From a0ea3f2c608a8935890264e6fb92218e49bee299 Mon Sep 17 00:00:00 2001
From: Trevor Spiteri <tspiteri@ieee.org>
Date: Tue, 22 Sep 2020 19:09:41 +0200
Subject: [PATCH] loosen Rhs constraint on MulAssign

---
 README.md    | 10 +++++++---
 RELEASES.md  |  5 +++++
 src/arith.rs | 17 +++++++++++++----
 3 files changed, 25 insertions(+), 7 deletions(-)

diff --git a/README.md b/README.md
index 8bdaf6e..5bb75e2 100644
--- a/README.md
+++ b/README.md
@@ -80,8 +80,11 @@ The conversions supported cover the following cases.
 
 ### Version 1.3.0 news (unreleased)
 
+  * The [`MulAssign`] implementation on fixed-point numbers now
+    accepts an rhs fixed-point number with a different number of
+    fractional bits from `self`.
   * The following methods were added to all fixed-point types, to the
-    [`Fixed`][tf-1-3] trait, and to the [`Wrapping`][w-1-3] wrapper:
+    [`Fixed`][tf-ma-1-3] trait, and to the [`Wrapping`][w-ma-1-3] wrapper:
       * [`mul_add`][f-ma-1-3], [`checked_mul_add`][f-cma-1-3],
         [`saturating_mul_add`][f-sma-1-3],
         [`wrapping_mul_add`][f-wma-1-3],
@@ -90,14 +93,15 @@ The conversions supported cover the following cases.
     providing arithmetic methods that panic on overflow even when
     debug assertions are disabled.
 
+[`MulAssign`]: https://doc.rust-lang.org/nightly/core/ops/trait.MulAssign.html
 [f-cma-1-3]: https://tspiteri.gitlab.io/fixed/dev/fixed/struct.FixedI32.html#method.checked_mul_add
 [f-ma-1-3]: https://tspiteri.gitlab.io/fixed/dev/fixed/struct.FixedI32.html#method.mul_add
 [f-oma-1-3]: https://tspiteri.gitlab.io/fixed/dev/fixed/struct.FixedI32.html#method.overflowing_mul_add
 [f-sma-1-3]: https://tspiteri.gitlab.io/fixed/dev/fixed/struct.FixedI32.html#method.saturating_mul_add
 [f-wma-1-3]: https://tspiteri.gitlab.io/fixed/dev/fixed/struct.FixedI32.html#method.wrapping_mul_add
 [feat-un-1-3]: https://tspiteri.gitlab.io/fixed/dev/fixed/#experimental-optional-features
-[tf-1-3]: https://tspiteri.gitlab.io/fixed/dev/fixed/traits/trait.Fixed.html
-[w-1-3]: https://tspiteri.gitlab.io/fixed/dev/fixed/struct.Wrapping.html
+[tf-ma-1-3]: https://tspiteri.gitlab.io/fixed/dev/fixed/traits/trait.Fixed.html#tymethod.mul_add
+[w-ma-1-3]: https://tspiteri.gitlab.io/fixed/dev/fixed/struct.Wrapping.html#method.mul_add
 
 ### Version 1.2.0 news (2020-09-02)
 
diff --git a/RELEASES.md b/RELEASES.md
index 23610f7..2e89a39 100644
--- a/RELEASES.md
+++ b/RELEASES.md
@@ -8,6 +8,9 @@ as-is, without any warranty. -->
 Version 1.3.0 (unreleased)
 ==========================
 
+  * The [`MulAssign`] implementation on fixed-point numbers now
+    accepts an rhs fixed-point number with a different number of
+    fractional bits from `self`.
   * The following methods were added to all fixed-point types, to the
     [`Fixed`][tf-1-3] trait, and to the [`Wrapping`][w-1-3] wrapper:
       * [`mul_add`][f-ma-1-3], [`checked_mul_add`][f-cma-1-3],
@@ -407,3 +410,5 @@ Version 0.1.0 (2018-08-10)
     for the number of fractional bits.
   * Many methods and trait implementations available for primitive
     integers are now also supported by the fixed-point numbers.
+
+[`MulAssign`]: https://doc.rust-lang.org/nightly/core/ops/trait.MulAssign.html
diff --git a/src/arith.rs b/src/arith.rs
index 2d4caaa..1c21e23 100644
--- a/src/arith.rs
+++ b/src/arith.rs
@@ -238,14 +238,23 @@ macro_rules! fixed_arith {
 
         refs! { impl Mul for $Fixed($LeEqU) { mul } }
 
-        impl<Frac: $LeEqU> MulAssign<$Fixed<Frac>> for $Fixed<Frac> {
+        impl<Frac, RhsFrac: $LeEqU> MulAssign<$Fixed<RhsFrac>> for $Fixed<Frac> {
             #[inline]
-            fn mul_assign(&mut self, rhs: $Fixed<Frac>) {
-                *self = (*self).mul(rhs)
+            fn mul_assign(&mut self, rhs: $Fixed<RhsFrac>) {
+                let (ans, overflow) = self.to_bits().mul_overflow(rhs.to_bits(), RhsFrac::U32);
+                debug_assert!(!overflow, "overflow");
+                *self = Self::from_bits(ans);
             }
         }
 
-        refs_assign! { impl MulAssign for $Fixed($LeEqU) { mul_assign } }
+        impl<Frac, RhsFrac: $LeEqU> MulAssign<&'_ $Fixed<RhsFrac>> for $Fixed<Frac> {
+            #[inline]
+            fn mul_assign(&mut self, rhs: &$Fixed<RhsFrac>) {
+                let (ans, overflow) = self.to_bits().mul_overflow(rhs.to_bits(), RhsFrac::U32);
+                debug_assert!(!overflow, "overflow");
+                *self = Self::from_bits(ans);
+            }
+        }
 
         impl<Frac: $LeEqU> Div<$Fixed<Frac>> for $Fixed<Frac> {
             type Output = $Fixed<Frac>;