Berry import strict now detects useless expr without side effects

[s-hadinger]

Description:

Berry: new test added when compiling in strict mode import strict. Now the parser detects when statements or expression don't have any side effect, which means that they do nothing. It's most certainly a mistake.

This detects for example self.attr 0 which should have been self.attr = 0, we was silently accepted as two useless expression.

> def f() 1 end
syntax_error: stdin:1: strict: expression without side effect detected

> def f(x) x 0 end
syntax_error: stdin:1: strict: expression without side effect detected

> def f(x) return x end     # this is ok

Checklist:

• The pull request is done against the latest development branch
• Only relevant files were touched
• Only one feature/fix was added per PR and the code change compiles without warnings
• The code change is tested and works with Tasmota core ESP8266 V.2.7.4.9
• The code change is tested and works with Tasmota core ESP32 V.2.0.10
• I accept the CLA.

NOTE: The code change must pass CI tests. Your PR cannot be merged unless tests pass

[sfromis]

Good improvement :-)

One funny edge case:

var a,b
def f() b:=a end
BRY: Exception> 'syntax_error' - input:1: strict: expression without side effect detected

Not a very intuitive message, when the walrus purpose is to actually have a side effect. Of course, not very useful to use walrus when a simple assignment would be the thing to do, hence I do not really see this as a "bug".

[s-hadinger]

Nah this is a bug. I thought I marked a walrus assignment as a side effect.

[s-hadinger]

Fixed in #18999