Turn SystemError.from_errno into a macro

This ensures we capture `Errno.value` right away and it cannot be
influenced by other code producing the arguments.

Author: straight-shoota

spec/std/system_error_spec.cr

@@ -19,7 +19,7 @@ describe SystemError do
     it "avoid reset from message" do
       Errno.value = :ENOENT
       error = ::RuntimeError.from_errno(message: "foobar".tap{ Errno.value = :EPERM })
-      error.os_error.should eq Errno::EPERM # This should be ENOENT
+      error.os_error.should eq Errno::ENOENT
     end
   end
 end

src/system_error.cr

@@ -40,6 +40,21 @@
 module SystemError
   macro included
     extend ::SystemError::ClassMethods
+
+    # Builds an instance of the exception from the current system error value (`Errno.value`).
+    #
+    # The system message corresponding to the OS error value amends the *message*.
+    # Additional keyword arguments are forwarded to the exception initializer `.new_from_os_error`.
+    macro from_errno(message, **opts)
+      # This is a macro in order to retrieve `Errno.value` first before evaluating `message` and `opts`.
+      %errno = Errno.value
+      ::\{{@type}}.from_os_error(\{{ message }}, %errno, \{{ opts.double_splat }})
+    end
+
+    @[Deprecated("Use `.from_os_error` instead")]
+    macro from_errno(message = nil, errno = nil, **opts)
+      ::\{{@type}}.from_os_error(\{{ message }}, \{{ errno }}, \{{ opts.double_splat }})
+    end
   end
 
   # The original system error wrapped by this exception
@@ -68,19 +83,6 @@ module SystemError
       end
     end
 
-    # Builds an instance of the exception from the current system error value (`Errno.value`).
-    #
-    # The system message corresponding to the OS error value amends the *message*.
-    # Additional keyword arguments are forwarded to the exception initializer `.new_from_os_error`.
-    def from_errno(message : String, **opts)
-      from_os_error(message, Errno.value, **opts)
-    end
-
-    @[Deprecated("Use `.from_os_error` instead")]
-    def from_errno(message : String? = nil, errno : Errno? = nil, **opts)
-      from_os_error(message, errno, **opts)
-    end
-
     # Prepares the message that goes before the system error description.
     #
     # By default it returns the original message unchanged. But that could be