core: do not memoize constraint strings

This PR causes Nomad to no longer memoize the String value of
a Constraint. The private memoized variable may or may not be
initialized at any given time, which means a reflect.DeepEqual
comparison between two jobs (e.g. during Plan) may return incorrect
results.

Fixes #10836

nomad/structs/structs.go

@@ -8245,17 +8245,17 @@ type Constraint struct {
 	LTarget string // Left-hand target
 	RTarget string // Right-hand target
 	Operand string // Constraint operand (<=, <, =, !=, >, >=), contains, near
-	str     string // Memoized string
 }
 
-// Equals checks if two constraints are equal
+// Equals checks if two constraints are equal.
 func (c *Constraint) Equals(o *Constraint) bool {
 	return c == o ||
 		c.LTarget == o.LTarget &&
 			c.RTarget == o.RTarget &&
 			c.Operand == o.Operand
 }
 
+// Equal is like Equals but with one less s.
 func (c *Constraint) Equal(o *Constraint) bool {
 	return c.Equals(o)
 }
@@ -8264,17 +8264,15 @@ func (c *Constraint) Copy() *Constraint {
 	if c == nil {
 		return nil
 	}
-	nc := new(Constraint)
-	*nc = *c
-	return nc
+	return &Constraint{
+		LTarget: c.LTarget,
+		RTarget: c.RTarget,
+		Operand: c.Operand,
+	}
 }
 
 func (c *Constraint) String() string {
-	if c.str != "" {
-		return c.str
-	}
-	c.str = fmt.Sprintf("%s %s %s", c.LTarget, c.Operand, c.RTarget)
-	return c.str
+	return fmt.Sprintf("%s %s %s", c.LTarget, c.Operand, c.RTarget)
 }
 
 func (c *Constraint) Validate() error {