From 01d7f87d8ee16915d8f663f44a5587ec45f54f76 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?D=C3=A1niel=20Buga?= <bugadani@gmail.com>
Date: Sun, 20 Dec 2020 00:50:06 +0100
Subject: [PATCH] Reserve necessary space for params in generics_of

---
 compiler/rustc_typeck/src/collect.rs | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/compiler/rustc_typeck/src/collect.rs b/compiler/rustc_typeck/src/collect.rs
index 543e39af66944..8f4f4fdcbe787 100644
--- a/compiler/rustc_typeck/src/collect.rs
+++ b/compiler/rustc_typeck/src/collect.rs
@@ -1369,7 +1369,11 @@ fn generics_of(tcx: TyCtxt<'_>, def_id: DefId) -> ty::Generics {
         generics.parent_count + generics.params.len()
     });
 
-    let mut params: Vec<_> = opt_self.into_iter().collect();
+    let mut params: Vec<_> = Vec::with_capacity(ast_generics.params.len() + has_self as usize);
+
+    if let Some(opt_self) = opt_self {
+        params.push(opt_self);
+    }
 
     let early_lifetimes = early_bound_lifetimes_from_generics(tcx, ast_generics);
     params.extend(early_lifetimes.enumerate().map(|(i, param)| ty::GenericParamDef {