syntax: Add labeled block break (#60481)
This is a redo of #60367 taking into account various feedback on that
PR. In particular, it seemed like people strongly disliked the `break
break` syntax for two primary reasons:
1. It scales poorly to multiple loops `break break break break`
2. It introduced footguns if extra loops are introduced between the loop
and the break.
Instead, the consensus seemed to be that labeled break should be the
only facility available. Thus, this implements a proper labeled break
facility that looks as follows:
```
@label :name for i = 1:10
for j = 1:10
break :name (i, j)
end
end # evaluate to `(1,1)
```
The idea is to re-use the `@label` macro for now, but possibly promote
this to syntax in a future version if it gains widespread use. Note that
parser changes are still required, since the `break` syntax is currently
an error. However, compat.jl could provide `@goto break name val`, which
parses fine.
`continue` is extended with label support as well:
```
@label :outer while true
for i = 1:10
a[i] && continue :outer
end
[...]
end
```
However, the feature is not restricted to loops. A particular use case
is to replace cleanup blocks written like
```
cond1 && @goto error
cond2 && @goto error
return true
@label error
error("foo")
```
by a labeled begin/end block:
```
@label :error begin
cond1 && break :error
cond2 && break :error
return true
end
error("foo")
```
This doesn't save much typing work, but it makes this kind of pattern
much more structured, e.g. for code-folding in IDEs.
A similar pattern replaces the `for-then` construction originally
proposed:
```
result = @label :result begin
for x in arr
pred(x) && break :result x
end
default
end
```
For convenience, the label may be ommitted, in which case it defaults to
`_`, i.e. the above can be written as:
```
result = @label begin
for x in arr
pred(x) && break _ x
end
default
end
```
I've taken the liberty of converting some base code for testing and to
give an idea of what the syntax looks like in practice, but didn't go
through particularly comprehensively. These changes should be considered
extended usage examples.
Largely written by Claude, and I haven't looked at the implementation
particularly carefully yet - for now I'm just interested in discussion
of the syntax.
---------
Co-authored-by: Keno Fischer <Keno@users.noreply.github.com>
