buildconfig: error out on unknown toml keys

This should remove some confusion when people use old bib with
recently added customizations.

bib/internal/buildconfig/config.go

@@ -59,9 +59,15 @@ func decodeTomlBuildConfig(r io.Reader, what string) (*BuildConfig, error) {
 	dec := toml.NewDecoder(r)
 
 	var conf BuildConfig
-	if _, err := dec.Decode(&conf); err != nil {
+	metadata, err := dec.Decode(&conf)
+	if err != nil {
 		return nil, fmt.Errorf("cannot decode %q: %w", what, err)
 	}
+
+	if len(metadata.Undecoded()) > 0 {
+		return nil, fmt.Errorf("cannot decode %q: unknown keys found: %v", what, metadata.Undecoded())
+	}
+
 	return &conf, nil
 }
 

bib/internal/buildconfig/config_test.go

@@ -132,3 +132,28 @@ func TestReadLegacyJSONConfig(t *testing.T) {
 	assert.NoError(t, err)
 	assert.Equal(t, expectedBuildConfig, cfg)
 }
+
+func TestTomlUnknownKeysError(t *testing.T) {
+	fakeUserCnfPath := makeFakeConfig(t, "config.toml", `
+[[birds]]
+name = "toucan"
+`)
+	_, err := buildconfig.ReadWithFallback(fakeUserCnfPath)
+
+	assert.ErrorContains(t, err, "unknown keys found: [birds birds.name]")
+}
+
+func TestJsonUnknownKeysError(t *testing.T) {
+	fakeUserCnfPath := makeFakeConfig(t, "config.json", `
+{
+  "birds": [
+	{
+	  "name": "toucan"
+	}
+  ]
+}
+`)
+	_, err := buildconfig.ReadWithFallback(fakeUserCnfPath)
+
+	assert.ErrorContains(t, err, `json: unknown field "birds"`)
+}