Build and check Swift snippets

[nickpdemarco]

This PR adds support for checking Swift examples, copying the Rust-mdbook experience as much as possible: lines beginning with # will be omitted from the book output, but are used to compile the examples. Additionally, as a preprocessor step, all { ... } blocks are replaced with { fatalError() }.

# let a = [1, 2, 3]
# let x = 1
var i = 0
while (i != a.count && a[i] != x) {
  i += 1
}

A block marked with swift,ignore will not be built.

The examples can be checked with mdbook build. Several examples were slightly changed to pass the check. Without the two # lines above, the following error is emitted.

$ mdbook build better-code 
2026-02-03 16:23:39 [INFO] (mdbook::book): Book building has started
2026-02-03 16:23:39 [INFO] (mdbook::book): Running the html backend
2026-02-03 16:23:40 [INFO] (mdbook::book): Running the swift-test backend
2026-02-03 16:23:40 [INFO] (mdbook::renderer): Invoking the "swift-test" renderer
============================================================
Swift Code Example Testing
============================================================

Results: 19 passed, 1 failed, 20 total

------------------------------------------------------------
FAILURES:
------------------------------------------------------------

FAIL: chapter-2-contracts.md:238
/var/folders/zm/ll4504dd6dgbvy20y9ls2_hr0000gq/T/tmpwu0wf27f.swift:2:13: error: cannot find 'a' in scope
1 | var i = 0
2 | while (i != a.count && a[i] != x) {
  |             `- error: cannot find 'a' in scope
3 |   i += 1
4 | }

/var/folders/zm/ll4504dd6dgbvy20y9ls2_hr0000gq/T/tmpwu0wf27f.swift:2:24: error: cannot find 'a' in scope
1 | var i = 0
2 | while (i != a.count && a[i] != x) {
  |                        `- error: cannot find 'a' in scope
3 |   i += 1
4 | }

/var/folders/zm/ll4504dd6dgbvy20y9ls2_hr0000gq/T/tmpwu0wf27f.swift:2:32: error: cannot find 'x' in scope
1 | var i = 0
2 | while (i != a.count && a[i] != x) {
  |                                `- error: cannot find 'x' in scope
3 |   i += 1
4 | }

============================================================
2026-02-03 16:23:44 [ERROR] (mdbook::renderer): Renderer exited with non-zero return code.
2026-02-03 16:23:44 [ERROR] (mdbook::utils): Error: Rendering failed
2026-02-03 16:23:44 [ERROR] (mdbook::utils):    Caused By: The "swift-test" renderer failed

[dabrahams]

SwiftyLab/setup-swift is the reliable Swift installation action we've been using for Hylo, FWIW.

[sean-parent]

Can you use the built-in mdbook support?

[output.html.code.hidelines]
swift = "#"

[dabrahams]

Swift has # directives so might need a different character

[nickpdemarco]

It seems to work! Huge simplification, thanks for pointing that out.

[dabrahams]

This line isn't adding anything

[dabrahams]

Raw loop should be algorithms and/or list comprehensions

[dabrahams]

Content free comment

[dabrahams]

Content free comment

[dabrahams]

What the heck is this??

[dabrahams]

I got partway through reading the actual code which began to smell sloppy, and when I reached the point that it decided there was a case it wasn't going to support, and it would just exit with success (the output is not HTML), that was an almost certain sign that this was generated with AI and not carefully reviewed. Please forgive me if I've gotten that wrong, but Everything we check in here, even tooling should uphold the principles of better code. that includes real meaningful contract documentation for example.

[nickpdemarco]

Thanks for that feedback @dabrahams - I'll take a closer look and address what you've mentioned. In hindsight I should have made this a draft PR; it wasn't my intention to request a review, just share progress.