math/big: shallow copies of Int/Rat/Float are not supported (documentation)

Fixes #28423.

Change-Id: Ie57ade565d0407a4bffaa86fb4475ff083168e79
Reviewed-on: https://go-review.googlesource.com/c/145537
Reviewed-by: Ian Lance Taylor <iant@golang.org>

1 files changed, 7 insertions, 1 deletions

diff --git a/src/math/big/float.go b/src/math/big/float.go
index 6b0cb3f1ed..d5e801b2c8 100644
--- a/src/math/big/float.go
+++ b/src/math/big/float.go
@@ -43,7 +43,7 @@ const debugFloat = false // enable for debugging
 // precision of the argument with the largest precision value before any
 // rounding takes place, and the rounding mode remains unchanged. Thus,
 // uninitialized Floats provided as result arguments will have their
-// precision set to a reasonable value determined by the operands and
+// precision set to a reasonable value determined by the operands, and
 // their mode is the zero value for RoundingMode (ToNearestEven).
 //
 // By setting the desired precision to 24 or 53 and using matching rounding
@@ -56,6 +56,12 @@ const debugFloat = false // enable for debugging
 // The zero (uninitialized) value for a Float is ready to use and represents
 // the number +0.0 exactly, with precision 0 and rounding mode ToNearestEven.
 //
+// Operations always take pointer arguments (*Float) rather
+// than Float values, and each unique Float value requires
+// its own unique *Float pointer. To "copy" a Float value,
+// an existing (or newly allocated) Float must be set to
+// a new value using the Float.Set method; shallow copies
+// of Floats are not supported and may lead to errors.
 type Float struct {
 	prec uint32
 	mode RoundingMode