feat(gnovm): align Gno constant handling with Go specifications

[omarsy]

Related Issues:
#2628

This PR aims to align Gno's constant handling with the Go specification regarding constant expressions (see Go Specification on Constants).

1. Primitive Type Requirement

   • Should Work:

     const t = 1

   • Should Return an Error:

     const t = []string{"1"}

     Error:

     (const (slice[("1" string)] []string)) (value of type []string) is not constant
     

2. Function Calls Disallowed
   Only built-in functions should be allowed.

   • Should Work:

     const t = len("s")

   • Should Return an Error:

     func v() string {
         return ""
     }
     const t = v()

     Error:

     v<VPBlock(3,0)>() (value of type string) is not constant
     

3. Constant Operands Requirement
   Constant expressions may contain only constant operands and are evaluated at compile time.

   • Should Work:

     const t = 1
     const v = t

   • Should Raise an Error:

     t := 1
     const v = t

     Error:

     t (variable of type int) is not constant
     

4. Type Assertion Forbidden

   • This code:

     var i interface{} = 1
     const t, ok = i.(int)

   • Should Raise This Error:

     i.(int) (comma, ok expression of type int) is not constant
     

5. Index Expression Forbidden

   • This code:

     var i = []string{}
     const t, ok = i[0]

   • Should Return This Error:

     i[0] (variable of type string) is not constant
     

Contributors' checklist...

• Added new tests, or not needed, or not feasible
• Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• Updated the official documentation or not needed
• No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• Added references to related issues and PRs
• Provided any useful hints for running manual tests
• Added new benchmarks to generated graphs, if any. More info here.

[codecov]

Codecov Report

Attention: Patch coverage is 78.07018% with 25 lines in your changes missing coverage. Please review.

Project coverage is 63.40%. Comparing base (1a57e81) to head (3a52e14).
Report is 11 commits behind head on master.

Files with missing lines	Patch %	Lines
gnovm/pkg/gnolang/type_check.go	77.77%	19 Missing and 3 partials ⚠️
gnovm/pkg/gnolang/preprocess.go	80.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2828      +/-   ##
==========================================
+ Coverage   63.37%   63.40%   +0.03%     
==========================================
  Files         561      565       +4     
  Lines       79199    80149     +950     
==========================================
+ Hits        50189    50821     +632     
- Misses      25625    25912     +287     
- Partials     3385     3416      +31     

Flag	Coverage Δ
contribs/gnodev	60.57% <ø> (ø)
contribs/gnofaucet	15.77% <ø> (ø)
gno.land	67.37% <ø> (-0.01%)	⬇️
gnovm	67.94% <78.07%> (+<0.01%)	⬆️
misc/genstd	79.72% <ø> (ø)
tm2	62.45% <ø> (+0.19%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

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[jefft0]

@omarsy , can you fix the lint-pr-title CI check?

[ltzmaxwell]

consider this one, it should be handled:

package main

var x = 1

const ff = +(1 << x)

func main() {
	println("ok")
}

and this one:

package main

const a = func() { println("hey") }

func main() {
	println("ok")
}

this triggers an error message like:
"func func(){ (const (println func(xs ...interface{})()))((const ("hey" string))) } is not a type",
which is not the intended outcome.

[ltzmaxwell]

a better msg? a test?

[omarsy]

Improved the message, but testing this branch is on hold until we resolve #3006.
related commit: b80a890

[jefft0]

Removed the review/triage-pending label because all CI tests passed, this has been declared as a priority, and core dev ltzmaxwell is reviewing it.

[ltzmaxwell]

consider this one as a special case:

package main

var x = 1
var y = 1

const b = x == y

func main() {
	println("ok")
}

[omarsy]

this one return an error in my PR, I think. Do you want I added as a test ?

[ltzmaxwell]

please see this: https://go.dev/play/p/dhxEqNA7p60

[omarsy]

I think that when you run it, you'll encounter an error like x (variable of type int) is not constant. While it's not the same error, I think the description of the error in my PR better clarifies the issue we have.

[ltzmaxwell]

IMO, it would be more natural that the RHS is not const, rather than its child node.

[omarsy]

I find the current error message a bit unclear. For example, consider the following code:

package main

const x = 1

var y = 1

const b = x == y

func main() {
	println("ok")
}

In Go, this will produce the error: x == y (untyped bool value) is not constant. However, in the PR, the error we get is: y (variable of type int) is not constant.

This difference suggests that we should change our code to make it work, like so:

package main

const x = 1

const y = 1

const b = x == y

func main() {
	println("ok")
}

The Go error message is more informative, indicating that the expression itself is not considered constant rather than simply pointing out that y is a variable.

[omarsy]

But I can the logic of the code if you think it is a blocker (to be more go like)

[ltzmaxwell]

more tests on this?

[omarsy]

right, done here: e74e6e9

[ltzmaxwell]

Partial, will have a final one after.

[omarsy]

consider this one, it should be handled:

package main

var x = 1

const ff = +(1 << x)

func main() {
	println("ok")
}

and this one:

package main

const a = func() { println("hey") }

func main() {
	println("ok")
}

this triggers an error message like: "func func(){ (const (println func(xs ...interface{})()))((const ("hey" string))) } is not a type", which is not the intended outcome.

nice catch @ltzmaxwell ^^
improved the error message here 90c8c5b

[petar-dambovaliev]

If real and imag are added later, it would need to be handled here as well.
Can you add a comment for this?