# Sema: rework \`switch_block\[\_ref/\_err_union\]\` logic · gitcafe/zig

[View on GitCafe](https://git.cafe/gitcafe/zig/commit/b79bd313566347873e022adcd61891339735d9a6)

Repository: [gitcafe/zig](https://git.cafe/gitcafe/zig)

Visibility: public

Requested revision: b79bd313566347873e022adcd61891339735d9a6

Requested commit: b79bd313566347873e022adcd61891339735d9a6

Commit: b79bd313566347873e022adcd61891339735d9a6

Tree: eeebc217c745a964d6f72788cf25df8ad2cbe9e9

Author: Justus Klausecker

Committer: Matthew Lugg

## Message

```
Sema: rework `switch_block[_ref/_err_union]` logic

This commit aims to simplify and de-duplicate the logic required for
semantically analyzing `switch` expressions.

The core logic has been moved to `analyzeSwitchBlock`, `resolveSwitchBlock`
and `finishSwitchBlock` and has been rewritten around the new iterator-based
API exposed by `Zir.UnwrappedSwitchBlock`.

All validation logic and switch prong item resolution have been moved to
`validateSwitchBlock`, which produces a `ValidatedSwitchBlock` containing
all the necessary information for further analysis.

`Zir.UnwrappedSwitchBlock`, `ValidatedSwitchBlock` and `SwitchOperand`
replace `SwitchProngAnalysis` while adding more flexibility, mainly for
better integration with `switch_block_err_union`.

`analyzeSwitchBlock` has an explicit code path for OPV types which lowers
them to either a `block`-`br` or a `loop`-`repeat` construct instead of a
switch. Backends expect `switch` to actually have an operand that exists
at runtime, so this is a bug fix and avoids further special cases in the
rest of the switch logic.
`resolveSwitchBlock` and `finishSwitchBr` exclusively deal with operands
which can have more than one value, at comptime and at runtime respectively.

This commit also reworks `RangeSet` to be an unmanaged container and adds
`Air.SwitchBr.BranchHints` to offload some complexity from Sema to there
and save a few bytes of memory in the process.

Additionally, some new features have been implemented:
- decl literals and everything else requiring a result type (`@enumFromInt`!)
  may now be used as switch prong items
- union tag captures are now allowed for all prongs, not just `inline` ones
- switch prongs may contain errors which are not in the error set being
  switched on, if these prongs contain `=> comptime unreachable`

and some bugs have been fixed:
- lots of issues with switching on OPV types are now fixed
- the rules around unreachable `else` prongs when switching on errors now
  apply to *any* switch on an error, not just to `switch_block_err_union`,
  and are applied properly based on the AST
- switching on `void` no longer requires an `else` prong unconditionally
- lazy values are properly resolved before any comparisons with prong items
- evaluation order between all kinds of switch statements is now the same,
  with or without label

```

## Parents

- [d94137d23fff3958e003c7d6d70a5d1087a1e98c](https://git.cafe/gitcafe/zig/commit/d94137d23fff3958e003c7d6d70a5d1087a1e98c?format=markdown)

[Source at this commit](https://git.cafe/gitcafe/zig/tree/b79bd313566347873e022adcd61891339735d9a6?format=markdown)