Rollup merge of #79525 - jyn514:feature-gate-normalize, r=GuillaumeGomez

Add -Z normalize-docs and enable it for compiler docs

Works around https://github.com/rust-lang/rust/issues/79459 by only enabling normalization for the compiler itself (and anyone who opts-in on nightly). Eventually I want to remove this and enable normalization by default, but that's turned out to be [really hard](https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/How.20do.20I.20normalize.20projection.20types.20to.20a.20single.20type.3F/near/218125195).

This uses a command line option instead of a feature gate so it's easier to pass it to all crates at once. Theoretically it's better to use a feature gate instead so that it's easier for people to use on docs.rs, but I'm also not terribly concerned with how easy it to use a temporary hack.

Addresses https://github.com/rust-lang/rust/issues/77459.

compiler/rustc_session/src/options.rs

@@ -996,6 +996,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         "run LLVM in non-parallel mode (while keeping codegen-units and ThinLTO)"),
     no_profiler_runtime: bool = (false, parse_no_flag, [TRACKED],
         "prevent automatic injection of the profiler_builtins crate"),
+    normalize_docs: bool = (false, parse_bool, [TRACKED],
+        "normalize associated items in rustdoc when generating documentation"),
     osx_rpath_install_name: bool = (false, parse_bool, [TRACKED],
         "pass `-install_name @rpath/...` to the macOS linker (default: no)"),
     panic_abort_tests: bool = (false, parse_bool, [TRACKED],

src/bootstrap/builder.rs

@@ -731,6 +731,7 @@ impl<'a> Builder<'a> {
             .env("CFG_RELEASE_CHANNEL", &self.config.channel)
             .env("RUSTDOC_REAL", self.rustdoc(compiler))
             .env("RUSTC_BOOTSTRAP", "1")
+            .arg("-Znormalize_docs")
             .arg("-Winvalid_codeblock_attributes");
         if self.config.deny_warnings {
             cmd.arg("-Dwarnings");

src/bootstrap/doc.rs

@@ -527,6 +527,7 @@ impl Step for Rustc {
         cargo.rustdocflag("--document-private-items");
         cargo.rustdocflag("--enable-index-page");
         cargo.rustdocflag("-Zunstable-options");
+        cargo.rustdocflag("-Znormalize-docs");
         compile::rustc_cargo(builder, &mut cargo, target);
 
         // Only include compiler crates, no dependencies of those, such as `libc`.

src/librustdoc/clean/mod.rs

@@ -1515,9 +1515,12 @@ impl Clean<Type> for hir::Ty<'_> {
 }
 
 /// Returns `None` if the type could not be normalized
-#[allow(unreachable_code, unused_variables)]
 fn normalize(cx: &DocContext<'tcx>, ty: Ty<'_>) -> Option<Ty<'tcx>> {
-    return None; // HACK: low-churn fix for #79459 while we wait for a trait normalization fix
+    // HACK: low-churn fix for #79459 while we wait for a trait normalization fix
+    if !cx.tcx.sess.opts.debugging_opts.normalize_docs {
+        return None;
+    }
+
     use crate::rustc_trait_selection::infer::TyCtxtInferExt;
     use crate::rustc_trait_selection::traits::query::normalize::AtExt;
     use rustc_middle::traits::ObligationCause;

src/test/rustdoc/normalize-assoc-item.rs

@@ -1,7 +1,7 @@
 // ignore-tidy-linelength
 // aux-build:normalize-assoc-item.rs
 // build-aux-docs
-// ignore-test
+// compile-flags:-Znormalize-docs
 
 pub trait Trait {
     type X;