message: use strip_prefix where it makes sense
diff: #![feature(rustc_private)]
  +#![feature(str_strip)]
  extern crate rustc_middle;
  extern crate rustc_driver;
  @@ -208,7 +209,7 @@ fn main() {
  if seed.is_some() {
  panic!("Cannot specify -Zmiri-seed multiple times!");
  }
  - let seed_raw = hex::decode(arg.trim_start_matches("-Zmiri-seed="))
  + let seed_raw = hex::decode(arg.strip_prefix("-Zmiri-seed=").unwrap())
  .unwrap_or_else(|err| match err {
  FromHexError::InvalidHexCharacter { .. } => panic!(
  "-Zmiri-seed should only contain valid hex digits [0-9a-fA-F]"
  @@ -230,10 +231,10 @@ fn main() {
  }
  arg if arg.starts_with("-Zmiri-env-exclude=") => {
  excluded_env_vars
  - .push(arg.trim_start_matches("-Zmiri-env-exclude=").to_owned());
  + .push(arg.strip_prefix("-Zmiri-env-exclude=").unwrap().to_owned());
  }
  arg if arg.starts_with("-Zmiri-track-pointer-tag=") => {
  - let id: u64 = match arg.trim_start_matches("-Zmiri-track-pointer-tag=").parse()
  + let id: u64 = match arg.strip_prefix("-Zmiri-track-pointer-tag=").unwrap().parse()
  {
  Ok(id) => id,
  Err(err) => panic!(
  @@ -248,7 +249,7 @@ fn main() {
  }
  }
  arg if arg.starts_with("-Zmiri-track-alloc-id=") => {
  - let id: u64 = match arg.trim_start_matches("-Zmiri-track-alloc-id=").parse()
  + let id: u64 = match arg.strip_prefix("-Zmiri-track-alloc-id=").unwrap().parse()
  {
  Ok(id) => id,
  Err(err) => panic!(
  