cgo: add support for #cgo noescape lines

Here is the proposal:
golang/go#56378

They are documented here:
https://pkg.go.dev/cmd/cgo@master#hdr-Optimizing_calls_of_C_code

This would have been very useful to fix
tinygo-org/bluetooth#176 in a nice way. That
bug is now fixed in a different way using a wrapper function, but once
this new noescape pragma gets included in TinyGo we could remove the
workaround and use `#cgo noescape` instead.

cgo/cgo.go

@@ -18,6 +18,7 @@ import (
 	"go/scanner"
 	"go/token"
 	"path/filepath"
+	"sort"
 	"strconv"
 	"strings"
 
@@ -42,6 +43,7 @@ type cgoPackage struct {
 	fset            *token.FileSet
 	tokenFiles      map[string]*token.File
 	definedGlobally map[string]ast.Node
+	noescapingFuncs map[string]*noescapingFunc // #cgo noescape lines
 	anonDecls       map[interface{}]string
 	cflags          []string // CFlags from #cgo lines
 	ldflags         []string // LDFlags from #cgo lines
@@ -80,6 +82,13 @@ type bitfieldInfo struct {
 	endBit   int64 // may be 0 meaning "until the end of the field"
 }
 
+// Information about a #cgo noescape line in the source code.
+type noescapingFunc struct {
+	name string
+	pos  token.Pos
+	used bool // true if used somewhere in the source (for proper error reporting)
+}
+
 // cgoAliases list type aliases between Go and C, for types that are equivalent
 // in both languages. See addTypeAliases.
 var cgoAliases = map[string]string{
@@ -186,6 +195,7 @@ func Process(files []*ast.File, dir, importPath string, fset *token.FileSet, cfl
 		fset:            fset,
 		tokenFiles:      map[string]*token.File{},
 		definedGlobally: map[string]ast.Node{},
+		noescapingFuncs: map[string]*noescapingFunc{},
 		anonDecls:       map[interface{}]string{},
 		visitedFiles:    map[string][]byte{},
 	}
@@ -344,6 +354,22 @@ func Process(files []*ast.File, dir, importPath string, fset *token.FileSet, cfl
 		})
 	}
 
+	// Show an error when a #cgo noescape line isn't used in practice.
+	// This matches upstream Go. I think the goal is to avoid issues with
+	// misspelled function names, which seems very useful.
+	var unusedNoescapeLines []*noescapingFunc
+	for _, value := range p.noescapingFuncs {
+		if !value.used {
+			unusedNoescapeLines = append(unusedNoescapeLines, value)
+		}
+	}
+	sort.SliceStable(unusedNoescapeLines, func(i, j int) bool {
+		return unusedNoescapeLines[i].pos < unusedNoescapeLines[j].pos
+	})
+	for _, value := range unusedNoescapeLines {
+		p.addError(value.pos, fmt.Sprintf("function %#v in #cgo noescape line is not used", value.name))
+	}
+
 	// Print the newly generated in-memory AST, for debugging.
 	//ast.Print(fset, p.generated)
 
@@ -409,6 +435,33 @@ func (p *cgoPackage) parseCGoPreprocessorLines(text string, pos token.Pos) strin
 		}
 		text = text[:lineStart] + string(spaces) + text[lineEnd:]
 
+		allFields := strings.Fields(line[4:])
+		switch allFields[0] {
+		case "noescape":
+			// The code indicates that pointer parameters will not be captured
+			// by the called C function.
+			if len(allFields) < 2 {
+				p.addErrorAfter(pos, text[:lineStart], "missing function name in #cgo noescape line")
+				continue
+			}
+			if len(allFields) > 2 {
+				p.addErrorAfter(pos, text[:lineStart], "multiple function names in #cgo noescape line")
+				continue
+			}
+			name := allFields[1]
+			p.noescapingFuncs[name] = &noescapingFunc{
+				name: name,
+				pos:  pos,
+				used: false,
+			}
+			continue
+		case "nocallback":
+			// We don't do anything special when calling a C function, so there
+			// appears to be no optimization that we can do here.
+			// Accept, but ignore the parameter for compatibility.
+			continue
+		}
+
 		// Get the text before the colon in the #cgo directive.
 		colon := strings.IndexByte(line, ':')
 		if colon < 0 {

