-        s1, s2, s3, s4, s5, s6, s7, s8, s9, s10, s11, s12, s13, s14, s15, s16, s17, s18, s19,
+        "[{}] [{}] [{}] [{}] [{}] [{}] [{}] [{}] [{}] [{:?}] [{:?}] [{:?}] [{:?}] [{:?}] [{:?}] [{:?}] [{:?}] [{:?}] [{:?}] [{:?}]",
+        s1, s2, s3, s4, s5, s6, s7, s8, s9, s10, s11, s12, s13, s14, s15, s16, s17, s18, s19, s20,
     );
 }
 
@@ -86,3 +91,9 @@ struct ArrayDerivedDefault {
 
 #[derive(Debug, Default)]
 struct TupleStructDerivedDefault(String);
+
+#[derive(Debug, Default)]
+struct UpdateSyntax {
+    pub s: &'static str,
+    pub u: u64,
+}

tests/ui/default_trait_access.rs

@@ -46,9 +46,14 @@ fn main() {
 
     let s19 = <DerivedDefault as Default>::default();
 
+    let s20 = UpdateSyntax {
+        s: "foo",
+        ..Default::default()
+    };
+
     println!(
-        "[{}] [{}] [{}] [{}] [{}] [{}] [{}] [{}] [{}] [{:?}] [{:?}] [{:?}] [{:?}] [{:?}] [{:?}] [{:?}] [{:?}] [{:?}], [{:?}]",
-        s1, s2, s3, s4, s5, s6, s7, s8, s9, s10, s11, s12, s13, s14, s15, s16, s17, s18, s19,
+        "[{}] [{}] [{}] [{}] [{}] [{}] [{}] [{}] [{}] [{:?}] [{:?}] [{:?}] [{:?}] [{:?}] [{:?}] [{:?}] [{:?}] [{:?}] [{:?}] [{:?}]",
+        s1, s2, s3, s4, s5, s6, s7, s8, s9, s10, s11, s12, s13, s14, s15, s16, s17, s18, s19, s20,
     );
 }
 
@@ -86,3 +91,9 @@ struct ArrayDerivedDefault {
 
 #[derive(Debug, Default)]
 struct TupleStructDerivedDefault(String);
+
+#[derive(Debug, Default)]
+struct UpdateSyntax {
+    pub s: &'static str,
+    pub u: u64,
+}