Add an increment of a global variable after each op to ensure the Go compiler isn't being clever about optimisation.

bm_test.go

package bm

import (
	"testing"
)

var mb = map[string]bool{
	"alpha":   true,
	"beta":    true,
	"gamma":   true,
	"delta":   true,
	"epsilon": true,
	"zeta":    true,
	"eta":     true,
	"theta":   true,
	"iota":    true,
	"kappa":   true,
	"lambda":  true,
	"mu":      true,
	"nu":      true,
	"xi":      true,
	"omicron": true,
	"pi":      true,
	"rho":     true,
	"sigma":   true,
	"tau":     true,
	"upsilon": true,
	"phi":     true,
	"chi":     true,
	"psi":     true,
	"omega":   true,
}
var ms = map[string]struct{}{
	"alpha":   struct{}{},
	"beta":    struct{}{},
	"gamma":   struct{}{},
	"delta":   struct{}{},
	"epsilon": struct{}{},
	"zeta":    struct{}{},
	"eta":     struct{}{},
	"theta":   struct{}{},
	"iota":    struct{}{},
	"kappa":   struct{}{},
	"lambda":  struct{}{},
	"mu":      struct{}{},
	"nu":      struct{}{},
	"xi":      struct{}{},
	"omicron": struct{}{},
	"pi":      struct{}{},
	"rho":     struct{}{},
	"sigma":   struct{}{},
	"tau":     struct{}{},
	"upsilon": struct{}{},
	"phi":     struct{}{},
	"chi":     struct{}{},
	"psi":     struct{}{},
	"omega":   struct{}{},
}

var bb int

func inc(truthy bool) {
	if truthy {
		bb += 1
	}
}

func BenchmarkNoOp(B *testing.B) {
	bb = 0
	for i := 0; i < B.N; i++ {
		inc(true)
		inc(true)
		inc(true)
		inc(true)
		inc(true)
	}
}

func BenchmarkMapBool(B *testing.B) {
	bb = 0
	var b bool
	for i := 0; i < B.N; i++ {
		b = mb["alpha"]
		inc(b)
		b = mb["gamma"]
		inc(b)
		b = mb["mu"]
		inc(b)
		b = mb["pi"]
		inc(b)
		b = mb["omega"]
		inc(b)
	}
}

func BenchmarkMapStruct(B *testing.B) {
	bb = 0
	var b bool
	for i := 0; i < B.N; i++ {
		_, b = ms["alpha"]
		inc(b)
		_, b = ms["gamma"]
		inc(b)
		_, b = ms["mu"]
		inc(b)
		_, b = ms["pi"]
		inc(b)
		_, b = ms["omega"]
		inc(b)
	}
}

results.txt

➜  gobench  go test -bench=. -benchtime=10s
testing: warning: no tests to run
PASS
BenchmarkNoOp	2000000000	         8.55 ns/op
BenchmarkMapBool	100000000	       115 ns/op
BenchmarkMapStruct	100000000	       112 ns/op
ok  	_/Users/mp/gobench	41.027s

➜  gobench  go test -bench=. -benchtime=10s
testing: warning: no tests to run
PASS
BenchmarkNoOp	2000000000	         8.77 ns/op
BenchmarkMapBool	100000000	       114 ns/op
BenchmarkMapStruct	100000000	       118 ns/op
ok  	_/Users/mp/gobench	41.909s