Allow no-argument methods in capacity suggestions

Author: NathanBaulch

pkg/prealloc.go

@@ -837,6 +837,11 @@ func hasCall(expr ast.Expr) bool {
 					// allow cheap pure built-in functions
 					return true
 				}
+			case *ast.SelectorExpr:
+				// allow argument-less methods
+				if len(call.Args) == 0 {
+					return true
+				}
 			}
 			found = true
 		}

testdata/for.go

@@ -1,5 +1,7 @@
 package test
 
+import "bytes"
+
 func forInfinite() {
 	var x []int
 	for {
@@ -226,6 +228,14 @@ func forTypeConvert() {
 	}
 }
 
+func forNoArgMethod() {
+	var buf bytes.Buffer
+	var x []int // want "Consider preallocating x with capacity buf\\.Len\\(\\)$"
+	for i := 0; i < buf.Len(); i++ {
+		x = append(x, i)
+	}
+}
+
 func forMultipleConjunctiveUpperLimits() {
 	m := 7
 	n := 6

testdata/range.go

@@ -1,6 +1,9 @@
 package test
 
-import "sort"
+import (
+	"bytes"
+	"sort"
+)
 
 func rangeZero() {
 	var x []int
@@ -182,6 +185,14 @@ func rangeIntTypeConvert() {
 	}
 }
 
+func rangeIntNoArgMethod() {
+	var buf bytes.Buffer
+	var x []int // want "Consider preallocating x with capacity buf\\.Len\\(\\)$"
+	for i := range buf.Len() {
+		x = append(x, i)
+	}
+}
+
 func rangeMultiple() {
 	var x []int // want "Consider preallocating x with capacity 5 \\+ n \\+ len\\(s\\) \\+ \\(n \\+ 1 - m\\)$"
 	for i := range 5 {