Rollup merge of #121160 - fmease:rustdoc-fix-n-refactor-html-rendering, r=GuillaumeGomez

rustdoc: fix and refactor HTML rendering a bit

* refactoring: get rid of a bunch of manual `f.alternate()` branches
  * not sure why this wasn't done so already, is this perf-sensitive?
* fix an ICE in debug builds of rustdoc
  * rustdoc used to crash on empty outlives-bounds: `where 'a:`
* properly escape const generic defaults
* actually print empty trait and outlives-bounds (doesn't work for cross-crate reexports yet, will fix that at some other point) since they can have semantic significance
  * outlives-bounds: forces lifetime params to be early-bound instead of late-bound which is technically speaking part of the public API
  * trait-bounds: can affect the well-formedness, consider
    * makeshift “const-evaluatable” bounds under `generic_const_exprs`
    * bounds to force wf-checking in light of #100041 (quite artificial I know, I couldn't figure out something better), see https://github.com/rust-lang/rust/pull/121160#discussion_r1491563816

2 files changed, 13 insertions, 1 deletions

diff --git a/tests/rustdoc/bounds-in-multiple-parts.rs b/tests/rustdoc/bounds.rs
index 279e3c14888..da09e3f2a52 100644
--- a/tests/rustdoc/bounds-in-multiple-parts.rs
+++ b/tests/rustdoc/bounds.rs
@@ -18,3 +18,15 @@ pub trait T2 {
     fn f<T: Eq>()
         where Self: Eq, Self: Eq2, T: Eq2;
 }
+
+// Checking that we support empty bounds (we used to crash on empty outlives-bounds).
+// Note that we don't want to hide them since they have a semantic effect.
+// For outlives-bounds, they force the lifetime param to be early-bound instead of late-bound.
+// For trait bounds, it can affect well-formedness (see `ClauseKind::WellFormed`).
+// @has 'foo/fn.empty.html'
+// @has - '//pre[@class="rust item-decl"]' "empty<'a, T>()where T:, 'a:,"
+pub fn empty<'a, T>()
+    where
+        T:,
+        'a:,
+{}
diff --git a/tests/rustdoc/const-generics/generic_const_exprs.rs b/tests/rustdoc/const-generics/generic_const_exprs.rs
index e23b3006da6..2d2d31d7231 100644
--- a/tests/rustdoc/const-generics/generic_const_exprs.rs
+++ b/tests/rustdoc/const-generics/generic_const_exprs.rs
@@ -3,5 +3,5 @@
 #![allow(incomplete_features)]
 // make sure that `ConstEvaluatable` predicates dont cause rustdoc to ICE #77647
 // @has foo/struct.Ice.html '//pre[@class="rust item-decl"]' \
-//      'pub struct Ice<const N: usize>;'
+//      'pub struct Ice<const N: usize> where [(); { _ }]:;'
 pub struct Ice<const N: usize> where [(); N + 1]:;