core: Fix to_str_exact for floats with no decimal component

src/libcore/float.rs

@@ -52,7 +52,7 @@ fn to_str_common(num: float, digits: uint, exact: bool) -> str {
     let trunc = num as uint;
     let frac = num - (trunc as float);
     accum += uint::str(trunc);
-    if frac < epsilon || digits == 0u { ret accum; }
+    if (frac < epsilon && !exact) || digits == 0u { ret accum; }
     accum += ".";
     let i = digits;
     let epsilon = 1. / pow_uint_to_uint_as_float(10u, i);
@@ -83,6 +83,12 @@ fn to_str_exact(num: float, digits: uint) -> str {
     to_str_common(num, digits, true)
 }
 
+#[test]
+fn test_to_str_exact_do_decimal() {
+    let s = to_str_exact(5.0, 4u);
+    assert s == "5.0000";
+}
+
 /*
 Function: to_str
 

src/test/run-pass/syntax-extension-fmt.rs

@@ -136,6 +136,7 @@ fn part4() {
     test(#fmt["%.5t", 3u], "00011");
     test(#fmt["%.5c", 'A'], "A");
     test(#fmt["%.5f", 5.82], "5.82000");
+    test(#fmt["%.5f", 5.0], "5.00000");
     // Bool precision. I'm not sure if it's good or bad to have bool
     // conversions support precision - it's not standard printf so we
     // can do whatever. For now I'm making it behave the same as string