Add support to generate documentation for tests

The new option --document-tests is unstable and documented as such.
In order to use it is needed to add `--cfg test` and in case the tests
are not marked public to add `--document-private-items`.
The implementation hide the auto generate main test function and
constants.

Author: ifxfrancois

Diff for: src/doc/rustdoc/src/unstable-features.md

@@ -830,3 +830,34 @@ will be split as follows:
     "you today?",
 ]
 ```
+
+### `--document-tests`: show test items
+
+Using this flag looks like this:
+
+```bash
+$ rustdoc src/lib.rs -Z unstable-options --cfg test --document-private-items --document-tests
+```
+
+By default, `rustdoc` does not document test items.
+
+```rust
+/// by default this test function would not be documented
+#[test]
+fn test_in_module() {
+     assert_eq!(2, 1 + 1);
+}
+/// by default this test module would not be documented
+#[cfg(test)]
+mod tests {
+    /// by default this test function would not be documented
+    #[test]
+    fn test_in_a_test_module() {
+        assert_eq!(2, 1 + 1);
+    }
+}
+```
+
+Note:
+* `--cfg test` must be set because tests are guarded by #[cfg(test)].
+* `--document-private-items` is typically required because it is standard practice to keep test items private. By enabling this option, you ensure that private items, including tests, are documented as needed while maintaining their non-public status.

Diff for: src/librustdoc/clean/mod.rs

@@ -1040,7 +1040,11 @@ fn clean_fn_or_proc_macro<'tcx>(
         None => {
             let mut func = clean_function(cx, sig, generics, FunctionArgs::Body(body_id));
             clean_fn_decl_legacy_const_generics(&mut func, attrs);
-            FunctionItem(func)
+            if cx.cache.document_tests && cx.cache.tests.contains(&item.owner_id.to_def_id()) {
+                TestItem(func)
+            } else {
+                FunctionItem(func)
+            }
         }
     }
 }

Diff for: src/librustdoc/clean/types.rs

@@ -697,7 +697,8 @@ impl Item {
             }
             ItemKind::FunctionItem(_)
             | ItemKind::MethodItem(_, _)
-            | ItemKind::RequiredMethodItem(_) => {
+            | ItemKind::RequiredMethodItem(_)
+            | ItemKind::TestItem(_) => {
                 let def_id = self.def_id().unwrap();
                 build_fn_header(def_id, tcx, tcx.asyncness(def_id))
             }
@@ -871,6 +872,7 @@ pub(crate) enum ItemKind {
     UnionItem(Union),
     EnumItem(Enum),
     FunctionItem(Box<Function>),
+    TestItem(Box<Function>),
     ModuleItem(Module),
     TypeAliasItem(Box<TypeAlias>),
     StaticItem(Static),
@@ -931,6 +933,7 @@ impl ItemKind {
             ExternCrateItem { .. }
             | ImportItem(_)
             | FunctionItem(_)
+            | TestItem(_)
             | TypeAliasItem(_)
             | StaticItem(_)
             | ConstantItem(_)

Diff for: src/librustdoc/clean/types/tests.rs

@@ -71,7 +71,7 @@ fn should_not_trim() {
 fn is_same_generic() {
     use crate::clean::types::{PrimitiveType, Type};
     use crate::formats::cache::Cache;
-    let cache = Cache::new(false, false);
+    let cache = Cache::new(false, false, false);
     let generic = Type::Generic(rustc_span::symbol::sym::Any);
     let unit = Type::Primitive(PrimitiveType::Unit);
     assert!(!generic.is_doc_subtype_of(&unit, &cache));

Diff for: src/librustdoc/config.rs

@@ -280,6 +280,8 @@ pub(crate) struct RenderOptions {
     pub(crate) document_private: bool,
     /// Document items that have `doc(hidden)`.
     pub(crate) document_hidden: bool,
+    /// Document tests.
+    pub(crate) document_tests: bool,
     /// If `true`, generate a JSON file in the crate folder instead of HTML redirection files.
     pub(crate) generate_redirect_map: bool,
     /// Show the memory layout of types in the docs.
