fix(gnovm): Prevent use of blank identifier as Value or Type

[notJoon]

Description

Closes #1946

The isNamedConversion function now includes a safety check to prevent the use of blank identifiers ("_") as values or types. If both xt and t are nil, the function assumes that a blank identifier is being used inappropriately and panics with an error message that includes the location of the issue.

Variable Explanations

• xt (Expression Type): Represents the type of the right-hand side of an assignment or expression. It's the type resulting from evaluating an expression.
• t (Target Type): Represents the type of the left-hand side of an assignment. It's the variable or field that will receive the value.

Checks if a named conversion is needed when assigning a value of type xt to a variable of type t.

Preprocess

Added some checks to prevent the disallowd usage of blank identifiers in Preprocess function level. Theses checks are performed at different stages of the preprocessing:

1. TRANS_ENTER for AssignStmt:
   • Checks if both LHS and RHS are blank identifiers in a DEFINE statement.
2. TRANS_LEAVE for NameExpr:
   • Checks if blank identifier is used as a value in disallowed contexts (excluding TRANS_ASSIGN_LHS, TRANS_RANGE_KEY and TRANS_RANGE_VALUE).
3. TRANS_LEAVE for AssignStmt:
   • Checks if RHS is a blank identifier when LHS is not, in a DEFINE statement.

When any of these conditions are met, the function throws an panics like go message.

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 94.11765% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
gnovm/pkg/gnolang/preprocess.go	92.85%	0 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!

[deelawn]

Thanks Joon. Looks mostly good as it fixes the example in the issue. Can you also make a change that panics if the RHS identifier is _ during a DEFINE assignment? Maybe assert it is a named expression and compare its name to blankIdentifier if it is.

package main

func main() {
	a := _
}

And while you are making changes regarding the blank identifier, maybe we want to handle this case as well. It is technically a valid expression, though I can't think of a use case, so I'm not sure if we should fix it for any reason other than to more strictly adhere to the go spec.

package main

type zilch interface{}

func main() {
	_ = zilch(nil)
}

It would be good to get @ltzmaxwell 's review on this as well.

[notJoon]

@deelawn I've just finished up due to working on other tasks. Now I've modified the code based on your review.

This cases seemed to require handling at the Preprocess function level, so i update that function. Additionally, for the last, case, I also couldn't think of a use case ither, but I thought it would be better to follow the go spec. So I fixed that as well.

[ltzmaxwell]

consider this one:

package main

func main() {
	var a = _
}

[ltzmaxwell]

there should be a better PR title too.

[notJoon]

@ltzmaxwell I apologize for the delay. I've made some changes to reflect your comments. Still, I can't think of a good idea for the PR title for now. Thanks!

[ltzmaxwell]

hey @notJoon, do you have time to take a look at the comments, so we can push this forward? thank you.

[mvertes]

Ok to me