cmd/compile: read safemode bit from package header

Ignore respective bit in export data, but leave the info to
minimize format changes for 1.7. Scheduled to remove by 1.8.

For #15772.

Change-Id: Ifb3beea655367308a4e2d5dc8cb625915f904287
Reviewed-on: https://go-review.googlesource.com/23285
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/bimport.go

@@ -103,7 +103,9 @@ func Import(in *bufio.Reader) {
 	// --- compiler-specific export data ---
 
 	// read compiler-specific flags
-	importpkg.Safe = p.bool()
+
+	// read but ignore safemode bit (see issue #15772)
+	p.bool() // formerly: importpkg.Safe = p.bool()
 
 	// phase 2
 	objcount = 0

src/cmd/compile/internal/gc/main.go

@@ -784,6 +784,21 @@ func importfile(f *Val, indent []byte) {
 		}
 	}
 
+	// process header lines
+	for {
+		p, err = imp.ReadString('\n')
+		if err != nil {
+			log.Fatalf("reading input: %v", err)
+		}
+		if p == "\n" {
+			break // header ends with blank line
+		}
+		if strings.HasPrefix(p, "safe") {
+			importpkg.Safe = true
+			break // ok to ignore rest
+		}
+	}
+
 	// assume files move (get installed)
 	// so don't record the full path.
 	linehistpragma(file[len(file)-len(path_)-2:]) // acts as #pragma lib

src/cmd/compile/internal/gc/parser.go

@@ -398,11 +398,8 @@ func (p *parser) import_package() {
 		p.import_error()
 	}
 
-	importsafe := false
+	// read but skip "safe" bit (see issue #15772)
 	if p.tok == LNAME {
-		if p.sym_.Name == "safe" {
-			importsafe = true
-		}
 		p.next()
 	}
 	p.want(';')
@@ -413,7 +410,6 @@ func (p *parser) import_package() {
 	} else if importpkg.Name != name {
 		Yyerror("conflicting names %s and %s for package %q", importpkg.Name, name, importpkg.Path)
 	}
-	importpkg.Safe = importsafe
 
 	typecheckok = true
 	defercheckwidth()