@@ -808,6 +810,7 @@ impl Options {
         }
 
         let scrape_examples_options = ScrapeExamplesOptions::new(matches, dcx);
+        let document_tests = matches.opt_present("document-tests");
         let with_examples = matches.opt_strs("with-examples");
         let call_locations = crate::scrape_examples::load_call_locations(with_examples, dcx);
         let doctest_compilation_args = matches.opt_strs("doctest-compilation-args");
@@ -881,6 +884,7 @@ impl Options {
             markdown_playground_url,
             document_private,
             document_hidden,
+            document_tests,
             generate_redirect_map,
             show_type_layout,
             unstable_features,

Diff for: src/librustdoc/core.rs

@@ -251,7 +251,8 @@ pub(crate) fn create_config(
     } else {
         ResolveDocLinks::Exported
     };
-    let test = scrape_examples_options.map(|opts| opts.scrape_tests).unwrap_or(false);
+    let test = scrape_examples_options.map(|opts| opts.scrape_tests).unwrap_or(false)
+        || (cfgs.iter().any(|cfg| cfg == "test") && render_options.document_tests);
     // plays with error output here!
     let sessopts = config::Options {
         maybe_sysroot,
@@ -370,7 +371,11 @@ pub(crate) fn run_global_ctxt(
         impl_trait_bounds: Default::default(),
         generated_synthetics: Default::default(),
         auto_traits,
-        cache: Cache::new(render_options.document_private, render_options.document_hidden),
+        cache: Cache::new(
+            render_options.document_private,
+            render_options.document_hidden,
+            render_options.document_tests,
+        ),
         inlined: FxHashSet::default(),
         output_format,
         render_options,

Diff for: src/librustdoc/fold.rs

@@ -79,6 +79,7 @@ pub(crate) trait DocFolder: Sized {
             ExternCrateItem { src: _ }
             | ImportItem(_)
             | FunctionItem(_)
+            | TestItem(_)
             | StaticItem(_)
             | ConstantItem(..)
             | TraitAliasItem(_)

Diff for: src/librustdoc/formats/cache.rs

@@ -92,6 +92,11 @@ pub(crate) struct Cache {
     /// Whether to document hidden items.
     /// This is stored in `Cache` so it doesn't need to be passed through all rustdoc functions.
     pub(crate) document_hidden: bool,
+    /// Whether to document tests.
+    /// This is stored in `Cache` so it doesn't need to be passed through all rustdoc functions.
+    pub(crate) document_tests: bool,
+    /// DefIds of all functions which are tests.
+    pub(crate) tests: FxHashSet<DefId>,
 
     /// Crates marked with [`#[doc(masked)]`][doc_masked].
     ///
@@ -144,8 +149,8 @@ struct CacheBuilder<'a, 'tcx> {
 }
 
 impl Cache {
-    pub(crate) fn new(document_private: bool, document_hidden: bool) -> Self {
-        Cache { document_private, document_hidden, ..Cache::default() }
+    pub(crate) fn new(document_private: bool, document_hidden: bool, document_tests: bool) -> Self {
+        Cache { document_private, document_hidden, document_tests, ..Cache::default() }
     }
 
     /// Populates the `Cache` with more data. The returned `Crate` will be missing some data that was
@@ -302,6 +307,7 @@ impl DocFolder for CacheBuilder<'_, '_> {
             | clean::TraitItem(..)
             | clean::TraitAliasItem(..)
             | clean::FunctionItem(..)
+            | clean::TestItem(..)
             | clean::ModuleItem(..)
             | clean::ForeignFunctionItem(..)
             | clean::ForeignStaticItem(..)

