0e+10 is invalid whereas 00e+10 is valid

[qnighy]

I found this line in the lexer prevents 0e+10 to be parsed as a float, whereas 00e+10 is allowed anyway. Is this a bug or an intended behavior?

This is a code I tested on Rust Playground:

fn main() {
  // OK
  println!("{}", 0);
  // OK
  println!("{}", 00e+10);
  // Parse Error
  println!("{}", 0e+10);
}

It seems natural that both of them are banned or both of them are allowed.

[nagisa]

It is a bug.

[nagisa]

IIRC at least.

[nagisa]

This is the relevant section in the reference: https://doc.rust-lang.org/reference.html#floating-point-literals

It says:

A floating-point literal has one of two forms:

• A decimal literal followed by a period character U+002E (.). This is optionally followed by another decimal literal, with an optional exponent.
• A single decimal literal followed by an exponent.

and

A decimal literal starts with a decimal digit and continues with any mixture of decimal digits and underscores.

i.e.

LIT_DECIMAL := DEC_DIGIT (DEC_DIGIT | "_")+
EXPONENT := /* snip */
LIT_FLOAT_FORM1 := LIT_DECIMAL "." [LIT_DECIMAL [EXPONENT]]
LIT_FLOAT_FORM2 := LIT_DECIMAL EXPONENT
LIT_FLOAT := LIT_FLOAT_FORM1 | LIT_FLOAT_FORM2

so all forms should parse.

[qnighy]

I'm going to close this issue, because #40589 fixed it.