message: subscriber: don't enable `tracing-core` features by default
  This attempts to address by disabling the `tracing-core`
  crate's default features in `tracing-subscriber`'s dependency.
  Now, `tracing-core`'s optional "alloc" feature is only enabled
  when "tracing-subscriber/alloc" is enabled.
  Closes
diff: @@ -25,7 +25,7 @@ rust-version = "1.49.0"
  [features]
  default = ["smallvec", "fmt", "ansi", "tracing-log", "std"]
  -alloc = []
  +alloc = ["tracing-core/alloc"]
  std = ["alloc", "tracing-core/std"]
  env-filter = ["matchers", "regex", "lazy_static", "tracing", "std", "thread_local"]
  fmt = ["registry", "std"]
  @@ -37,7 +37,7 @@ json = ["tracing-serde", "serde", "serde_json"]
  local-time = ["time/local-offset"]
  [dependencies]
  -tracing-core = { path = "../tracing-core", version = "0.2" }
  +tracing-core = { path = "../tracing-core", version = "0.2", default-features = false }
  # only required by the `env-filter` feature
  tracing = { optional = true, path = "../tracing", version = "0.2", default-features = false }
  