Cgo takes a different approach. It determines the meaning of C
identifiers not by parsing C code but by feeding carefully constructed
programs into the system C compiler and interpreting the generated
error messages, debug information, and object files. In practice,
parsing these is significantly less work and more robust than parsing
C source.

Cgo first invokes gcc -E -dM on the preamble, in order to find out

}

var (
	swigOnce sync.Once
	haveSwig bool
)

func mustHaveSwig(t *testing.T) {
	swigOnce.Do(func() {
		mustHaveSwigOnce(t)
		haveSwig = true
	})
	// The first call will skip t with a nice message. On later calls, we just skip.
	if !haveSwig {
		t.Skip("swig not found")
	}
}

func mustHaveSwigOnce(t *testing.T) {
	swig, err := exec.LookPath("swig")
	if err != nil {
		t.Skipf("swig not in PATH: %s", err)

func (p *Parser) line(scratch [][]lex.Token) (word, cond string, operands [][]lex.Token, ok bool) {
next:
	// Skip newlines.
	var tok lex.ScanToken
	for {
		tok = p.nextToken()
		// We save the line number here so error messages from this instruction
		// are labeled with this line. Otherwise we complain after we've absorbed
		// the terminating newline and the line numbers are off by one in errors.
		p.lineNum = p.lex.Line()
		switch tok {

		return needType
	}

	// Coerce gcc into telling us whether each name is a type, a value, or undeclared.
	// For names, find out whether they are integer constants.
	// We used to look at specific warning or error messages here, but that tied the
	// behavior too closely to specific versions of the compilers.
	// Instead, arrange that we can infer what we need from only the presence or absence
	// of an error on a specific line.
	//

	for _, f := range strings.Split(s, " or ") {
		if f == "" || len(f)%2 != 0 || strings.TrimLeft(f, "0123456789abcdef") != "" {
			return false
		}
	}
	return true
}

// It would be nice if the error messages always began with
// the standard file:line: prefix,
// but that's not where we are today.
// It might be at the beginning but it might be in the middle of the printed instruction.

	// VEX-encoded VSIB can't use High-16 registers as index (unlike EVEX).
	// TODO(quasilyte): improve error message (#21860).
	VPGATHERQQ X2, (BP)(X20*2), X3  // ERROR "invalid instruction"
	VPGATHERQQ Y2, (BP)(Y20*2), Y3  // ERROR "invalid instruction"
	// YzrMulti4 expects exactly 4 registers referenced by REG_LIST.
	// TODO(quasilyte): improve error message (#21860).
	V4FMADDPS (AX), [Z0-Z4], K1, Z7  // ERROR "invalid instruction"

<h3 id="Type_unification_rules">Type unification rules</h3>

<p>
The type unification rules describe if and how two types unify.
The precise details are relevant for Go implementations,
affect the specifics of error messages (such as whether
a compiler reports a type inference or other error),
and may explain why type inference fails in unusual code situations.
But by and large these rules can be ignored when writing Go code:

// Copyright 2015 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package flags implements top-level flags and the usage message for the assembler.
package flags

import (
	"cmd/internal/obj"
	"cmd/internal/objabi"
	"flag"
	"fmt"
	"os"
	"path/filepath"
	"strings"
)

var (

pkg log/slog, type LogValuer interface, LogValue() Value #56345
pkg log/slog, type Record struct #56345
pkg log/slog, type Record struct, Level Level #56345
pkg log/slog, type Record struct, Message string #56345
pkg log/slog, type Record struct, PC uintptr #56345
pkg log/slog, type Record struct, Time time.Time #56345
pkg log/slog, type Source struct #59280
pkg log/slog, type Source struct, File string #59280