Auto merge of #92395 - Kobzol:rustdoc-bindings-thin-vec, r=camelid

Rustdoc: use ThinVec for GenericArgs bindings

The bindings are almost always empty. This reduces the size of `PathSegment` and `GenericArgs` by about one fourth.

compiler/rustc_data_structures/src/thin_vec.rs

@@ -5,7 +5,7 @@ use std::iter::FromIterator;
 /// A vector type optimized for cases where this size is usually 0 (cf. `SmallVec`).
 /// The `Option<Box<..>>` wrapping allows us to represent a zero sized vector with `None`,
 /// which uses only a single (null) pointer.
-#[derive(Clone, Encodable, Decodable, Debug)]
+#[derive(Clone, Encodable, Decodable, Debug, Hash, Eq, PartialEq)]
 pub struct ThinVec<T>(Option<Box<Vec<T>>>);
 
 impl<T> ThinVec<T> {
@@ -20,6 +20,13 @@ impl<T> ThinVec<T> {
     pub fn iter_mut(&mut self) -> std::slice::IterMut<'_, T> {
         self.into_iter()
     }
+
+    pub fn push(&mut self, item: T) {
+        match *self {
+            ThinVec(Some(ref mut vec)) => vec.push(item),
+            ThinVec(None) => *self = vec![item].into(),
+        }
+    }
 }
 
 impl<T> From<Vec<T>> for ThinVec<T> {
@@ -101,10 +108,7 @@ impl<T> Extend<T> for ThinVec<T> {
     }
 
     fn extend_one(&mut self, item: T) {
-        match *self {
-            ThinVec(Some(ref mut vec)) => vec.push(item),
-            ThinVec(None) => *self = vec![item].into(),
-        }
+        self.push(item)
     }
 
     fn extend_reserve(&mut self, additional: usize) {

src/librustdoc/clean/inline.rs

@@ -5,6 +5,7 @@ use std::sync::Arc;
 
 use rustc_ast as ast;
 use rustc_data_structures::fx::FxHashSet;
+use rustc_data_structures::thin_vec::ThinVec;
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::DefId;
@@ -540,7 +541,7 @@ fn build_module(
                                     name: prim_ty.as_sym(),
                                     args: clean::GenericArgs::AngleBracketed {
                                         args: Vec::new(),
-                                        bindings: Vec::new(),
+                                        bindings: ThinVec::new(),
                                     },
                                 }],
                             },

src/librustdoc/clean/types.rs

@@ -2060,14 +2060,14 @@ rustc_data_structures::static_assert_size!(GenericArg, 80);
 
 #[derive(Clone, PartialEq, Eq, Debug, Hash)]
 crate enum GenericArgs {
-    AngleBracketed { args: Vec<GenericArg>, bindings: Vec<TypeBinding> },
+    AngleBracketed { args: Vec<GenericArg>, bindings: ThinVec<TypeBinding> },
     Parenthesized { inputs: Vec<Type>, output: Option<Box<Type>> },
 }
 
 // `GenericArgs` is in every `PathSegment`, so its size can significantly
 // affect rustdoc's memory usage.
 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
-rustc_data_structures::static_assert_size!(GenericArgs, 56);
+rustc_data_structures::static_assert_size!(GenericArgs, 40);
 
 #[derive(Clone, PartialEq, Eq, Debug, Hash)]
 crate struct PathSegment {
@@ -2078,7 +2078,7 @@ crate struct PathSegment {
 // `PathSegment` usually occurs multiple times in every `Path`, so its size can
 // significantly affect rustdoc's memory usage.
 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
-rustc_data_structures::static_assert_size!(PathSegment, 64);
+rustc_data_structures::static_assert_size!(PathSegment, 48);
 
 #[derive(Clone, Debug)]
 crate struct Typedef {

src/librustdoc/clean/utils.rs

@@ -10,6 +10,7 @@ use crate::visit_lib::LibEmbargoVisitor;
 
 use rustc_ast as ast;
 use rustc_ast::tokenstream::TokenTree;
+use rustc_data_structures::thin_vec::ThinVec;
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
@@ -108,7 +109,7 @@ fn external_generic_args(
     if cx.tcx.fn_trait_kind_from_lang_item(did).is_some() {
         let inputs = match ty_kind.unwrap() {
             ty::Tuple(tys) => tys.iter().map(|t| t.expect_ty().clean(cx)).collect(),
-            _ => return GenericArgs::AngleBracketed { args, bindings },
+            _ => return GenericArgs::AngleBracketed { args, bindings: bindings.into() },
         };
         let output = None;
         // FIXME(#20299) return type comes from a projection now
@@ -118,7 +119,7 @@ fn external_generic_args(
         // };
         GenericArgs::Parenthesized { inputs, output }
     } else {
-        GenericArgs::AngleBracketed { args, bindings }
+        GenericArgs::AngleBracketed { args, bindings: bindings.into() }
     }
 }
 
@@ -143,7 +144,7 @@ pub(super) fn external_path(
 /// Remove the generic arguments from a path.
 crate fn strip_path_generics(mut path: Path) -> Path {
     for ps in path.segments.iter_mut() {
-        ps.args = GenericArgs::AngleBracketed { args: vec![], bindings: vec![] }
+        ps.args = GenericArgs::AngleBracketed { args: vec![], bindings: ThinVec::new() }
     }
 
     path