Rollup merge of rust-lang#55542 - ljedrz:syntax_alloc_improvements, r=kennytm

syntax: improve a few allocations

Add 2 `reserve`s and a `with_capacity` where the final vector size is known.

Author: pietroalbini

src/libsyntax/ext/expand.rs

@@ -387,6 +387,8 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
                         add_derived_markers(&mut self.cx, item.span(), &traits, item.clone());
                     let derives = derives.entry(invoc.expansion_data.mark).or_default();
 
+                    derives.reserve(traits.len());
+                    invocations.reserve(traits.len());
                     for path in &traits {
                         let mark = Mark::fresh(self.cx.current_expansion.mark);
                         derives.push(mark);

src/libsyntax_ext/deriving/generic/mod.rs

@@ -1384,7 +1384,7 @@ impl<'a> MethodDef<'a> {
             // let __self2_vi = unsafe {
             //     std::intrinsics::discriminant_value(&arg2) } as i32;
             // ```
-            let mut index_let_stmts: Vec<ast::Stmt> = Vec::new();
+            let mut index_let_stmts: Vec<ast::Stmt> = Vec::with_capacity(vi_idents.len() + 1);
 
             // We also build an expression which checks whether all discriminants are equal
             // discriminant_test = __self0_vi == __self1_vi && __self0_vi == __self2_vi && ...