Use AttrVec in more places.

In some places we use `Vec<Attribute>` and some places we use
`ThinVec<Attribute>` (a.k.a. `AttrVec`). This results in various points
where we have to convert between `Vec` and `ThinVec`.

This commit changes the places that use `Vec<Attribute>` to use
`AttrVec`. A lot of this is mechanical and boring, but there are
some interesting parts:
- It adds a few new methods to `ThinVec`.
- It implements `MapInPlace` for `ThinVec`, and introduces a macro to
  avoid the repetition of this trait for `Vec`, `SmallVec`, and
  `ThinVec`.

Overall, it makes the code a little nicer, and has little effect on
performance. But it is a precursor to removing
`rustc_data_structures::thin_vec::ThinVec` and replacing it with
`thin_vec::ThinVec`, which is implemented more efficiently.

src/attr.rs

@@ -49,10 +49,7 @@ pub(crate) fn get_span_without_attrs(stmt: &ast::Stmt) -> Span {
 }
 
 /// Returns attributes that are within `outer_span`.
-pub(crate) fn filter_inline_attrs(
-    attrs: &[ast::Attribute],
-    outer_span: Span,
-) -> Vec<ast::Attribute> {
+pub(crate) fn filter_inline_attrs(attrs: &[ast::Attribute], outer_span: Span) -> ast::AttrVec {
     attrs
         .iter()
         .filter(|a| outer_span.lo() <= a.span.lo() && a.span.hi() <= outer_span.hi())

src/imports.rs

@@ -116,7 +116,7 @@ pub(crate) struct UseTree {
     // Additional fields for top level use items.
     // Should we have another struct for top-level use items rather than reusing this?
     visibility: Option<ast::Visibility>,
-    attrs: Option<Vec<ast::Attribute>>,
+    attrs: Option<ast::AttrVec>,
 }
 
 impl PartialEq for UseTree {
@@ -417,7 +417,7 @@ impl UseTree {
         list_item: Option<ListItem>,
         visibility: Option<ast::Visibility>,
         opt_lo: Option<BytePos>,
-        attrs: Option<Vec<ast::Attribute>>,
+        attrs: Option<ast::AttrVec>,
     ) -> UseTree {
         let span = if let Some(lo) = opt_lo {
             mk_sp(lo, a.span.hi())

src/modules.rs

@@ -26,7 +26,7 @@ type FileModMap<'ast> = BTreeMap<FileName, Module<'ast>>;
 pub(crate) struct Module<'a> {
     ast_mod_kind: Option<Cow<'a, ast::ModKind>>,
     pub(crate) items: Cow<'a, Vec<rustc_ast::ptr::P<ast::Item>>>,
-    inner_attr: Vec<ast::Attribute>,
+    inner_attr: ast::AttrVec,
     pub(crate) span: Span,
 }
 
@@ -35,7 +35,7 @@ impl<'a> Module<'a> {
         mod_span: Span,
         ast_mod_kind: Option<Cow<'a, ast::ModKind>>,
         mod_items: Cow<'a, Vec<rustc_ast::ptr::P<ast::Item>>>,
-        mod_attrs: Cow<'a, Vec<ast::Attribute>>,
+        mod_attrs: Cow<'a, ast::AttrVec>,
     ) -> Self {
         let inner_attr = mod_attrs
             .iter()
@@ -158,7 +158,7 @@ impl<'ast, 'sess, 'c> ModResolver<'ast, 'sess> {
                         module_item.item.span,
                         Some(Cow::Owned(sub_mod_kind.clone())),
                         Cow::Owned(vec![]),
-                        Cow::Owned(vec![]),
+                        Cow::Owned(ast::AttrVec::new()),
                     ),
                 )?;
             }
@@ -185,7 +185,7 @@ impl<'ast, 'sess, 'c> ModResolver<'ast, 'sess> {
                         span,
                         Some(Cow::Owned(sub_mod_kind.clone())),
                         Cow::Owned(vec![]),
-                        Cow::Owned(vec![]),
+                        Cow::Owned(ast::AttrVec::new()),
                     ),
                 )?;
             }

src/parse/parser.rs

@@ -109,7 +109,7 @@ impl<'a> Parser<'a> {
         sess: &'a ParseSess,
         path: &Path,
         span: Span,
-    ) -> Result<(Vec<ast::Attribute>, Vec<ptr::P<ast::Item>>, Span), ParserError> {
+    ) -> Result<(ast::AttrVec, Vec<ptr::P<ast::Item>>, Span), ParserError> {
         let result = catch_unwind(AssertUnwindSafe(|| {
             let mut parser = new_parser_from_file(sess.inner(), path, Some(span));
             match parser.parse_mod(&TokenKind::Eof) {