Fix #1081 #1204
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Fix #1081 #1204
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Varriount
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Clyybber
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## Summary
Make turning copy and move assignments into `=copy` and `=sink` calls a
standalone MIR pass; semantics don't change, but higher quality MIR
code is produced. This is progress towards fully decoupling lifetime
hooks from the move analyzer / destructor injection.
## Details
### General architecture
* the `injectdestructors` pass/module only collapses `sink` assignments
and updates the MIR tree accordingly
* turning copy assignments into `=copy` and sink assignments into
`=sink` calls is done by the new `injecthooks` pass/module
* reporting errors and warnings regarding hooks is also made part of
the `injecthooks` pass
* the `injecthooks` is run after the `injectdestructors` pass
* handling of `--expandArc` is moved to `backends.process`, so that it
can happen after hook injection
### `injectdestructors` Pass
* assignments are not replaced with `=copy` or `=sink` calls
* `expandAsgn` is renamed to `specializeAsgn`
* `sink` assignments are turned into either `copy`, `move`, or
destructive `move` assignments (same as before)
* assignments to locations that don't yet store a value are turned into
*initializing* assignments (this is necessary for the later `=sink`
injection to work)
* compared to before, *all* assignments are processed by
`specializeAsgn`, not only those for locations with lifetime hooks
* `=destroy` hooks are still injected
### `injecthooks` Pass
* the pass looks for `move` and `copy` assignment modifiers
* if the involved type has lifetime hooks, the assignment is
replaced with a call to the appropriate hook
* the same injection rules as used previously apply
* the error detection and reporting logic is moved over - without
change - from `injectdestructors`
* an adapter procedure to the legacy pass managements is provided
(needed by `backends`)
With less surrounding complexity, more effort is spent on using less
temporaries for the hook call injection:
* no temporary is used for the destination operand
* no temporary is used for the source operand when it's guaranteed to
not alias the destination. This is the case for:
* `move` assignments
* define with `copy` (e.g., `def x = copy y`)
* initial assignments with `copy` (e.g., `x := copy y`)
Less temporaries means less work for the pass eliminating unnecessary
temporaries.
### Technical Correctness
* `mirconstr.inline` is only usable with lvalue expressions and always
creates a non-owning temporary (instead of an owning temporary)
* the remaining usage of `mirconstr.inline` in
`injectdestructors.lowerBranchSwitch`, where an owning temporary is
required, is replaced with using `wrapTemp`
---------
Co-authored-by: Saem Ghani <saemghani+github@gmail.com>
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