This is a sub-issue of the larger tracking issue devoted to the ? in main RFC. It is specifically proposing that we stabilize the behavior that permits main to have a return type other than (). This return type must implement the Termination trait. The details of the trait itself (including its name and so forth) are not stabilized.

This commits us to the following (each link is to a test):

• fn main() -> i32
  • return value is the exit code of the process
• fn main() -> ! (no test, but here is the impl -- a test is being added in Termination trait in tests #48143)
  • should never return
• fn main() -> Result<(), E> where E: Debug
  • on Err, a Debug printout occurs
  • these impls are modified in Restrict the Termination impls to simplify stabilization #48497
• fn main() -> Result<!, E> where E: Debug
  • these impls are modified in Restrict the Termination impls to simplify stabilization #48497

The changes from the RFC are as follows:

• we modified the Result impl to use Debug

The following unresolved questions will be resolved:

• The precise initial implementations
  • as above
• exit codes (discussion)
  • these are defined as libc::EXIT_SUCCESS for "Ok" and libc::EXIT_FAILURE for "Err"
  • except for wasm, where they are currently defined as 0 or 1 for wasm -- this is because those constants are/were not available for wasm
  • (they also appear not to be publicly exported? but that is not part of this stabilization)

But the following is not stabilized and free to change:

• the name/location of the Termination trait and its methods
  • likely changing to std::process::Termination
• the usage of -> Result<...> in unit tests (not yet landed)
• the ErrCode newtype wrapper for returning a custom error code

--

UPDATE 1: Restricted the set of termination impls as proposed by @scottmcm in #48497.