Auto merge of #34096 - eddyb:launch, r=nikomatsakis

Switch to MIR-based translation by default.

This patch makes `-Z orbit` default to "on", which means that by default, functions will be translated from Rust to LLVM IR through the upcoming MIR backend, instead of the antiquated AST backend.

This switch is made possible by the recently merged #33622, #33905 and smaller fixes.

If you experience any issues, please file a report for each of them. You can switch to the old backend to work around problems by either setting `RUSTFLAGS="-Zorbit=off"` or by annotating specific functions with `#[rustc_no_mir]` (which requires `#![feature(rustc_attrs)]` at the crate-level).

I would like this PR to get into nightly soon so that we can get early feedback in this release cycle and focus on correctness fixes and performance improvements, with the potential for removing the old backend implementation before beta branches off.

cc @rust-lang/compiler

configure

@@ -609,7 +609,7 @@ opt dist-host-only 0 "only install bins for the host architecture"
 opt inject-std-version 1 "inject the current compiler version of libstd into programs"
 opt llvm-version-check 1 "check if the LLVM version is supported, build anyway"
 opt rustbuild 0 "use the rust and cargo based build system"
-opt orbit 0 "get MIR where it belongs - everywhere; most importantly, in orbit"
+opt orbit 1 "get MIR where it belongs - everywhere; most importantly, in orbit"
 opt codegen-tests 1 "run the src/test/codegen tests"
 opt option-checking 1 "complain about unrecognized options in this configure script"
 opt ninja 0 "build LLVM using the Ninja generator (for MSVC, requires building in the correct environment)"
@@ -733,7 +733,7 @@ if [ -n "$CFG_ENABLE_DEBUG_ASSERTIONS" ]; then putvar CFG_ENABLE_DEBUG_ASSERTION
 if [ -n "$CFG_ENABLE_DEBUGINFO" ]; then putvar CFG_ENABLE_DEBUGINFO; fi
 if [ -n "$CFG_ENABLE_DEBUG_JEMALLOC" ]; then putvar CFG_ENABLE_DEBUG_JEMALLOC; fi
 
-if [ -n "$CFG_ENABLE_ORBIT" ]; then putvar CFG_ENABLE_ORBIT; fi
+if [ -n "$CFG_DISABLE_ORBIT" ]; then putvar CFG_DISABLE_ORBIT; fi
 
 step_msg "looking for build programs"
 

mk/main.mk

@@ -162,9 +162,9 @@ ifdef CFG_ENABLE_DEBUGINFO
   CFG_RUSTC_FLAGS += -g
 endif
 
-ifdef CFG_ENABLE_ORBIT
-  $(info cfg: launching MIR (CFG_ENABLE_ORBIT))
-  CFG_RUSTC_FLAGS += -Z orbit
+ifdef CFG_DISABLE_ORBIT
+  $(info cfg: HOLD HOLD HOLD (CFG_DISABLE_ORBIT))
+  CFG_RUSTC_FLAGS += -Z orbit=off
 endif
 
 ifdef SAVE_TEMPS

src/doc/book/lang-items.md

@@ -19,6 +19,8 @@ sugar for dynamic allocations via `malloc` and `free`:
 #![feature(lang_items, box_syntax, start, libc)]
 #![no_std]
 
+# #![feature(panic_unwind)]
+# extern crate unwind;
 extern crate libc;
 
 extern {

src/librustc/session/config.rs

@@ -756,7 +756,7 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
           "dump MIR state at various points in translation"),
     dump_mir_dir: Option<String> = (None, parse_opt_string,
           "the directory the MIR is dumped into"),
-    orbit: bool = (false, parse_bool,
+    orbit: bool = (true, parse_bool,
           "get MIR where it belongs - everywhere; most importantly, in orbit"),
 }