rustdoc: Note why rustdoc::html::markdown is public

[camelid]

Almost all of the modules are crate-private, except for
rustdoc::json::types, which I believe is intended to be for public
use; and rustdoc::html::markdown, which is used externally by the
error-index generator and so has to be public.

r? @GuillaumeGomez

[rust-log-analyzer]

The job x86_64-gnu-llvm-9 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

 finished in 0.426 seconds
Check compiletest suite=assembly mode=assembly (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)

running 29 tests
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Suite("src/test/incremental") not skipped for "bootstrap::test::Incremental" -- not in ["src/tools/tidy"]
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Suite("src/test/debuginfo") not skipped for "bootstrap::test::Debuginfo" -- not in ["src/tools/tidy"]
Check compiletest suite=debuginfo mode=debuginfo (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)

running 116 tests
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test result: ok. 78 passed; 0 failed; 38 ignored; 0 measured; 0 filtered out; finished in 2.29s

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Suite("src/test/ui-fulldeps") not skipped for "bootstrap::test::UiFullDeps" -- not in ["src/tools/tidy"]
---
running 1 test
F
failures:

---- html/markdown.rs - html::markdown (line 5) stdout ----
error[E0603]: module `html` is private
  --> html/markdown.rs:11:14
   |
9  | use rustdoc::html::markdown::{IdMap, Markdown, ErrorCodes};
   |              ^^^^ private module
   |
note: the module `html` is defined here
   |
   |
87 | mod html;

error: aborting due to previous error

For more information about this error, try `rustc --explain E0603`.
---

error: test failed, to rerun pass '--doc'


command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "test" "--target" "x86_64-unknown-linux-gnu" "-Zbinary-dep-depinfo" "-j" "16" "--release" "--locked" "--color" "always" "--manifest-path" "/checkout/src/tools/rustdoc/Cargo.toml" "-p" "rustdoc:0.0.0" "--" "--quiet"


failed to run: /checkout/obj/build/bootstrap/debug/bootstrap --stage 2 test --exclude src/tools/tidy
Build completed unsuccessfully in 0:27:37

[camelid]

This doctest is failing as a result of this change:

---- html/markdown.rs - html::markdown (line 5) stdout ----
error[E0603]: module `html` is private
  --> html/markdown.rs:11:14
   |
9  | use rustdoc::html::markdown::{IdMap, Markdown, ErrorCodes};
   |              ^^^^ private module

Should I just mark it as ignore?

[bugadani]

Should I just mark it as ignore?

It certainly doesn't seem to test anything, so I think you should.

[GuillaumeGomez]

Yes, you can ignore it.

[camelid]

It turns out rustdoc::html is used by error_index_generator, so I don't think we can make it private. (That may also be why there was a doctest — to test that the basic interface stayed somewhat stable, or to show that it's used externally.) I think I'll make it public again and un-ignore the doctest.

[camelid]

At this point, I'm no longer changing the visibility of any the modules, so I updated the commit message and PR title and description :)

[GuillaumeGomez]

Well, more doc is always a good thing, so thanks!

@bors: r+ rollup

[bors]

📌 Commit 82010e8 has been approved by GuillaumeGomez