compile: Change name of result var for wasm binary

Previously it was being bound to `$result` but this was then stripped
out of the resultset by the planner. To avoid this issue we need the
variable to not be seen as a wildcard or generated var. The new one
is just `result`.

The documentation is also now updated to show this behavior. The
various client SDK's can strip it out as needed.

The idea is that this is going to just be a part of the built WASM
binary format. Anyone building with the lower level API's using
ad-hoc queries will not need to worry about anything changing.

Fixes: #2441
Signed-off-by: Patrick East <east.patrick@gmail.com>

compile/compile.go

@@ -34,6 +34,8 @@ const (
 	TargetWasm = "wasm"
 )
 
+const wasmResultVar = ast.Var("result")
+
 var validTargets = map[string]struct{}{
 	TargetRego: struct{}{},
 	TargetWasm: struct{}{},
@@ -300,7 +302,7 @@ func (c *Compiler) compileWasm(ctx context.Context) error {
 	}
 
 	store := inmem.NewFromObject(c.bundle.Data)
-	resultSym := ast.VarTerm(ast.WildcardPrefix + "__result__")
+	resultSym := ast.NewTerm(wasmResultVar)
 
 	cr, err := rego.New(
 		rego.ParsedQuery(ast.NewBody(ast.Equality.Expr(resultSym, c.entrypointrefs[0]))),

docs/content/wasm.md

@@ -33,14 +33,40 @@ You can compile Rego policies into Wasm modules using the `opa build` subcommand
 
 For example, the `opa build` command below compiles the `example.rego` file into a
 Wasm module and packages it into an OPA bundle. The `wasm` target requires exactly
-one entrypoint (specified by `-e`).
+one entrypoint rule (specified by `-e`).
 
 ```bash
 opa build -t wasm -e example/allow example.rego
 ```
 
+The output of a Wasm module built this way contain the `result` of evaluating the
+entrypoint rule. For example:
+```json
+[
+  {
+    "result": <value of data.example.allow>
+  }
+]
+```
+
+The output of policy evaluation is a set of variable assignments. The variable
+assignments specify values that satisfy the expressions in the policy query
+(i.e., if the variables in the query are replaced with the values from the
+assignments, all of the expressions in the query would be defined and not
+false.)
+
+When policies are compiled into Wasm, the user provides the path of the policy
+decision that should be exposed by the Wasm module. The policy decision is
+assigned to a variable named `result`. The policy decision can be ANY JSON value
+(boolean, string, object, etc.) but there will be at-most-one assignment. This
+means that callers should first check if the set of variable assignments is
+empty (indicating an undefined policy decision) otherwise they should select the
+`"result"` key out of the variable assignment set.
+
 > For more information on `opa build` run `opa build --help`.
 
+## Advanced Compiling Options
+
 You can also compile Rego policies into Wasm modules from Go using the lower-level
 [rego](https://godoc.org/github.com/open-policy-agent/opa/rego#Rego.Compile) API
 that produces raw Wasm executables and the higher-level [compile]() API that