Clarify rational behind current implementation of `.mod_euc()`

2 files changed, 8 insertions, 0 deletions

diff --git a/src/libstd/f32.rs b/src/libstd/f32.rs
index 24204670229..8e8340b3ed9 100644
--- a/src/libstd/f32.rs
+++ b/src/libstd/f32.rs
@@ -258,6 +258,10 @@ impl f32 {
     /// most cases. However, due to a floating point round-off error it can
     /// result in `r == rhs.abs()`, violating the mathematical definition, if
     /// `self` is much smaller than `rhs.abs()` in magnitude and `self < 0.0`.
+    /// This result is not an element of the function's codomain, but it is the
+    /// closest floating point number in the real numbers and thus fulfills the
+    /// property `self == self.div_euc(rhs) * rhs + self.mod_euc(rhs)`
+    /// approximatively.
     ///
     /// # Examples
     ///
diff --git a/src/libstd/f64.rs b/src/libstd/f64.rs
index bd7ef2eb1a4..6880294afca 100644
--- a/src/libstd/f64.rs
+++ b/src/libstd/f64.rs
@@ -234,6 +234,10 @@ impl f64 {
     /// most cases.  However, due to a floating point round-off error it can
     /// result in `r == rhs.abs()`, violating the mathematical definition, if
     /// `self` is much smaller than `rhs.abs()` in magnitude and `self < 0.0`.
+    /// This result is not an element of the function's codomain, but it is the
+    /// closest floating point number in the real numbers and thus fulfills the
+    /// property `self == self.div_euc(rhs) * rhs + self.mod_euc(rhs)`
+    /// approximatively.
     ///
     /// # Examples
     ///