Rollup merge of #73001 - ilya-bobyr:master, r=dtolnay

Free `default()` forwarding to `Default::default()`

It feels a bit redundant to have to say `Default::default()` every time I need a new value of a type that has a `Default` instance.
Especially so, compared to Haskell, where the same functionality is called `def`.
Providing a free `default()` function that forwards to `Default::default()` seems to improve the situation.
The trait is still there, so if someone wants to be explicit and to say `Default::default()` - it still works, but if imported as `std::default::default;`, then the free function reduces typing and visual noise.

Author: RalfJung

src/doc/unstable-book/src/library-features/default-free-fn.md

@@ -0,0 +1,45 @@
+# `default_free_fn`
+
+The tracking issue for this feature is: [#73014]
+
+[#73014]: https://github.com/rust-lang/rust/issues/73014
+
+------------------------
+
+Adds a free `default()` function to the `std::default` module.  This function
+just forwards to [`Default::default()`], but may remove repetition of the word
+"default" from the call site.
+
+Here is an example:
+
+```rust
+#![feature(default_free_fn)]
+use std::default::default;
+
+#[derive(Default)]
+struct AppConfig {
+    foo: FooConfig,
+    bar: BarConfig,
+}
+
+#[derive(Default)]
+struct FooConfig {
+    foo: i32,
+}
+
+#[derive(Default)]
+struct BarConfig {
+    bar: f32,
+    baz: u8,
+}
+
+fn main() {
+    let options = AppConfig {
+        foo: default(),
+        bar: BarConfig {
+            bar: 10.1,
+            ..default()
+        },
+    };
+}
+```

src/libcore/default.rs

@@ -115,6 +115,50 @@ pub trait Default: Sized {
     fn default() -> Self;
 }
 
+/// Return the default value of a type according to the `Default` trait.
+///
+/// The type to return is inferred from context; this is equivalent to
+/// `Default::default()` but shorter to type.
+///
+/// For example:
+/// ```
+/// #![feature(default_free_fn)]
+///
+/// use std::default::default;
+///
+/// #[derive(Default)]
+/// struct AppConfig {
+///     foo: FooConfig,
+///     bar: BarConfig,
+/// }
+///
+/// #[derive(Default)]
+/// struct FooConfig {
+///     foo: i32,
+/// }
+///
+/// #[derive(Default)]
+/// struct BarConfig {
+///     bar: f32,
+///     baz: u8,
+/// }
+///
+/// fn main() {
+///     let options = AppConfig {
+///         foo: default(),
+///         bar: BarConfig {
+///             bar: 10.1,
+///             ..default()
+///         },
+///     };
+/// }
+/// ```
+#[unstable(feature = "default_free_fn", issue = "73014")]
+#[inline]
+pub fn default<T: Default>() -> T {
+    Default::default()
+}
+
 /// Derive macro generating an impl of the trait `Default`.
 #[rustc_builtin_macro]
 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]

src/test/ui/resolve/issue-2356.stderr

@@ -14,7 +14,16 @@ error[E0425]: cannot find function `default` in this scope
   --> $DIR/issue-2356.rs:31:5
    |
 LL |     default();
-   |     ^^^^^^^ help: try: `Self::default`
+   |     ^^^^^^^
+   |
+help: try
+   |
+LL |     Self::default();
+   |     ^^^^^^^^^^^^^
+help: consider importing this function
+   |
+LL | use std::default::default;
+   |
 
 error[E0425]: cannot find value `whiskers` in this scope
   --> $DIR/issue-2356.rs:39:5