default_free_fn feature does not have the same diagnostics as using Default::default in recursing situations

[harudagondi]

Given the following code: Rust playground link

#![feature(default_free_fn)]

use std::default::default;

struct Foo {
    a: i32,
    b: String,
}

impl Default for Foo {
    fn default() -> Self {
        Self { a: 1, ..default() }
    }
}

fn main() {
    let foo = Foo::default();
}

The current output is:

   Compiling playground v0.0.1 (/playground)
warning: unused variable: `foo`
  --> src/main.rs:17:9
   |
17 |     let foo = Foo::default();
   |         ^^^ help: if this is intentional, prefix it with an underscore: `_foo`
   |
   = note: `#[warn(unused_variables)]` on by default

warning: fields `a` and `b` are never read
 --> src/main.rs:6:5
  |
5 | struct Foo {
  |        --- fields in this struct
6 |     a: i32,
  |     ^
7 |     b: String,
  |     ^
  |
  = note: `#[warn(dead_code)]` on by default

warning: `playground` (bin "playground") generated 2 warnings
    Finished dev [unoptimized + debuginfo] target(s) in 0.86s
     Running `target/debug/playground`

thread 'main' has overflowed its stack
fatal runtime error: stack overflow
timeout: the monitored command dumped core
/playground/tools/entrypoint.sh: line 11:     9 Aborted                 timeout --signal=KILL ${timeout} "$@"

Ideally the output should look like: (Using Default::default())

   Compiling playground v0.0.1 (/playground)
warning: unused variable: `foo`
  --> src/main.rs:16:9
   |
16 |     let foo = Foo::default();
   |         ^^^ help: if this is intentional, prefix it with an underscore: `_foo`
   |
   = note: `#[warn(unused_variables)]` on by default

warning: function cannot return without recursing
  --> src/main.rs:7:5
   |
7  |     fn default() -> Self {
   |     ^^^^^^^^^^^^^^^^^^^^ cannot return without recursing
...
10 |             ..Default::default()
   |               ------------------ recursive call site
   |
   = note: `#[warn(unconditional_recursion)]` on by default
   = help: a `loop` may express intention better if this is on purpose

warning: fields `a` and `b` are never read
 --> src/main.rs:2:5
  |
1 | struct Foo {
  |        --- fields in this struct
2 |     a: i32,
  |     ^
3 |     b: String,
  |     ^
  |
  = note: `#[warn(dead_code)]` on by default

warning: `playground` (bin "playground") generated 3 warnings
    Finished dev [unoptimized + debuginfo] target(s) in 0.86s
     Running `target/debug/playground`

thread 'main' has overflowed its stack
fatal runtime error: stack overflow
timeout: the monitored command dumped core
/playground/tools/entrypoint.sh: line 11:     8 Aborted                 timeout --signal=KILL ${timeout} "$@"

Using default() should also warn the user about unconditional_recursion.

[lukas-code]

Another instance of #57965.