Using stackalloc in nested expressions

With C# 7, we started using Span<T>, ReadOnlySpan<T>, and Memory<T> because they are ref struct instances that are guaranteed to be allocated on the stack, and therefore won't affect the garbage collector. Thanks to Span, it was also possible to avoid declaring the stackalloc statements that are directly assigned to Span or ReadOnlySpan as unsafe:

Span<int> nums = stackalloc int[10];

Starting from C# 8, the compiler widens the use of stackalloc to any expression expecting Span or ReadOnlySpan. In the following example, the test trims the input string from three special characters, obtaining the string specified in the expected variable:

string input = " this string can be trimmed \r\n";
var expected = "this string can be trimmed";
ReadOnlySpan<char> trimmedSpan = input.AsSpan()
    .Trim(stackalloc[] { ' ', '\r', '\n' });
string result...