POC for an analyzer that finds http requests that unnecessarily set a content-type header

http-req-content-type.go

package request

import (
	"fmt"
	"go/ast"

	"golang.org/x/tools/go/analysis"
	"golang.org/x/tools/go/analysis/passes/inspect"
	"golang.org/x/tools/go/ast/inspector"
)

var Analyzer = &analysis.Analyzer{
	Name:     "http-req-content-type",
	Doc:      "Finds setting of unnecessary content-type request headers",
	Run:      run,
	Requires: []*analysis.Analyzer{inspect.Analyzer},
}

func run(pass *analysis.Pass) (interface{}, error) {

	inspect := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector)

	nodeFilter := []ast.Node{
		(*ast.CallExpr)(nil),
	}

	inspect.Preorder(nodeFilter, func(n ast.Node) {
		// cast the node as a call expression
		ce := n.(*ast.CallExpr)
		// check if the call expression is a selector expression
		se, ok := ce.Fun.(*ast.SelectorExpr)
		if ok {
			// if is, verify we're selecting a header
			if pass.TypesInfo.TypeOf(se.X).String() == "net/http.Header" {
				// and that the selector name is "Set"
				if se.Sel.Name == "Set" {
					// then check if the first argument is a basic-literal string
					firstArgBasicLit, ok := ce.Args[0].(*ast.BasicLit)
					if ok {
						// and that it's setting "Content-Length"
						if fmt.Sprintf("%v", firstArgBasicLit.Value) == "\"Content-Length\"" {
							// so that we can warn about it
							pass.Reportf(ce.Pos(), "no need to set content-length for requests")
						}
					}
				}
			}
		}
	})

	return nil, nil
}