cgo/libclang.go

@@ -256,10 +256,18 @@ func (f *cgoFile) createASTNode(name string, c clangCursor) (ast.Node, any) {
 				},
 			},
 		}
+		var doc []string
 		if C.clang_isFunctionTypeVariadic(cursorType) != 0 {
+			doc = append(doc, "//go:variadic")
+		}
+		if _, ok := f.noescapingFuncs[name]; ok {
+			doc = append(doc, "//go:noescape")
+			f.noescapingFuncs[name].used = true
+		}
+		if len(doc) != 0 {
 			decl.Doc.List = append(decl.Doc.List, &ast.Comment{
 				Slash: pos - 1,
-				Text:  "//go:variadic",
+				Text:  strings.Join(doc, "\n"),
 			})
 		}
 		for i := 0; i < numArgs; i++ {

cgo/testdata/errors.go

@@ -10,6 +10,11 @@ typedef struct {
 
 typedef someType noType; // undefined type
 
+// Some invalid noescape lines
+#cgo noescape
+#cgo noescape foo bar
+#cgo noescape unusedFunction
+
 #define SOME_CONST_1 5) // invalid const syntax
 #define SOME_CONST_2 6) // const not used (so no error)
 #define SOME_CONST_3 1234 // const too large for byte

cgo/testdata/errors.out.go

@@ -1,14 +1,17 @@
 // CGo errors:
+//     testdata/errors.go:14:1: missing function name in #cgo noescape line
+//     testdata/errors.go:15:1: multiple function names in #cgo noescape line
 //     testdata/errors.go:4:2: warning: some warning
 //     testdata/errors.go:11:9: error: unknown type name 'someType'
-//     testdata/errors.go:26:5: warning: another warning
-//     testdata/errors.go:13:23: unexpected token ), expected end of expression
-//     testdata/errors.go:21:26: unexpected token ), expected end of expression
-//     testdata/errors.go:16:33: unexpected token ), expected end of expression
-//     testdata/errors.go:17:34: unexpected token ), expected end of expression
+//     testdata/errors.go:31:5: warning: another warning
+//     testdata/errors.go:18:23: unexpected token ), expected end of expression
+//     testdata/errors.go:26:26: unexpected token ), expected end of expression
+//     testdata/errors.go:21:33: unexpected token ), expected end of expression
+//     testdata/errors.go:22:34: unexpected token ), expected end of expression
 //     -: unexpected token INT, expected end of expression
-//     testdata/errors.go:30:35: unexpected number of parameters: expected 2, got 3
-//     testdata/errors.go:31:31: unexpected number of parameters: expected 2, got 1
+//     testdata/errors.go:35:35: unexpected number of parameters: expected 2, got 3
+//     testdata/errors.go:36:31: unexpected number of parameters: expected 2, got 1
+//     testdata/errors.go:3:1: function "unusedFunction" in #cgo noescape line is not used
 
 // Type checking errors after CGo processing:
 //     testdata/errors.go:102: cannot use 2 << 10 (untyped int constant 2048) as C.char value in variable declaration (overflows)

cgo/testdata/symbols.go

@@ -9,6 +9,10 @@ static void staticfunc(int x);
 
 // Global variable signatures.
 extern int someValue;
+
+void notEscapingFunction(int *a);
+
+#cgo noescape notEscapingFunction
 */
 import "C"
 
@@ -18,6 +22,7 @@ func accessFunctions() {
 	C.variadic0()
 	C.variadic2(3, 5)
 	C.staticfunc(3)
+	C.notEscapingFunction(nil)
 }
 
 func accessGlobals() {

cgo/testdata/symbols.out.go

@@ -67,5 +67,10 @@ func C.staticfunc!symbols.go(x C.int)
 
 var C.staticfunc!symbols.go$funcaddr unsafe.Pointer
 
+//export notEscapingFunction
+//go:noescape
+func C.notEscapingFunction(a *C.int)
+
+var C.notEscapingFunction$funcaddr unsafe.Pointer
 //go:extern someValue
 var C.someValue C.int