Diff for: src/librustdoc/formats/item_type.rs

@@ -57,6 +57,7 @@ pub(crate) enum ItemType {
     TraitAlias = 25,
     // This number is reserved for use in JavaScript
     // Generic = 26,
+    Test = 27,
 }
 
 impl Serialize for ItemType {
@@ -83,6 +84,7 @@ impl<'a> From<&'a clean::Item> for ItemType {
             clean::UnionItem(..) => ItemType::Union,
             clean::EnumItem(..) => ItemType::Enum,
             clean::FunctionItem(..) => ItemType::Function,
+            clean::TestItem(..) => ItemType::Test,
             clean::TypeAliasItem(..) => ItemType::TypeAlias,
             clean::StaticItem(..) => ItemType::Static,
             clean::ConstantItem(..) => ItemType::Constant,
@@ -178,6 +180,7 @@ impl ItemType {
             ItemType::Union => "union",
             ItemType::Enum => "enum",
             ItemType::Function => "fn",
+            ItemType::Test => "test",
             ItemType::TypeAlias => "type",
             ItemType::Static => "static",
             ItemType::Trait => "trait",

Diff for: src/librustdoc/html/render/mod.rs

@@ -341,6 +341,7 @@ struct AllTypes {
     attribute_macros: FxIndexSet<ItemEntry>,
     derive_macros: FxIndexSet<ItemEntry>,
     trait_aliases: FxIndexSet<ItemEntry>,
+    tests: FxIndexSet<ItemEntry>,
 }
 
 impl AllTypes {
@@ -360,6 +361,7 @@ impl AllTypes {
             attribute_macros: new_set(100),
             derive_macros: new_set(100),
             trait_aliases: new_set(100),
+            tests: new_set(100),
         }
     }
 
@@ -385,6 +387,7 @@ impl AllTypes {
                 }
                 ItemType::ProcDerive => self.derive_macros.insert(ItemEntry::new(new_url, name)),
                 ItemType::TraitAlias => self.trait_aliases.insert(ItemEntry::new(new_url, name)),
+                ItemType::Test => self.tests.insert(ItemEntry::new(new_url, name)),
                 _ => true,
             };
         }
@@ -414,6 +417,9 @@ impl AllTypes {
         if !self.functions.is_empty() {
             sections.insert(ItemSection::Functions);
         }
+        if !self.tests.is_empty() {
+            sections.insert(ItemSection::Tests);
+        }
         if !self.type_aliases.is_empty() {
             sections.insert(ItemSection::TypeAliases);
         }
@@ -432,6 +438,9 @@ impl AllTypes {
         if !self.trait_aliases.is_empty() {
             sections.insert(ItemSection::TraitAliases);
         }
+        if !self.tests.is_empty() {
+            sections.insert(ItemSection::Tests);
+        }
 
         sections
     }
@@ -470,6 +479,7 @@ impl AllTypes {
         print_entries(f, &self.attribute_macros, ItemSection::AttributeMacros);
         print_entries(f, &self.derive_macros, ItemSection::DeriveMacros);
         print_entries(f, &self.functions, ItemSection::Functions);
+        print_entries(f, &self.tests, ItemSection::Tests);
         print_entries(f, &self.type_aliases, ItemSection::TypeAliases);
         print_entries(f, &self.trait_aliases, ItemSection::TraitAliases);
         print_entries(f, &self.statics, ItemSection::Statics);
@@ -2380,6 +2390,7 @@ pub(crate) enum ItemSection {
     Statics,
     Traits,
     Functions,
+    Tests,
     TypeAliases,
     Unions,
     Implementations,
@@ -2412,6 +2423,7 @@ impl ItemSection {
             Statics,
             Traits,
             Functions,
+            Tests,
             TypeAliases,
             Unions,
             Implementations,
@@ -2437,6 +2449,7 @@ impl ItemSection {
             Self::Unions => "unions",
             Self::Enums => "enums",
             Self::Functions => "functions",
+            Self::Tests => "tests",
             Self::TypeAliases => "types",
             Self::Statics => "statics",
             Self::Constants => "constants",
@@ -2466,6 +2479,7 @@ impl ItemSection {
             Self::Unions => "Unions",
             Self::Enums => "Enums",
             Self::Functions => "Functions",
+            Self::Tests => "Tests",
             Self::TypeAliases => "Type Aliases",
             Self::Statics => "Statics",
             Self::Constants => "Constants",
@@ -2496,6 +2510,7 @@ fn item_ty_to_section(ty: ItemType) -> ItemSection {
         ItemType::Union => ItemSection::Unions,
         ItemType::Enum => ItemSection::Enums,
         ItemType::Function => ItemSection::Functions,
+        ItemType::Test => ItemSection::Tests,
         ItemType::TypeAlias => ItemSection::TypeAliases,
         ItemType::Static => ItemSection::Statics,
         ItemType::Constant => ItemSection::Constants,

