Missed unconditional recursion warning in Default trait implementation.

[a2aaron]

Given the following code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=97f86e28bd7cd07f59f1507f35a9c7a3

pub struct Foo {
    pub x: f32,
    pub y: f32,
}

impl Default for Foo {
    fn default() -> Self {
        Foo {
            x: 1.0f32.floor(),
            ..Default::default()
        }
    }
}

The current output is:

   Compiling playground v0.0.1 (/playground)
    Finished dev [unoptimized + debuginfo] target(s) in 0.83s

Ideally the output should look like:

   Compiling playground v0.0.1 (/playground)
warning: function cannot return without recursing
  --> src/lib.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

The above code should produce a warning because it is impossible for Foo::default() to return without recursing. This likely occurs because the compiler is conservative when it comes to function calls (ex: it's possible that 1.0f32.floor() could panic and hence never return) but this should still be rejected because any path that actually would return would require unconditional recursion.

Note that this does not occur if x doesn't make a function or method call. For example, this code properly warns about unconditional recursion.

impl Default for Foo {
    fn default() -> Self {
        Foo {
            x: 1.0,
            ..Default::default()
        }
    }
}

I ran into this accidentally when implementing default.

[ShikChen]

It's a common trap... #78474 was also for this issue.

[ehuss]

Closing as a duplicate of #78474.