Add libstd and libcore Cargo features "panic_immediate_abort"

It stop asserts and panics from libstd to automatically
include string output and formatting code.

Use case: developing static executables smaller than 50 kilobytes,
where usual formatting code is excessive while keeping debuggability
in debug mode.

May resolve #54981.

2 files changed, 25 insertions, 3 deletions

diff --git a/src/libcore/Cargo.toml b/src/libcore/Cargo.toml
index 0b01cfc488b..7fd61f07d5e 100644
--- a/src/libcore/Cargo.toml
+++ b/src/libcore/Cargo.toml
@@ -21,3 +21,7 @@ path = "../libcore/benches/lib.rs"
 
 [dev-dependencies]
 rand = "0.5"
+
+[features]
+# Make panics and failed asserts immediately abort without formatting any message
+panic_immediate_abort = []
diff --git a/src/libcore/panicking.rs b/src/libcore/panicking.rs
index 58407de9566..67c0b6ada90 100644
--- a/src/libcore/panicking.rs
+++ b/src/libcore/panicking.rs
@@ -39,9 +39,15 @@
 use fmt;
 use panic::{Location, PanicInfo};
 
-#[cold] #[inline(never)] // this is the slow path, always
+#[cold]
+// inline(never) is required even in panic_immediate_abort mode, lest linker error
+#[inline(never)]
 #[lang = "panic"]
 pub fn panic(expr_file_line_col: &(&'static str, &'static str, u32, u32)) -> ! {
+    if cfg!(feature = "panic_immediate_abort") {
+        unsafe { super::intrinsics::abort() }
+    };
+
     // Use Arguments::new_v1 instead of format_args!("{}", expr) to potentially
     // reduce size overhead. The format_args! macro uses str's Display trait to
     // write expr, which calls Formatter::pad, which must accommodate string
@@ -52,16 +58,28 @@ pub fn panic(expr_file_line_col: &(&'static str, &'static str, u32, u32)) -> ! {
     panic_fmt(fmt::Arguments::new_v1(&[expr], &[]), &(file, line, col))
 }
 
-#[cold] #[inline(never)]
+#[cold]
+// inline(never) is required even in panic_immediate_abort mode, lest linker error
+#[inline(never)]
 #[lang = "panic_bounds_check"]
 fn panic_bounds_check(file_line_col: &(&'static str, u32, u32),
                      index: usize, len: usize) -> ! {
+    if cfg!(feature = "panic_immediate_abort") {
+        unsafe { super::intrinsics::abort() }
+    };
+
     panic_fmt(format_args!("index out of bounds: the len is {} but the index is {}",
                            len, index), file_line_col)
 }
 
-#[cold] #[inline(never)]
+#[cold]
+#[cfg_attr(not(feature="panic_immediate_abort"),inline(never))]
+#[cfg_attr(    feature="panic_immediate_abort" ,inline)]
 pub fn panic_fmt(fmt: fmt::Arguments, file_line_col: &(&'static str, u32, u32)) -> ! {
+    if cfg!(feature = "panic_immediate_abort") {
+        unsafe { super::intrinsics::abort() }
+    };
+
     // NOTE This function never crosses the FFI boundary; it's a Rust-to-Rust call
     #[allow(improper_ctypes)] // PanicInfo contains a trait object which is not FFI safe
     extern "Rust" {