Removing cold/inline(never) from slice_error_fail causes a cycle error

[saethlin]

Apply this patch:

diff --git a/library/core/src/str/mod.rs b/library/core/src/str/mod.rs
index 4943bbc45d0..77ef7b9431d 100644
--- a/library/core/src/str/mod.rs
+++ b/library/core/src/str/mod.rs
@@ -80,8 +80,6 @@
 use iter::SplitInternal;
 use iter::{MatchesInternal, SplitNInternal};
 
-#[inline(never)]
-#[cold]
 #[track_caller]
 #[rustc_allow_const_fn_unstable(const_eval_select)]
 #[cfg(not(feature = "panic_immediate_abort"))]

And you'll get this query cycle error:

error[E0391]: cycle detected when optimizing MIR for `str::traits::<impl at library/core/src/str/traits.rs:330:1: 330:52>::index`
   --> library/core/src/str/traits.rs:363:5
    |
363  |     fn index(self, slice: &str) -> &Self::Output {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
note: ...which requires whether the item should be made inlinable across crates...
   --> library/core/src/str/mod.rs:86:1
    |
86   | const fn slice_error_fail(s: &str, begin: usize, end: usize) -> ! {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...which requires optimizing MIR for `str::slice_error_fail`...
   --> library/core/src/str/mod.rs:86:1
    |
86   | const fn slice_error_fail(s: &str, begin: usize, end: usize) -> ! {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...which requires whether the item should be made inlinable across crates...
   --> library/core/src/str/mod.rs:109:1
    |
109  | fn slice_error_fail_rt(s: &str, begin: usize, end: usize) -> ! {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...which requires optimizing MIR for `str::slice_error_fail_rt`...
   --> library/core/src/str/mod.rs:109:1
    |
109  | fn slice_error_fail_rt(s: &str, begin: usize, end: usize) -> ! {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: ...which again requires optimizing MIR for `str::traits::<impl at library/core/src/str/traits.rs:330:1: 330:52>::index`, completing the cycle
note: cycle used when optimizing MIR for `fmt::<impl at library/core/src/fmt/mod.rs:1238:1: 1238:23>::write_formatted_parts`
   --> library/core/src/fmt/mod.rs:1527:5
    |
1527 |     unsafe fn write_formatted_parts(&mut self, formatted: &numfmt::Formatted<'_>) -> Result {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information

There's a strange call graph here; slice_error_fail_rt which is responsible for emitting string slicing panics uses panicky string slicing. From reading the code, I'm pretty confident that the infinite recursion case is actually impossible, but our query cycle detection doesn't know or care, and is supposed to handle this case.

[tmiasko]

The cycle detection probably doesn't understand const_eval_select intrinsic. The intrinsic is lowered in optimized_mir query, while cycle detection operates on promoted_mir.

[saethlin]

That was my instinct as well. What's tripping me up mentally is that with this patch, I can x build library without a cycle error:

diff --git a/library/core/src/str/mod.rs b/library/core/src/str/mod.rs
index 4943bbc45d0..bf5539fd1ec 100644
--- a/library/core/src/str/mod.rs
+++ b/library/core/src/str/mod.rs
@@ -80,8 +80,7 @@
 use iter::SplitInternal;
 use iter::{MatchesInternal, SplitNInternal};
 
-#[inline(never)]
-#[cold]
+#[cfg_attr(bootstrap, inline(never))]
 #[track_caller]
 #[rustc_allow_const_fn_unstable(const_eval_select)]
 #[cfg(not(feature = "panic_immediate_abort"))]

There's no remap_mir_for_const_eval_select inside of the cycle detection logic. I think there should be. But I also think that since the latest beta, we accidentally fixed this somehow?

[tmiasko]

Nightly reproducer:

#![feature(core_intrinsics)]
#![feature(const_eval_select)]
use std::intrinsics::const_eval_select;
#[inline(always)]
pub const fn f() {
    const_eval_select((), g, g)
}
#[inline(always)]
pub const fn g() {
    const_eval_select((), f, f)
}

error[E0391]: cycle detected when optimizing MIR for `f`
 --> a.rs:5:1
  |
5 | pub const fn f() {
  | ^^^^^^^^^^^^^^^^
  |
note: ...which requires optimizing MIR for `g`...
 --> a.rs:9:1
  |
9 | pub const fn g() {
  | ^^^^^^^^^^^^^^^^
  = note: ...which again requires optimizing MIR for `f`, completing the cycle