Auto merge of rust-lang#132662 - RalfJung:const-panic-inlining, r=<try>

tweak attributes for const panic macro

Let's do some random mutations of this macro to see if that can re-gain the perf lost in rust-lang#132542.

Author: bors

library/core/src/char/methods.rs

@@ -1774,6 +1774,7 @@ const fn len_utf16(code: u32) -> usize {
 #[cfg_attr(bootstrap, rustc_const_stable(feature = "const_char_encode_utf8", since = "1.83.0"))]
 #[doc(hidden)]
 #[inline]
+#[rustc_allow_const_fn_unstable(const_eval_select)] // just used for panics
 pub const fn encode_utf8_raw(code: u32, dst: &mut [u8]) -> &mut [u8] {
     let len = len_utf8(code);
     match (len, &mut *dst) {
@@ -1826,6 +1827,7 @@ pub const fn encode_utf8_raw(code: u32, dst: &mut [u8]) -> &mut [u8] {
 )]
 #[doc(hidden)]
 #[inline]
+#[rustc_allow_const_fn_unstable(const_eval_select)] // just used for panics
 pub const fn encode_utf16_raw(mut code: u32, dst: &mut [u16]) -> &mut [u16] {
     let len = len_utf16(code);
     match (len, &mut *dst) {

library/core/src/intrinsics.rs

@@ -2965,14 +2965,35 @@ pub(crate) macro const_eval_select {
             $(#[$compiletime_attr:meta])* $compiletime:block
         else
             $(#[$runtime_attr:meta])* $runtime:block
+    ) => {
+        // Use the `noinline` arm, after adding explicit `inline` attributes
+        $crate::intrinsics::const_eval_select!(
+            @capture { $($arg : $ty = $val),* } $(-> $ret)? :
+            #[noinline]
+            if const
+                #[inline] // prevent codegen on this function
+                $(#[$compiletime_attr])*
+                $compiletime
+            else
+                #[inline] // avoid the overhead of an extra fn call
+                $(#[$runtime_attr])*
+                $runtime
+        )
+    },
+    // With a leading #[noinline], we don't add inline attributes
+    (
+        @capture { $($arg:ident : $ty:ty = $val:expr),* $(,)? } $( -> $ret:ty )? :
+        #[noinline]
+        if const
+            $(#[$compiletime_attr:meta])* $compiletime:block
+        else
+            $(#[$runtime_attr:meta])* $runtime:block
     ) => {{
-        #[inline] // avoid the overhead of an extra fn call
         $(#[$runtime_attr])*
         fn runtime($($arg: $ty),*) $( -> $ret )? {
             $runtime
         }
 
-        #[inline] // prevent codegen on this function
         $(#[$compiletime_attr])*
         const fn compiletime($($arg: $ty),*) $( -> $ret )? {
             // Don't warn if one of the arguments is unused.

library/core/src/num/mod.rs

@@ -1457,6 +1457,7 @@ pub const fn can_not_overflow<T>(radix: u32, is_signed_ty: bool, digits: &[u8])
 #[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[cold]
 #[track_caller]
+#[rustc_allow_const_fn_unstable(const_eval_select)] // just used for panics
 const fn from_str_radix_panic(radix: u32) -> ! {
     const_panic!(
         "from_str_radix_int: must lie in the range `[2, 36]`",

library/core/src/panic.rs

@@ -201,27 +201,17 @@ pub unsafe trait PanicPayload: crate::fmt::Display {
 #[unstable(feature = "panic_internals", issue = "none")]
 #[doc(hidden)]
 pub macro const_panic {
-    ($const_msg:literal, $runtime_msg:literal, $($arg:ident : $ty:ty = $val:expr),* $(,)?) => {{
-        // Wrap call to `const_eval_select` in a function so that we can
-        // add the `rustc_allow_const_fn_unstable`. This is okay to do
-        // because both variants will panic, just with different messages.
-        #[rustc_allow_const_fn_unstable(const_eval_select)]
-        #[inline(always)]
-        #[track_caller]
-        #[cfg_attr(bootstrap, rustc_const_stable(feature = "const_panic", since = "CURRENT_RUSTC_VERSION"))]
-        const fn do_panic($($arg: $ty),*) -> ! {
-            $crate::intrinsics::const_eval_select!(
-                @capture { $($arg: $ty),* } -> !:
-                if const #[track_caller] {
-                    $crate::panic!($const_msg)
-                } else #[track_caller] {
-                    $crate::panic!($runtime_msg)
-                }
-            )
-        }
-
-        do_panic($($val),*)
-    }},
+    ($const_msg:literal, $runtime_msg:literal, $($arg:ident : $ty:ty = $val:expr),* $(,)?) => {
+        $crate::intrinsics::const_eval_select!(
+            @capture { $($arg: $ty = $val),* } -> !:
+            #[noinline]
+            if const #[track_caller] {
+                $crate::panic!($const_msg)
+            } else #[track_caller] {
+                $crate::panic!($runtime_msg)
+            }
+        )
+    },
     // We support leaving away the `val` expressions for *all* arguments
     // (but not for *some* arguments, that's too tricky).
     ($const_msg:literal, $runtime_msg:literal, $($arg:ident : $ty:ty),* $(,)?) => {

library/core/src/slice/index.rs

@@ -31,6 +31,7 @@ where
 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never), cold)]
 #[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[track_caller]
+#[rustc_allow_const_fn_unstable(const_eval_select)] // just used for panics
 const fn slice_start_index_len_fail(index: usize, len: usize) -> ! {
     const_panic!(
         "slice start index is out of range for slice",
@@ -43,6 +44,7 @@ const fn slice_start_index_len_fail(index: usize, len: usize) -> ! {
 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never), cold)]
 #[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[track_caller]
+#[rustc_allow_const_fn_unstable(const_eval_select)] // just used for panics
 const fn slice_end_index_len_fail(index: usize, len: usize) -> ! {
     const_panic!(
         "slice end index is out of range for slice",
@@ -55,6 +57,7 @@ const fn slice_end_index_len_fail(index: usize, len: usize) -> ! {
 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never), cold)]
 #[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[track_caller]
+#[rustc_allow_const_fn_unstable(const_eval_select)] // just used for panics
 const fn slice_index_order_fail(index: usize, end: usize) -> ! {
     const_panic!(
         "slice index start is larger than end",