Diff for: src/librustdoc/html/render/print_item.rs

@@ -162,6 +162,7 @@ pub(super) fn print_item(cx: &Context<'_>, item: &clean::Item) -> impl fmt::Disp
                 }
             }
             clean::FunctionItem(..) | clean::ForeignFunctionItem(..) => "Function ",
+            clean::TestItem(..) => "Test ",
             clean::TraitItem(..) => "Trait ",
             clean::StructItem(..) => "Struct ",
             clean::UnionItem(..) => "Union ",
@@ -228,7 +229,7 @@ pub(super) fn print_item(cx: &Context<'_>, item: &clean::Item) -> impl fmt::Disp
             clean::ModuleItem(m) => {
                 write!(buf, "{}", item_module(cx, item, &m.items))
             }
-            clean::FunctionItem(f) | clean::ForeignFunctionItem(f, _) => {
+            clean::FunctionItem(f) | clean::ForeignFunctionItem(f, _) | clean::TestItem(f) => {
                 write!(buf, "{}", item_function(cx, item, f))
             }
             clean::TraitItem(t) => write!(buf, "{}", item_trait(cx, item, t)),
@@ -326,6 +327,7 @@ fn item_module(cx: &Context<'_>, item: &clean::Item, items: &[clean::Item]) -> i
                 ItemType::Static => 8,
                 ItemType::Trait => 9,
                 ItemType::Function => 10,
+                ItemType::Test => 11,
                 ItemType::TypeAlias => 12,
                 ItemType::Union => 13,
                 _ => 14 + ty as u8,

Diff for: src/librustdoc/html/static/js/main.js

@@ -656,6 +656,7 @@ function preLoadCss(cssUrl) {
             block("static", "static", "Statics");
             block("trait", "traits", "Traits");
             block("fn", "functions", "Functions");
+            block("test", "tests", "Tests");
             block("type", "types", "Type Aliases");
             block("union", "unions", "Unions");
             // No point, because these items don't appear in modules

Diff for: src/librustdoc/json/conversions.rs

@@ -312,7 +312,9 @@ fn from_clean_item(item: clean::Item, renderer: &JsonRenderer<'_>) -> ItemEnum {
         StructFieldItem(f) => ItemEnum::StructField(f.into_json(renderer)),
         EnumItem(e) => ItemEnum::Enum(e.into_json(renderer)),
         VariantItem(v) => ItemEnum::Variant(v.into_json(renderer)),
-        FunctionItem(f) => ItemEnum::Function(from_function(*f, true, header.unwrap(), renderer)),
+        FunctionItem(f) | TestItem(f) => {
+            ItemEnum::Function(from_function(*f, true, header.unwrap(), renderer))
+        }
         ForeignFunctionItem(f, _) => {
             ItemEnum::Function(from_function(*f, false, header.unwrap(), renderer))
         }
@@ -870,7 +872,7 @@ impl FromClean<ItemType> for ItemKind {
             Struct => ItemKind::Struct,
             Union => ItemKind::Union,
             Enum => ItemKind::Enum,
-            Function | TyMethod | Method => ItemKind::Function,
+            Function | Test | TyMethod | Method => ItemKind::Function,
             TypeAlias => ItemKind::TypeAlias,
             Static => ItemKind::Static,
             Constant => ItemKind::Constant,

Diff for: src/librustdoc/lib.rs

@@ -699,6 +699,7 @@ fn opts() -> Vec<RustcOptGroup> {
             "removed, see issue #44136 <https://github.com/rust-lang/rust/issues/44136> for more information",
             "[rust]",
         ),
+        opt(Unstable, FlagMulti, "", "document-tests", "Generate documentation for tests", ""),
     ]
 }
 

Diff for: src/librustdoc/passes/propagate_stability.rs

@@ -77,6 +77,7 @@ impl DocFolder for StabilityPropagator<'_, '_> {
                     | ItemKind::UnionItem(..)
                     | ItemKind::EnumItem(..)
                     | ItemKind::FunctionItem(..)
+                    | ItemKind::TestItem(..)
                     | ItemKind::ModuleItem(..)
                     | ItemKind::TypeAliasItem(..)
                     | ItemKind::StaticItem(..)

Diff for: src/librustdoc/passes/stripper.rs

@@ -57,6 +57,7 @@ impl DocFolder for Stripper<'_, '_> {
             | clean::EnumItem(..)
             | clean::TraitItem(..)
             | clean::FunctionItem(..)
+            | clean::TestItem(..)
             | clean::VariantItem(..)
             | clean::ForeignFunctionItem(..)
             | clean::ForeignStaticItem(..)

Diff for: src/librustdoc/visit.rs

@@ -31,6 +31,7 @@ pub(crate) trait DocVisitor<'a>: Sized {
             ExternCrateItem { src: _ }
             | ImportItem(_)
             | FunctionItem(_)
+            | TestItem(_)
             | TypeAliasItem(_)
             | StaticItem(_)
             | ConstantItem(..)