Unexpected truncated boolean when debug printing with limited float precision.

[CasperN]

I tried this code

fn main() {                                                                                                                                                                                                                                                                                                                                                                 
    println!("Hello, world! {:.3?}", false);                                                                                                                                                                                                                                                                                                                                
} 

Actual output

Hello, world! fal

Expected output, because a boolean isn't a decimal....

Hello, world! false

Version stuff:

$ rustc --version --verbose
rustc 1.40.0 (73528e339 2019-12-16)
binary: rustc
commit-hash: 73528e339aae0f17a15ffa49a8ac608f50c6cf14
commit-date: 2019-12-16
host: x86_64-apple-darwin
release: 1.40.0
LLVM version: 9.0

[SNCPlay42]

From the docs for format precision specifiers:

For non-numeric types, this can be considered a "maximum width". If the resulting string is longer than this width, then it is truncated down to this many characters and that truncated value is emitted with proper fill, alignment and width if those parameters are set.

For integral types, this is ignored.

For floating-point types, this indicates how many digits after the decimal point should be printed.

This is expected behaviour.

[CasperN]

The docs could be clearer on what constitutes "non-numeric types." I interpret that statement to be a suggestion as it depends on the type's Debug implementor whether they check formatter.precision(). Still, I think I find it somewhat surprising that booleans are truncated midway through the word. Feel free to close this as intended behavior I guess.

As a comparison:

#[derive(Debug)]                                                                                                                                                                                                                                                                                                                                                            
enum Bool { True, False }                                                                                                                                                                                                                                                                                                                                                   
                                                                                                                                                                                                                                                                                                                                                                            
fn main() {                                                                                                                                                                                                                                                                                                                                                                 
    println!("Hello, world! {:.3?}", false);                                                                                                                                                                                                                                                                                                                                
    println!("Hello, world! {:.3?}", Bool::False);                                                                                                                                                                                                                                                                                                                          
    println!("Hello, world! {:.3?}", &"false");                                                                                                                                                                                                                                                                                                                             
}   

has the following output

Hello, world! fal
Hello, world! False
Hello, world! "false"