disallow switch case capture discards #25467
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Previously Zig allowed you to write something like,
```zig
switch (x) {
.y => |_| {
```
This seems a bit strange because in other cases, such as when
capturing the tag in a switch case,
```zig
switch (x) {
.y => |_, _| {
```
this produces an error.
The only usecase I can think of for the previous behaviour is
if you wanted to assert that all union payloads are able
to coerce,
```zig
const X = union(enum) { y: u8, z: f32 };
switch (x) {
.y, .z => |_| {
```
This will compile-error with the `|_|` and pass without it.
I don't believe this usecase is strong enough to keep the current
behaviour; it was never used in the Zig codebase and I cannot
find a single usage of this behaviour in the real world, searching
through Sourcegraph.
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fixes: #24789 (by disallowing it)
also fixes another bug that I noticed, which remains allowed but Actually Works now.
Previously Zig allowed you to write something like,
This seems a bit strange because in other cases, such as when capturing the tag in a switch case,
this produces an error.
The only usecase I can think of for the previous behaviour is if you wanted to assert that all union payloads are able to coerce,
This will compile-error with the
|_|and pass without it.I don't believe this usecase is strong enough to keep the current behaviour; it was never used in the Zig codebase and I cannot find a single usage of this behaviour in the real world, searching through Sourcegraph.
To be clear, this PR does not remove the ability to do,