added rustdoc book documentation, improved behavior when unstable flag not present

Author: Goirad

src/doc/rustdoc/src/unstable-features.md

@@ -455,3 +455,53 @@ Some methodology notes about what rustdoc counts in this metric:
 
 Public items that are not documented can be seen with the built-in `missing_docs` lint. Private
 items that are not documented can be seen with Clippy's `missing_docs_in_private_items` lint.
+
+### `--enable-per-target-ignores`: allow `ignore-foo` style filters for doctests
+
+Using this flag looks like this:
+
+```bash
+$ rustdoc src/lib.rs -Z unstable-options --enable-per-target-ignores
+```
+
+This flag allows you to tag doctests with compiltest style `ignore-foo` filters that prevent
+rustdoc from running that test if the target triple string contains foo. For example:
+
+```rust
+///```ignore-foo,ignore-bar
+///assert!(2 == 2);
+///```
+struct Foo;
+```
+
+This will not be run when the build target is `super-awesome-foo` or `less-bar-awesome`.
+If the flag is not enabled, then rustdoc will consume the filter, but do nothing with it, and
+the above example will be run for all targets.
+If you want to preserve backwards compatibility for older versions of rustdoc, you can use
+
+```rust
+///```ignore,ignore-foo
+///assert!(2 == 2);
+///```
+struct Foo;
+```
+
+In older versions, this will be ignored on all targets, but on newer versions `ignore-gnu` will
+override `ignore`.
+
+### `--runtool`, `--runtool-arg`: program to run tests with; args to pass to it
+
+Using thses options looks like this:
+
+```bash
+$ rustdoc src/lib.rs -Z unstable-options --runtool runner --runtool-arg --do-thing --runtool-arg --do-other-thing
+```
+
+These options can be used to run the doctest under a program, and also pass arguments to
+that program. For example, if you want to run your doctests under valgrind you might run
+
+```bash
+$ rustdoc src/lib.rs -Z unstable-options --runtool valgrind
+```
+
+Another use case would be to run a test inside an emulator, or through a Virtual Machine.

src/librustdoc/html/markdown.rs

@@ -650,7 +650,7 @@ impl LangString {
                 }
                 "no_run" => { data.no_run = true; seen_rust_tags = !seen_other_tags; }
                 "ignore" => { data.ignore = Ignore::All; seen_rust_tags = !seen_other_tags; }
-                x if enable_per_target_ignores && x.starts_with("ignore-") => {
+                x if x.starts_with("ignore-") => if enable_per_target_ignores {
                     ignores.push(x.trim_start_matches("ignore-").to_owned());
                     seen_rust_tags = !seen_other_tags;
                 }
@@ -681,15 +681,9 @@ impl LangString {
                 _ => { seen_other_tags = true }
             }
         }
-
-        match data.ignore {
-            Ignore::All => {},
-            Ignore::None => {
-                if !ignores.is_empty() {
-                    data.ignore = Ignore::Some(ignores);
-                }
-            },
-            _ => unreachable!(),
+        // ignore-foo overrides ignore
+        if !ignores.is_empty() {
+            data.ignore = Ignore::Some(ignores);
         }
 
         data.rust &= !seen_other_tags || seen_rust_tags;

src/librustdoc/test.rs

@@ -58,8 +58,10 @@ pub fn run(options: Options) -> i32 {
             ..config::basic_debugging_options()
         },
         edition: options.edition,
+        target_triple: options.target.clone(),
         ..config::Options::default()
     };
+
     let config = interface::Config {
         opts: sessopts,
         crate_cfg: config::parse_cfgspecs(options.cfgs.clone()),