Printing using fmt::Display - printing

Question

I am trying to print an enum (or structure) using fmt::Display. Though the code compiles and gets to the display method, it doesn't print the value.
pub enum TestEnum<'a> {
Foo(&'a str),
Bar(f32)
}
impl<'b> fmt::Display for TestEnum <'b> {
fn fmt(&self, f : &mut fmt::Formatter) -> fmt::Result {
println!("Got this far");
match self{
&TestEnum::Foo(x) => write!(f,"{}",x),
&TestEnum::Bar(x) => write!(f,"{}",x),
}
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test_print() {
let cell = TestEnum::Str("foo");
println!("Printing");
println!("{}",cell); // No output here
}
}
I tried using {:?} and {} but to no avail.

Answer 1

This happens because Rust test program hides stdout of successful tests. You can disable this behavior passing --nocapture option to test binary or to cargo test command this way:
cargo test -- --nocapture
PS: your code is broken/incomplete

Answer 2

The test runner seems to divert the standard output; you should consider using assert!, assert_eq! or other panicky ways to test your assertions rather than printing in tests.
Besides, your code fails to compile due to mismatching names. I got it working as expected from main:
use std::fmt;
pub enum TestEnum<'a> {
Foo(&'a str),
Bar(f32)
}
impl<'b> fmt::Display for TestEnum <'b> {
fn fmt(&self, f : &mut fmt::Formatter) -> fmt::Result {
match self {
&TestEnum::Foo(x) => write!(f, "{}", x),
&TestEnum::Bar(x) => write!(f, "{}", x),
}
}
}
fn main() {
let cell = TestEnum::Foo("foo");
println!("Printing");
println!("{}", cell);
}

Answer 3

Test output is redirected to a buffer when the test succeeds as to not mangle up with the test "FAILED" or "ok" messages.
If you just want to test something while developing your test, you can always add a panic!() at the end of your test to make sure it keeps failing and outputting all logging. Or as #AndreaP notes in his answer, you can use cargo test -- --nocapture to display the standard output of all tests.
Usually a test should not write to stdout, but instead write to a buffer and check whether that buffer contains what it should:
let cell = TestEnum::Foo("foo");
let mut buf = Vec::new();
let _ = write!(buf, "{}\n", cell);
assert_eq!(&buf, b"foo\n");
If you truly want to output something, you need to write directly to stdout.
let _ = write!(io::stdout(), "{}\n", cell);
but this will mix with the test's output:
test tests::blub ... foo
ok
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