Auto merge of #144587 - petrochenkov:optstdprel, r=nnethercote

expand: Micro-optimize prelude injection

Use `splice` to avoid shifting the other items twice.
Put `extern crate std;` first so it's already resolved when we resolve `::std::prelude::rust_20XX`.

Author: bors

compiler/rustc_builtin_macros/src/standard_library_imports.rs

@@ -47,8 +47,6 @@ pub fn inject(
         ast::ItemKind::ExternCrate(None, Ident::new(name, ident_span)),
     );
 
-    krate.items.insert(0, item);
-
     let root = (edition == Edition2015).then_some(kw::PathRoot);
 
     let import_path = root
@@ -75,6 +73,6 @@ pub fn inject(
         }),
     );
 
-    krate.items.insert(0, use_item);
+    krate.items.splice(0..0, [item, use_item]);
     krate.items.len() - orig_num_items
 }

tests/pretty/asm.pp

@@ -1,9 +1,9 @@
 #![feature(prelude_import)]
 #![no_std]
-#[prelude_import]
-use ::std::prelude::rust_2015::*;
 #[macro_use]
 extern crate std;
+#[prelude_import]
+use ::std::prelude::rust_2015::*;
 //@ pretty-mode:expanded
 //@ pp-exact:asm.pp
 //@ only-x86_64

tests/pretty/cast-lt.pp

@@ -1,9 +1,9 @@
 #![feature(prelude_import)]
 #![no_std]
-#[prelude_import]
-use ::std::prelude::rust_2015::*;
 #[macro_use]
 extern crate std;
+#[prelude_import]
+use ::std::prelude::rust_2015::*;
 //@ pretty-compare-only
 //@ pretty-mode:expanded
 //@ pp-exact:cast-lt.pp

tests/pretty/dollar-crate.pp

@@ -1,9 +1,9 @@
 #![feature(prelude_import)]
 #![no_std]
-#[prelude_import]
-use ::std::prelude::rust_2015::*;
 #[macro_use]
 extern crate std;
+#[prelude_import]
+use ::std::prelude::rust_2015::*;
 //@ pretty-compare-only
 //@ pretty-mode:expanded
 //@ pp-exact:dollar-crate.pp

tests/pretty/expanded-and-path-remap-80832.pp

@@ -1,9 +1,9 @@
 #![feature(prelude_import)]
 #![no_std]
-#[prelude_import]
-use ::std::prelude::rust_2015::*;
 #[macro_use]
 extern crate std;
+#[prelude_import]
+use ::std::prelude::rust_2015::*;
 // Test for issue 80832
 //
 //@ pretty-mode:expanded

tests/pretty/format-args-str-escape.pp

@@ -1,9 +1,9 @@
 #![feature(prelude_import)]
 #![no_std]
-#[prelude_import]
-use ::std::prelude::rust_2015::*;
 #[macro_use]
 extern crate std;
+#[prelude_import]
+use ::std::prelude::rust_2015::*;
 //@ pretty-compare-only
 //@ pretty-mode:expanded
 //@ pp-exact:format-args-str-escape.pp

tests/pretty/hir-delegation.pp

@@ -4,10 +4,10 @@
 
 #![allow(incomplete_features)]
 #![feature(fn_delegation)]
-#[prelude_import]
-use ::std::prelude::rust_2015::*;
 #[attr = MacroUse {arguments: UseAll}]
 extern crate std;
+#[prelude_import]
+use ::std::prelude::rust_2015::*;
 
 fn b<C>(e: C) { }
 

tests/pretty/hir-fn-params.pp

@@ -1,7 +1,7 @@
-#[prelude_import]
-use ::std::prelude::rust_2015::*;
 #[attr = MacroUse {arguments: UseAll}]
 extern crate std;
+#[prelude_import]
+use ::std::prelude::rust_2015::*;
 //@ pretty-compare-only
 //@ pretty-mode:hir
 //@ pp-exact:hir-fn-params.pp

tests/pretty/hir-fn-variadic.pp

@@ -3,10 +3,10 @@
 //@ pp-exact:hir-fn-variadic.pp
 
 #![feature(c_variadic)]
-#[prelude_import]
-use ::std::prelude::rust_2015::*;
 #[attr = MacroUse {arguments: UseAll}]
 extern crate std;
+#[prelude_import]
+use ::std::prelude::rust_2015::*;
 
 extern "C" {
     unsafe fn foo(x: i32, va1: ...);

tests/pretty/hir-if-else.pp

@@ -1,7 +1,7 @@
-#[prelude_import]
-use ::std::prelude::rust_2015::*;
 #[attr = MacroUse {arguments: UseAll}]
 extern crate std;
+#[prelude_import]
+use ::std::prelude::rust_2015::*;
 //@ pretty-compare-only
 //@ pretty-mode:hir
 //@ pp-exact:hir-if-else.pp