pkg/encoding: add tests that Unmarshal rejects trailing input

json correctly rejects trailing input whether it is valid or invalid,
like encoding/json.Unmarshal does, but yaml does not yet,
as already pointed out in the TODO I added to internal/encoding/yaml.
The following CL will fix the yaml package to do the right thing.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I01b381259ca0251bdb6e46dab60ca943226fe793
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1193241
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Roger Peppe <rogpeppe@gmail.com>

pkg/encoding/json/testdata/gen.txtar

@@ -42,6 +42,10 @@ unmarshal: [string]: X={
 	#result: json.Unmarshal(X)
 }
 unmarshal: t1: "1"
+unmarshal: {
+	trailingValid:   #"{"a": 1}{"b": 2}"#
+	trailingInvalid: #"{"a": 1}}invalid json"#
+}
 
 marshalStream: [string]: X={
 	[...] // input
@@ -88,6 +92,10 @@ validate.t2.result: error in call to encoding/json.Validate: invalid value 10 (o
     ./in.cue:6:10
     ./in.cue:9:27
     json.Validate:1:6
+unmarshal.trailingInvalid.#result: error in call to encoding/json.Unmarshal: json: invalid JSON:
+    ./in.cue:39:11
+unmarshal.trailingValid.#result: error in call to encoding/json.Unmarshal: json: invalid JSON:
+    ./in.cue:39:11
 
 Result:
 import "encoding/json"
@@ -147,6 +155,12 @@ unmarshal: {
 		"1"
 		#result: 1
 	}
+	trailingValid: {
+		#result: _|_ // unmarshal.trailingValid.#result: error in call to encoding/json.Unmarshal: json: invalid JSON
+	}
+	trailingInvalid: {
+		#result: _|_ // unmarshal.trailingInvalid.#result: error in call to encoding/json.Unmarshal: json: invalid JSON
+	}
 }
 marshalStream: {
 	t1: {

pkg/encoding/yaml/testdata/gen.txtar

@@ -27,6 +27,11 @@ unmarshalStream: {
 	null2: yaml.UnmarshalStream('1\n---\n---\n2')
 	null2: yaml.UnmarshalStream("1\n---\n---\n2")
 }
+
+unmarshalTrailingInput: {
+	valid:   yaml.Unmarshal("1\n---\n2")
+	invalid: yaml.Unmarshal("1\n---\n}invalid yaml")
+}
 -- out/yaml --
 Errors:
 t1: error in call to encoding/yaml.Validate: invalid value 4 (out of bound <3):
@@ -75,3 +80,7 @@ unmarshalStream: {
 	null1: [1, null, 2]
 	null2: [1, null, 2]
 }
+unmarshalTrailingInput: {
+	valid:   1
+	invalid: 1
+}