fmt: use the minimum parts array size

The comments for flt2dec::to_shortest_str says that we only need a slice
of length 5 for the parts array.  Initializing a 16-part array is just
wasted effort and wasted stack space.  Other functions in the flt2dec
module have similar comments, so we adjust the parts arrays passed to
those functions accordingly.

Author: froydnj

src/libcore/fmt/float.rs

@@ -19,7 +19,7 @@ fn float_to_decimal_common_exact<T>(fmt: &mut Formatter, num: &T,
     where T: flt2dec::DecodableFloat
 {
     let mut buf = [0; 1024]; // enough for f32 and f64
-    let mut parts = [flt2dec::Part::Zero(0); 16];
+    let mut parts = [flt2dec::Part::Zero(0); 5];
     let formatted = flt2dec::to_exact_fixed_str(flt2dec::strategy::grisu::format_exact,
                                                 *num, sign, precision,
                                                 false, &mut buf, &mut parts);
@@ -34,7 +34,7 @@ fn float_to_decimal_common_shortest<T>(fmt: &mut Formatter,
     where T: flt2dec::DecodableFloat
 {
     let mut buf = [0; flt2dec::MAX_SIG_DIGITS]; // enough for f32 and f64
-    let mut parts = [flt2dec::Part::Zero(0); 16];
+    let mut parts = [flt2dec::Part::Zero(0); 5];
     let formatted = flt2dec::to_shortest_str(flt2dec::strategy::grisu::format_shortest,
                                              *num, sign, 0, false, &mut buf, &mut parts);
     fmt.pad_formatted_parts(&formatted)
@@ -68,7 +68,7 @@ fn float_to_exponential_common_exact<T>(fmt: &mut Formatter, num: &T,
     where T: flt2dec::DecodableFloat
 {
     let mut buf = [0; 1024]; // enough for f32 and f64
-    let mut parts = [flt2dec::Part::Zero(0); 16];
+    let mut parts = [flt2dec::Part::Zero(0); 7];
     let formatted = flt2dec::to_exact_exp_str(flt2dec::strategy::grisu::format_exact,
                                               *num, sign, precision,
                                               upper, &mut buf, &mut parts);
@@ -84,7 +84,7 @@ fn float_to_exponential_common_shortest<T>(fmt: &mut Formatter,
     where T: flt2dec::DecodableFloat
 {
     let mut buf = [0; flt2dec::MAX_SIG_DIGITS]; // enough for f32 and f64
-    let mut parts = [flt2dec::Part::Zero(0); 16];
+    let mut parts = [flt2dec::Part::Zero(0); 7];
     let formatted = flt2dec::to_shortest_exp_str(flt2dec::strategy::grisu::format_shortest, *num,
                                                  sign, (0, 0), upper, &mut buf, &mut parts);
     fmt.pad_formatted_parts(&formatted)