Handling Fastly TLS termination in nginx with map: http://nginx.org/en/docs/http/ngx_http_map_module.html

nginx.conf

# Declares a new variable, $forwarded_scheme, that contains the URL scheme
# used by a user even if Fastly terminates your TLS connections.
#
# Will use the scheme extracted from the request URI by default or return
# "https" if the Fastly-SSL HTTP header is found.
map $http_fastly_ssl $forwarded_scheme {
  default $scheme;
  "1"     "https";
}

# And then elsewhere...

server {
  server_name example.com;
  
  return 301 $forwarded_scheme://www.example.com$request_uri; 
}