Instead of defining a named struct, we can use `std::iter::from_fn` and store
intermediate state in a closure.

This function already has access to the MIR body, so instead of taking a
reference to a terminator, it's simpler and easier to pass in a basic block
index.

There is no need to box the returned iterator if we instead add appropriate
lifetime captures, since `short_circuit_preorder` is now generic over the type
of iterator it expects.

We can also greatly simplify the function's implementation by observing that
the only difference between its two cases is whether we take all of a BB's
successors, or just the first one.

use env variable to control thread ids in rustc_log

Currently, when parallel rustc is enabled, even if the number of threads is 1, the thread ID will be included before all the logs.
E.g.
`WARN rustc_mir_build::thir::pattern::const_to_pat ...`
=>
`2:rustcWARN rustc_mir_build::thir::pattern::const_to_pat ...`
This makes the logs confusing and results in inconsistent UI test results for serial and parallel rustc. Therefore I think we should let users decide whether thread id information is needed through explicit control.

coverage: Unbox and simplify `bcb_filtered_successors`

This is a small cleanup in the coverage instrumentor's graph-building code.

---
This function already has access to the MIR body, so instead of taking a reference to a terminator, it's simpler and easier to pass in a basic block index.

There is no need to box the returned iterator if we instead add appropriate lifetime captures, and make `short_circuit_preorder` generic over the type of iterator it expects.

We can also greatly simplify the function's implementation by observing that the only difference between its two cases is whether we take all of a BB's successors, or just the first one.

---

`@rustbot` label  A-code-coverage

-Zmir-enable-passes: document that this may enable unsound passes

also add some comments explaining why MIR opts are marked as unsound

…errors

reorder files in solve

r? `@compiler-errors`