Erik Eckstein 5bc036661c SIL optimizer: add the LetPropertyLowering pass ...

It lowers let property accesses of classes.
Lowering consists of two tasks:

* In class initializers, insert `end_init_let_ref` instructions at places where all let-fields are initialized.
  This strictly separates the life-range of the class into a region where let fields are still written during
  initialization and a region where let fields are truly immutable.

* Add the `[immutable]` flag to all `ref_element_addr` instructions (for let-fields) which are in the "immutable"
  region. This includes the region after an inserted `end_init_let_ref` in an class initializer, but also all
  let-field accesses in other functions than the initializer and the destructor.

This pass should run after DefiniteInitialization but before RawSILInstLowering (because it relies on `mark_uninitialized` still present in the class initializer).
Note that it's not mandatory to run this pass. If it doesn't run, SIL is still correct.

Simplified example (after lowering):

  bb0(%0 : @owned C):                           // = self of the class initializer
    %1 = mark_uninitialized %0
    %2 = ref_element_addr %1, #C.l              // a let-field
    store %init_value to %2
    %3 = end_init_let_ref %1                    // inserted by lowering
    %4 = ref_element_addr [immutable] %3, #C.l  // set to immutable by lowering
    %5 = load %4

2023-09-19 15:10:30 +02:00

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