Arduino library detection works differently than "enableWiFiAtBootTime()" expects

[dok-net]

PR #7902 suggests that only adding one call, enableWiFiAtBootTime(), is sufficient to restore previous behavior. This is underlined by the commit log:

• mv enableWiFiAtBootTime() to core_esp8266_features.h

suggesting that no additional includes from any library, in particular #include <ESP8266WiFi.h> is required for this to work.

Unfortunately, this is not how the Arduino library resolver works, at least in my installation.

MCVE does not pass the link phase:

void setup()
{
#ifdef WIFI_IS_OFF_AT_BOOT
	enableWiFiAtBootTime();
#endif
}

void loop()
{
}

Result:

Compiling 'mcve' for 'LOLIN(WEMOS) D1 mini Pro'
ld.exe: mcve.cpp.o:(.text.setup+0x0): undefined reference to enableWiFiAtBootTime
 
ld.exe: mcve.cpp.o: in function setup
mcve.ino:4: undefined reference to enableWiFiAtBootTime
 
collect2.exe*: error: ld returned 1 exit status

Error linking for board LOLIN(WEMOS) D1 mini Pro
Build failed for project 'mcve'

Adding #include <ESP8266WiFi.h> helps, but in that case please

--- a/cores/esp8266/core_esp8266_features.h
+++ b/cores/esp8266/core_esp8266_features.h
@@ -105,8 +105,6 @@ uint64_t micros64(void);
 void delay(unsigned long);
 void delayMicroseconds(unsigned int us);
 
-void enableWiFiAtBootTime (void) __attribute__((noinline));
-
 #if defined(F_CPU) || defined(CORE_MOCK)
 #ifdef __cplusplus
 constexpr

and

--- a/libraries/ESP8266WiFi/src/ESP8266WiFi.h
+++ b/libraries/ESP8266WiFi/src/ESP8266WiFi.h
@@ -53,6 +53,7 @@ extern "C" {
 #define DEBUG_WIFI(...) do { (void)0; } while (0)
 #endif
 
+extern "C" void enableWiFiAtBootTime (void) __attribute__((noinline));
 
 class ESP8266WiFiClass : public ESP8266WiFiGenericClass, public ESP8266WiFiSTAClass, public ESP8266WiFiScanClass, public ESP8266WiFiAPClass {
     public:

because compiler errror messages are usually more easily understood by users than linker errors.

In the line of business finding: The #define WIFI_HAS_EVENT_CALLBACK from commit d9a7a81 seems to have no known use, as such, could it be (re)moved, because it intertwines ESP8266WiFi with the core without need?

[dok-net]

@d-a-v I see you've picked up on this already, and your proposed change makes sense too, could you just please give my proposal to keep as much out of core as possible a reading, too? Declaring a symbol in core's includes that is defined and only available if and when the library is explicitly referenced anyway, seems confusing. I'm sure you will agree.