Ruleguard by example: Multi-statement patterns

package gorules

import "github.com/quasilyte/go-ruleguard/dsl"

// Sometimes you want to match a statement that is **followed** or **preceded** by another statement.
// For this case we'll need a multi-statement patterns that gogrep does recognize.

func useParallelAssignment(m dsl.Matcher) {
	// If we want to specify $x statement that is followed by $y, we use `$x; $y` notation.
	// Here we match three statements:
	// 1. $x assigned to a temporary $tmp
	// 2. $x is re-assigned to $y
	// 3. $y is re-assigned to $tmp
	m.Match(`$tmp := $x; $x = $y; $y = $tmp`).
		Report(`use parallel assignent: $x, $y = $y, $x`)
}

$ ruleguard -c 0 -rules rules.go main.go
main.go:9:2: use parallel assignent: obj1.value, obj2.value = obj2.value, obj1.value
9		tmp := obj1.value

main.go

package main

func main() {
	var obj1, obj2 struct {
		value int
	}

	// Bad swapping choice:
	tmp := obj1.value
	obj1.value = obj2.value
	obj2.value = tmp

	// This is the right way to do it:
	obj1.value, obj2.value = obj2.value, obj1.value
}

---

Notes:

• $*_ can be used to describe "any number of statements before this node": $*_; $x
• $*_ can be used to describe "any number of statements after this node": $x; $*_
• Multi-statement patterns are especially useful when matching blocks: {$*_; $x; $*_}

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