cmd/go: avoid type names in __debug__modinfo__ variable injected in package main

If we use the name 'string' to refer to the built-in type, that name
can be shadowed by a local declaration. Use a string constant instead,
but keep the init function to populate it so that //go:linkname will
still work properly.

Fixes #27584.

Change-Id: I850cad6663e566f70fd123107d2e4e742c93b450
Reviewed-on: https://go-review.googlesource.com/134915Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/internal/modload/build.go

@@ -232,11 +232,16 @@ func findModule(target, path string) module.Version {
 }
 
 func ModInfoProg(info string) []byte {
+	// Inject a variable with the debug information as runtime/debug.modinfo,
+	// but compile it in package main so that it is specific to the binary.
+	// Populate it in an init func so that it will work with go:linkname,
+	// but use a string constant instead of the name 'string' in case
+	// package main shadows the built-in 'string' with some local declaration.
 	return []byte(fmt.Sprintf(`
 		package main
 		import _ "unsafe"
 		//go:linkname __debug_modinfo__ runtime/debug.modinfo
-		var __debug_modinfo__ string
+		var __debug_modinfo__ = ""
 		func init() {
 			__debug_modinfo__ = %q
 		}

src/cmd/go/testdata/script/mod_string_alias.txt

+[short] skip
+
+env GO111MODULE=on
+
+go mod init golang.org/issue/27584
+
+go build .
+
+-- main.go --
+package main
+
+type string = []int
+
+func main() {}