Auto merge of #51859 - kennytm:rollup, r=kennytm

Rollup of 7 pull requests

Successful merges:

 - #49987 (Add str::split_ascii_whitespace.)
 - #50342 (Document round-off error in `.mod_euc()`-method, see issue #50179)
 - #51658 (Only do sanity check with debug assertions on)
 - #51799 (Lower case some feature gate error messages)
 - #51800 (Add a compiletest header for edition)
 - #51824 (Fix the error reference for LocalKey::try_with)
 - #51842 (Document that Layout::from_size_align does not allow align=0)

Failed merges:

r? @ghost

3 files changed, 21 insertions, 3 deletions

diff --git a/src/libstd/f32.rs b/src/libstd/f32.rs
index ae30321f46d..8e8340b3ed9 100644
--- a/src/libstd/f32.rs
+++ b/src/libstd/f32.rs
@@ -254,7 +254,14 @@ impl f32 {
 
     /// Calculates the Euclidean modulo (self mod rhs), which is never negative.
     ///
-    /// In particular, the result `n` satisfies `0 <= n < rhs.abs()`.
+    /// In particular, the return value `r` satisfies `0.0 <= r < rhs.abs()` in
+    /// 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
     ///
@@ -266,6 +273,8 @@ impl f32 {
     /// assert_eq!((-a).mod_euc(b), 1.0);
     /// assert_eq!(a.mod_euc(-b), 3.0);
     /// assert_eq!((-a).mod_euc(-b), 1.0);
+    /// // limitation due to round-off error
+    /// assert!((-std::f32::EPSILON).mod_euc(3.0) != 0.0);
     /// ```
     #[inline]
     #[unstable(feature = "euclidean_division", issue = "49048")]
diff --git a/src/libstd/f64.rs b/src/libstd/f64.rs
index 7950d434b77..6880294afca 100644
--- a/src/libstd/f64.rs
+++ b/src/libstd/f64.rs
@@ -230,7 +230,14 @@ impl f64 {
 
     /// Calculates the Euclidean modulo (self mod rhs), which is never negative.
     ///
-    /// In particular, the result `n` satisfies `0 <= n < rhs.abs()`.
+    /// In particular, the return value `r` satisfies `0.0 <= r < rhs.abs()` in
+    /// 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
     ///
@@ -242,6 +249,8 @@ impl f64 {
     /// assert_eq!((-a).mod_euc(b), 1.0);
     /// assert_eq!(a.mod_euc(-b), 3.0);
     /// assert_eq!((-a).mod_euc(-b), 1.0);
+    /// // limitation due to round-off error
+    /// assert!((-std::f64::EPSILON).mod_euc(3.0) != 0.0);
     /// ```
     #[inline]
     #[unstable(feature = "euclidean_division", issue = "49048")]
diff --git a/src/libstd/thread/local.rs b/src/libstd/thread/local.rs
index 40d3280baa6..a170abb2628 100644
--- a/src/libstd/thread/local.rs
+++ b/src/libstd/thread/local.rs
@@ -276,7 +276,7 @@ impl<T: 'static> LocalKey<T> {
     ///
     /// This will lazily initialize the value if this thread has not referenced
     /// this key yet. If the key has been destroyed (which may happen if this is called
-    /// in a destructor), this function will return a `ThreadLocalError`.
+    /// in a destructor), this function will return an [`AccessError`](struct.AccessError.html).
     ///
     /// # Panics
     ///