Some invalid syntax in format spec in format! is ignored

[alco]

These examples demonstrate the issue:

rusti: format!("A{:6>}A", "hi")                
Notice(rusti): AhiA             

rusti: format!("A{:>6}A", "hi")  
Notice(rusti): A    hiA      

rusti: format!("A{:^6}A", "hi")
Notice(rusti): http://j.mp/1v4FzyA

Per the format spec grammar, the alignment character should go on the left of the width specifier.

[alexcrichton]

In you first example, {:6>} corresponds to a fill character of 6 and a right alignment, for example:

format!("A{:6>6}A", "hi") // => "A6666hiA"

Your second example seems to behave as expected, and your last example also seems to be working as intended. The fill character is only consumed if an accepted alignment follows, and rust, unlike python, does not have the ^ alignment specifier.

Does that resolve the ambiguity here?

[alco]

That explains it. I just want to add that format!("{:*>}", ...) where * denotes any character is basically a no-op. You have to also specify the width for both the filling character and the alignment spec to take effect.