timespan 0.2.1

A simple timespan for chrono times.
Documentation 
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// timespan - A simple timespan for chrono times.
//
// Copyright (C) 2017
//     Fin Christensen <fin.christensen@posteo.de>
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program.  If not, see <http://www.gnu.org/licenses/>.

use crate::Formatable;
use crate::Span;
use crate::Spanable;
use std;

/// An instance of this type gets created when issuing a `format` on a span.
///
/// In theory this approach for formatting reduces the amount of memory used but the
/// implementation is currently using more memory than it should.
///
/// Therefore this only eases the handling when formatting spans with a custom format
/// string.
pub struct DelayedFormat<'a, T> {
    pub span: Span<T>,
    pub fmt: &'a str,
    pub start: &'a str,
    pub end: &'a str,
}

impl<'a, T> std::fmt::Display for DelayedFormat<'a, T>
where
    T: Spanable + Formatable,
{
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        use std::fmt::Write;

        // FIXME: this uses way too much memory

        let mut start_fmt = String::new();
        start_fmt.write_fmt(format_args!("{}", self.span.start.format(self.start)))?;
        start_fmt.shrink_to_fit();

        let mut end_fmt = String::new();
        end_fmt.write_fmt(format_args!("{}", self.span.end.format(self.end)))?;
        end_fmt.shrink_to_fit();

        let r1 = self.fmt.replace("{start}", start_fmt.as_str());
        let r2 = r1.replace("{end}", end_fmt.as_str());

        write!(f, "{}", r2)
    }
}