feat: control precision via standard format options (#84)

Co-authored-by: Rob Ede <robjtede@icloud.com>

Author: weihanglo

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## Unreleased
 
+- Add support for precision in `Display` implementations.
+
 ## v2.0.0
 
 - Add support for `no_std` targets.

ensure-no-std/Cargo.lock

@@ -128,6 +128,7 @@ impl fmt::Display for Display {
         let unit_prefixes = self.format.unit_prefixes();
         let unit_separator = self.format.unit_separator();
         let unit_suffix = self.format.unit_suffix();
+        let precision = f.precision().unwrap_or(1);
 
         if bytes < unit {
             write!(f, "{bytes}{unit_separator}B")?;
@@ -144,7 +145,7 @@ impl fmt::Display for Display {
 
             write!(
                 f,
-                "{:.1}{unit_separator}{unit_prefix}{unit_suffix}",
+                "{:.precision$}{unit_separator}{unit_prefix}{unit_suffix}",
                 (size / unit.pow(exp as u32) as f64),
             )?;
         }
@@ -185,7 +186,7 @@ fn ideal_unit_std(size: f64, unit_base: f64) -> usize {
 
 #[cfg(test)]
 mod tests {
-    use alloc::string::ToString as _;
+    use alloc::{format, string::ToString as _};
 
     use super::*;
 
@@ -293,4 +294,12 @@ mod tests {
         assert_to_string("540.9 PiB", ByteSize::pb(609), Format::Iec);
         assert_to_string("609.0 PB", ByteSize::pb(609), Format::Si);
     }
+
+    #[test]
+    fn precision() {
+        let size = ByteSize::mib(1908);
+        assert_eq!("1.9 GiB".to_string(), format!("{}", size));
+        assert_eq!("2 GiB".to_string(), format!("{:.0}", size));
+        assert_eq!("1.86328 GiB".to_string(), format!("{:.5}", size));
+    }
 }

src/display.rs

@@ -236,9 +236,9 @@ impl fmt::Display for ByteSize {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let display = self.display();
 
-        if f.width().is_none() && f.precision().is_none() {
+        if f.width().is_none() {
             // allocation-free fast path for when no formatting options are specified
-            write!(f, "{display}")
+            display.fmt(f)
         } else {
             f.pad(&display.to_string())
         }