rustc should warn when the implementation of Display requires to display self

[rgiot]

rustc should fail or warn when the implementation of the Display trait of a strcut requires to display it. Indeed there is a stack overflow as the code is recursively called.
rustc already detect recursion issues in other cases.
Such mistake can happen when using "{}" instead of "{:?}" for debug purpose.

This code:

use std::fmt;

struct Dummy {
}

impl fmt::Display for Dummy {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "{}", self) // XXX Should fail or warn at compilation there
    }
}


impl Dummy {
    fn recursive(&self) {
        self.recursive(); /// XXX rustc warn there
    }
}

#[cfg(test)]
mod test {
    #[test]
    fn illustrate() {
        let dummy = crate::Dummy{};
        println!("{}", dummy);
    }
}

producde this compilation message and test error:

cargo test
   Compiling tmp v0.1.0 (/tmp)
warning: function cannot return without recursing
  --> src/lib.rs:13:5
   |
13 |     fn recursive(&self) {
   |     ^^^^^^^^^^^^^^^^^^^ cannot return without recursing
14 |         self.recursive();
   |         ---------------- recursive call site
   |
   = note: #[warn(unconditional_recursion)] on by default
   = help: a `loop` may express intention better if this is on purpose

warning: struct is never constructed: `Dummy`
 --> src/lib.rs:3:1
  |
3 | struct Dummy {
  | ^^^^^^^^^^^^
  |
  = note: #[warn(dead_code)] on by default

warning: method is never used: `recursive`
  --> src/lib.rs:13:5
   |
13 |     fn recursive(&self) {
   |     ^^^^^^^^^^^^^^^^^^^

warning: function cannot return without recursing
  --> src/lib.rs:13:5
   |
13 |     fn recursive(&self) {
   |     ^^^^^^^^^^^^^^^^^^^ cannot return without recursing
14 |         self.recursive();
   |         ---------------- recursive call site
   |
   = note: #[warn(unconditional_recursion)] on by default
   = help: a `loop` may express intention better if this is on purpose

warning: method is never used: `recursive`
  --> src/lib.rs:13:5
   |
13 |     fn recursive(&self) {
   |     ^^^^^^^^^^^^^^^^^^^
   |
   = note: #[warn(dead_code)] on by default

    Finished dev [unoptimized + debuginfo] target(s) in 0.23s
     Running target/debug/deps/tmp-dea596419d86ba6c

running 1 test

thread 'test::illustrate' has overflowed its stack
fatal runtime error: stack overflow

Although to have this crash is normal, the compiler should raise a warning exactly as it does for the recursive function.

With this version

impl fmt::Display for Dummy {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        unimplemented!("{}", self) // XXX Should fail or warn at compilation there
    }
}

although the crash is slightly different:

thread panicked while processing panic. aborting.
error: process didn't exit successfully: `/tmp/target/debug/deps/tmp-dea596419d86ba6c` (signal: 4, SIGILL: illegal instruction)

compilation warning should be the same.

I'm writting this issue because I guess it can be detected by the compiler as Display (or Debug) belong to the language.

Meta

rustc --version --verbose
rustc 1.34.0-nightly (147311c5f 2019-01-30)
binary: rustc
commit-hash: 147311c5fc62537da8eb9c6f69536bec6719d534
commit-date: 2019-01-30
host: x86_64-unknown-linux-gnu
release: 1.34.0-nightly
LLVM version: 8.0

[jonas-schievink]

One of the many instances of #57965

(the case for anything using std::fmt is more difficult since IIRC that heavily uses trait objects, though)

[estebank]

Direct dupe of #45838, but there are multiple cases as pointed out by @jonas-schievink.