util/StringBuilder: do not null-terminate the buffer

All users should switch to ToStringView() instead of assuming the
buffer is null-terminated.

Author: MaxKellermann

src/util/StringBuilder.cxx

@@ -10,15 +10,13 @@ void
 BasicStringBuilder<T>::UnsafeAppend(std::basic_string_view<T> src) noexcept
 {
 	p = std::copy(src.begin(), src.end(), p);
-	*p = SENTINEL;
 }
 
 template<typename T>
 void
 BasicStringBuilder<T>::UnsafeFill(T ch, std::size_t n) noexcept
 {
 	p = std::fill_n(p, n, ch);
-	*p = SENTINEL;
 }
 
 template class BasicStringBuilder<char>;

src/util/StringBuilder.hxx

@@ -26,8 +26,6 @@ class BasicStringBuilder {
 	pointer p;
 	const pointer end;
 
-	static constexpr value_type SENTINEL = '\0';
-
 public:
 	[[nodiscard]]
 	explicit constexpr BasicStringBuilder(std::span<value_type> b) noexcept
@@ -42,7 +40,7 @@ public:
 	}
 
 	constexpr bool IsFull() const noexcept {
-		return p >= end - 1;
+		return p >= end;
 	}
 
 	[[nodiscard]]
@@ -55,7 +53,7 @@ public:
 	}
 
 	constexpr bool CanAppend(size_type length) const noexcept {
-		return p + length < end;
+		return p + length <= end;
 	}
 
 	constexpr void CheckAppend(size_type length) const {
@@ -65,7 +63,6 @@ public:
 
 	constexpr void UnsafeAppend(T ch) noexcept {
 		*p++ = ch;
-		*p = SENTINEL;
 	}
 
 	[[nodiscard]]