Stack overflow while running rust program

[Harshit933]

I tried this code:

#[derive(Debug, Clone, PartialEq, Eq)]
pub enum TokenType {
  LEFT_PAREN
}

impl std::fmt::Display for TokenType {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "TokenType: {}", self)
    }
}

#[derive(Debug, Clone)]
pub struct Token {
    pub token_type: TokenType,
    pub lexeme: String,
    pub line: u16,
}

impl Token {
    pub fn new(token_type: TokenType, lexeme: String, line: u16) -> Self {
        Self {
            token_type,
            lexeme,
            line,
        }
    }
}

impl ToString for Token {
    fn to_string(&self) -> String {
        format!(
            "token_type: {}, lexeme: {}, line: {}",
            self.token_type, self.lexeme, self.line
        )
    }
}

fn main() {
    let token_new = Token::new(TokenType::LEFT_PAREN, "NIL".to_string(), 12);
    println!("TOKEN: {}", token_new.token_type);
}

I expected to see this happen: Some error like this

error[E0277]: `token_type` doesn't implement `std::fmt::Display`

Instead, this happened:

TOKEN: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: TokenType: ...
thread 'main' has overflowed its stack
fatal runtime error: stack overflow

Meta

rustc --version --verbose:

rustc 1.85.0 (4d91de4e4 2025-02-17)
binary: rustc
commit-hash: 4d91de4e48198da2e33413efdcd9cd2cc0c46688
commit-date: 2025-02-17
host: x86_64-unknown-linux-gnu
release: 1.85.0
LLVM version: 19.1.7

Backtrace

<backtrace>

[moxian]

Not a rustc bug.
You encounter the stack overflow at runtime, not during compilation.

If we exclude the Token type from your program we end with something like:

pub enum TokenType {
  LEFT_PAREN
}

impl std::fmt::Display for TokenType {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "TokenType: {}", self)
    }
}

fn main() {
    let tt = TokenType::LEFT_PAREN;
    println!("main: {}", tt);
}

which behaves in the same way as your original. Specifically the write!(f, "TokenType: {}", self) line calls std::fmt::Display on self for the {} format, recursively and unconditionally, leading to the behavior you see.

You probably meant to write the Display impl manually, i.e. something like:

#[derive(Debug)]
pub enum TokenType {
  LEFT_PAREN,
  RIGHT_PAREN
}

impl std::fmt::Display for TokenType {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self{
            TokenType::LEFT_PAREN =>  write!(f, "TokenType: (left paren)"),
            TokenType::RIGHT_PAREN =>  write!(f, "TokenType: (right paren)"),
        }
    }
}

fn main() {
    let tt = TokenType::LEFT_PAREN;
    println!("main: {}", tt);
}

that, or defer to Debug impl by using {:?} there instead:

#[derive(Debug)]
pub enum TokenType {
    LEFT_PAREN
  }
  
  impl std::fmt::Display for TokenType {
      fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
          write!(f, "TokenType: {:?}", self)
      }
  }
  
  fn main() {
      let tt = TokenType::LEFT_PAREN;
      println!("main: {}", tt);
  }

[Harshit933]

Oops sorry for this, I should have probably done something like you mentioned:

impl std::fmt::Display for TokenType {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self{
            TokenType::LEFT_PAREN =>  write!(f, "TokenType: (left paren)"),
            TokenType::RIGHT_PAREN =>  write!(f, "TokenType: (right paren)"),
        }
    }
}

which behaves in the same way as your original. Specifically the write!(f, "TokenType: {}", self) line calls std::fmt::Display on self for the {} format, recursively and unconditionally, leading to the behavior you see.

But I am still unclear why does write!() calls recursively. Maybe we should look into the implementation?

[y21]

But I am still unclear why does write!() calls recursively. Maybe we should look into the implementation?

This is just how the formatting macros work: {} always calls Display::fmt. If you pass self for the {}, which is a TokenType, it will call <TokenType as Display>::fmt. That is the very same function where this write! is, thus you end up with this recursive Display::fmt call which will forever keep trying to display format itself and eventually overflow the stack.

For what it's worth, Clippy is able to detect this

error: using `self` as `Display` in `impl Display` will cause infinite recursion
 --> src/main.rs:7:9
  |
7 |         write!(f, "TokenType: {}", self)
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#recursive_format_impl
  = note: `#[deny(clippy::recursive_format_impl)]` on by default
  = note: this error originates in the macro `write` (in Nightly builds, run with -Z macro-backtrace for more info)

[Harshit933]

This makes a lot of sense, thank you for the explanation

[y21]

I do think that the compiler should emit at least a warning for this mistake though, so it's probably worth keeping this open? Either the unconditonal_recursion lint could be extended to catch this or perhaps uplifting the already existing clippy lint. (I see @fmease already added the false-negative and L-unconditional-recursion labels)