Add nilness linter from govet (#7335)

Seeing @thevilledev adding this in conftest, and thinking we
had it here already. We didn't! Fixing the two issues it reported
where we checked for conditions that couldn't possibly be met.

Signed-off-by: Anders Eknert <anders@styra.com>

.golangci.yaml

@@ -143,6 +143,7 @@ linters-settings:
   govet:
     enable:
       - deepequalerrors
+      - nilness
   perfsprint:
     # only rule disabled by default, but it's a good one
     err-error: true

cmd/parse.go

@@ -133,10 +133,6 @@ func parse(args []string, params *parseParams, stdout io.Writer, stderr io.Write
 
 		_, _ = fmt.Fprint(stdout, string(bs)+"\n")
 	default:
-		if err != nil {
-			_, _ = fmt.Fprintln(stderr, err)
-			return 1
-		}
 		ast.Pretty(stdout, result.Parsed)
 	}
 

v1/storage/inmem/inmem.go

@@ -185,7 +185,9 @@ func (db *store) Truncate(ctx context.Context, txn storage.Transaction, params s
 		}
 	}
 
-	if err != nil && err != io.EOF {
+	// err is known not to be nil at this point, as it getting assigned
+	// a non-nil value is the only way the loop above can exit.
+	if err != io.EOF {
 		return err
 	}