Rollup merge of rust-lang#66694 - RalfJung:panic-comments, r=Dylan-DPC

Add some comments to panic runtime

Author: pietroalbini

src/libcore/panicking.rs

@@ -6,8 +6,7 @@
 //! interface for panicking is:
 //!
 //! ```
-//! # use std::fmt;
-//! fn panic_impl(fmt: fmt::Arguments, file_line_col: &(&'static str, u32, u32)) -> !
+//! fn panic_impl(pi: &core::panic::PanicInfo<'_>) -> !
 //! # { loop {} }
 //! ```
 //!
@@ -35,7 +34,7 @@ use crate::panic::{Location, PanicInfo};
 // never inline unless panic_immediate_abort to avoid code
 // bloat at the call sites as much as possible
 #[cfg_attr(not(feature="panic_immediate_abort"),inline(never))]
-#[lang = "panic"]
+#[lang = "panic"] // needed by codegen for panic on overflow and other `Assert` MIR terminators
 pub fn panic(expr: &str, location: &Location<'_>) -> ! {
     if cfg!(feature = "panic_immediate_abort") {
         unsafe { super::intrinsics::abort() }
@@ -52,7 +51,7 @@ pub fn panic(expr: &str, location: &Location<'_>) -> ! {
 
 #[cold]
 #[cfg_attr(not(feature="panic_immediate_abort"),inline(never))]
-#[lang = "panic_bounds_check"]
+#[lang = "panic_bounds_check"] // needed by codegen for panic on OOB array/slice access
 fn panic_bounds_check(location: &Location<'_>, index: usize, len: usize) -> ! {
     if cfg!(feature = "panic_immediate_abort") {
         unsafe { super::intrinsics::abort() }

src/libstd/panicking.rs

@@ -44,6 +44,8 @@ extern {
                                 data: *mut u8,
                                 data_ptr: *mut usize,
                                 vtable_ptr: *mut usize) -> u32;
+
+    /// `payload` is actually a `*mut &mut dyn BoxMeUp` but that would cause FFI warnings.
     #[unwind(allowed)]
     fn __rust_start_panic(payload: usize) -> u32;
 }
@@ -294,7 +296,7 @@ pub fn panicking() -> bool {
     update_panic_count(0) != 0
 }
 
-/// Entry point of panic from the libcore crate.
+/// Entry point of panic from the libcore crate (`panic_impl` lang item).
 #[cfg(not(test))]
 #[panic_handler]
 #[unwind(allowed)]
@@ -380,7 +382,7 @@ fn continue_panic_fmt(info: &PanicInfo<'_>) -> ! {
 #[unstable(feature = "libstd_sys_internals",
            reason = "used by the panic! macro",
            issue = "0")]
-#[cfg_attr(not(test), lang = "begin_panic")]
+#[cfg_attr(not(test), lang = "begin_panic")] // lang item for CTFE panic support
 // never inline unless panic_immediate_abort to avoid code
 // bloat at the call sites as much as possible
 #[cfg_attr(not(feature="panic_immediate_abort"),inline(never))]