Invalid plan arising from integer rounding

.changelog/190.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+tftypes: Prevented loss of number precision with integers between 54 and 64 bits
+```

CHANGELOG.md

@@ -1,3 +1,8 @@
+# 0.9.1 (May 12, 2022)
+
+BUG FIXES:
+* tftypes: Prevented loss of number precision with integers between 54 and 64 bits ([#190](https://github.com/hashicorp/terraform-plugin-go/issues/190))
+
 # 0.9.0 (April 13, 2022)
 
 NOTES:

tftypes/value.go

@@ -440,7 +440,7 @@ func (val Value) As(dst interface{}) error {
 		if !ok {
 			return fmt.Errorf("can't unmarshal %s into %T, expected *big.Float", val.Type(), dst)
 		}
-		target.Set(v)
+		target.Copy(v)
 		return nil
 	case **big.Float:
 		if val.IsNull() {

tftypes/value_msgpack.go

@@ -74,21 +74,21 @@ func msgpackUnmarshal(dec *msgpack.Decoder, typ Type, path *AttributePath) (Valu
 			if err != nil {
 				return Value{}, path.NewErrorf("couldn't decode number as int64: %w", err)
 			}
-			return NewValue(Number, big.NewFloat(float64(rv))), nil
+			return NewValue(Number, new(big.Float).SetInt64(rv)), nil
 		}
 		switch peek {
 		case msgpackCodes.Int8, msgpackCodes.Int16, msgpackCodes.Int32, msgpackCodes.Int64:
 			rv, err := dec.DecodeInt64()
 			if err != nil {
 				return Value{}, path.NewErrorf("couldn't decode number as int64: %w", err)
 			}
-			return NewValue(Number, big.NewFloat(float64(rv))), nil
+			return NewValue(Number, new(big.Float).SetInt64(rv)), nil
 		case msgpackCodes.Uint8, msgpackCodes.Uint16, msgpackCodes.Uint32, msgpackCodes.Uint64:
 			rv, err := dec.DecodeUint64()
 			if err != nil {
 				return Value{}, path.NewErrorf("couldn't decode number as uint64: %w", err)
 			}
-			return NewValue(Number, big.NewFloat(float64(rv))), nil
+			return NewValue(Number, new(big.Float).SetUint64(rv)), nil
 		case msgpackCodes.Float, msgpackCodes.Double:
 			rv, err := dec.DecodeFloat64()
 			if err != nil {

tftypes/value_msgpack_test.go

@@ -70,11 +70,36 @@ func TestValueFromMsgPack(t *testing.T) {
 			value: NewValue(Number, big.NewFloat(1)),
 			typ:   Number,
 		},
+		"int64-positive-number": {
+			hex:   "cf7fffffffffffffff",
+			value: NewValue(Number, new(big.Float).SetInt64(math.MaxInt64)),
+			typ:   Number,
+		},
+		"int64-negative-number": {
+			hex:   "d38000000000000000",
+			value: NewValue(Number, new(big.Float).SetInt64(math.MinInt64)),
+			typ:   Number,
+		},
+		"uint64-number": {
+			hex:   "b43138343436373434303733373039353531363135",
+			value: NewValue(Number, new(big.Float).SetUint64(math.MaxUint64)),
+			typ:   Number,
+		},
 		"float-number": {
 			hex:   "cb3ff8000000000000",
 			value: NewValue(Number, big.NewFloat(1.5)),
 			typ:   Number,
 		},
+		"float64-positive-number": {
+			hex:   "cb7fefffffffffffff",
+			value: NewValue(Number, new(big.Float).SetFloat64(math.MaxFloat64)),
+			typ:   Number,
+		},
+		"float64-negative-number": {
+			hex:   "cb0000000000000001",
+			value: NewValue(Number, new(big.Float).SetFloat64(math.SmallestNonzeroFloat64)),
+			typ:   Number,
+		},
 		"big-number": {
 			hex:   "d96439393939393939393939393939393939393939393939393939393939393939393939393939393939393939393939393939393939393939393939393939393939393939393939393939393939393939393939393939393939393939393939393939393939",
 			value: NewValue(Number, bigNumber),