`SmallString<SIZE>`: Added `extend_from_str()`: Shunts to heap when needed. `impl Extend<impl AsRef<str>> for SmallString<SIZE>`: Extends N strings.

Fortune for stack-str's current commit: Future small blessing − 末小吉

src/small.rs

@@ -60,6 +60,13 @@ impl HeapString {
 	}
     }
 
+    #[inline(always)]
+    pub fn as_mut_str(&mut self) -> &mut str
+    {
+	unsafe {
+	    std::str::from_utf8_unchecked_mut(self.as_bytes_mut())
+	}
+    }
     #[inline(always)]
     pub fn as_str(&self) -> &str
     {
@@ -297,6 +304,27 @@ impl<const SIZE: usize> SmallString<SIZE>
 	current
     }
 
+    /// Shunt the stack-allocated to heap along with 1 or more `strs`
+    ///
+    /// # Safety
+    /// * The caller must guarantee that the current invariant is `inner.stack`.
+    /// * the caller must recognise that the invariant after this function returns is `inner.heap`.
+    /// * the `fill_ptr` will be updated automacitally. The caller must guarantee that is is `> SIZE` when this function returns.
+    #[inline(always)] 
+    unsafe fn shunt_to_heap_with_unchecked<'i, I>(&'i mut self, strings: I) -> &'i mut HeapString
+    where I: IntoIterator<Item = &'i str>,
+    {
+	let string = {
+	    let mut string: String = self.inner.stack.as_str().into();
+	    string.extend(strings);
+	    string.into_boxed_str()
+	};
+
+	let heap = &mut self.inner.heap;
+	*heap = ManuallyDrop::new(HeapString::new(string));
+	heap
+    }
+
     /// Extends the memory inside `self` to fit more `bytes`.
     /// The data is moved to the heap first if `self.len() + bytes.len() > SIZE`.
     /// 
@@ -309,7 +337,7 @@ impl<const SIZE: usize> SmallString<SIZE>
     unsafe fn extend_from_bytes_unchecked<'i>(&'i mut self, bytes: &[u8]) -> &'i mut [u8]
     {
 	let len = self.len();
-	if bytes.len() + len > SIZE {
+	if !self.inner.is_heap() &&  bytes.len() + len > SIZE {
 	    return self.shunt_to_heap_unchecked().extend_from_bytes_unchecked(bytes);
 	}
 	match self.inner.get_stack_mut() {
@@ -325,7 +353,30 @@ impl<const SIZE: usize> SmallString<SIZE>
 	    }
 	}
     }
-    //TODO: extend_from_str()
+
+    #[inline] 
+    pub fn extend_from_str<'i, 'a: 'i>(&'i mut self, s: &'a str) -> &'i mut str
+    {
+	if self.inner.is_heap() {
+	    // Append to heap.
+	    unsafe {
+		std::str::from_utf8_unchecked_mut((&mut *(self.inner.heap)).extend_from_bytes_unchecked(s.as_bytes()))
+	    }
+	} else {
+	    // Attempt to append to stack
+	    let appended_stack = (unsafe { &mut self.inner.stack }).append_from_str(s);
+	    if appended_stack != s.len() {
+		// Shunt to heap, along with the rest of `s`.
+		let s = &s[appended_stack..];
+		unsafe {
+		    self.shunt_to_heap_with_unchecked(std::iter::once(s)).as_mut_str()
+		}
+	    } else {
+		// Fits in stack, return that.
+		(unsafe {&mut self.inner.stack}).as_mut_str()
+	    }
+	}
+    }
 }
 
 impl<const SIZE: usize> Borrow<str> for SmallString<SIZE>
@@ -414,4 +465,15 @@ impl<const SIZE: usize> fmt::Display for SmallString<SIZE>
     }
 }
 
+impl<const SIZE: usize, T: AsRef<str>> Extend<T> for SmallString<SIZE>
+{
+    #[inline] 
+    fn extend<U: IntoIterator<Item = T>>(&mut self, iter: U) {
+	for s in iter {
+	    //XXX: There's gotta be a more efficient way for this, like we have in `shunt_with_unchecked<I>()`.
+	    self.extend_from_str(s.as_ref());
+	}
+    }
+}
+
 //TODO: SmallString: fmt::Write, io::Write impls when extending functions have been written