Introduce more appropriate assertion mechanism

rust-analyzer is a long-running program, so we *should* handle assertion
failures.

See also https://www.sqlite.org/assert.html.

crates/stdx/src/macros.rs

@@ -1,4 +1,9 @@
 //! Convenience macros.
+
+use std::{
+    fmt, mem, panic,
+    sync::atomic::{AtomicUsize, Ordering::SeqCst},
+};
 #[macro_export]
 macro_rules! eprintln {
     ($($tt:tt)*) => {{
@@ -44,3 +49,50 @@ macro_rules! impl_from {
         )*
     }
 }
+
+/// A version of `assert!` macro which allows to handle an assertion failure.
+///
+/// In release mode, it returns the condition and logs an error.
+///
+/// ```
+/// if assert_never!(impossible) {
+///     // Heh, this shouldn't have happened, but lets try to soldier on...
+///     return None;
+/// }
+/// ```
+///
+/// Rust analyzer is a long-running process, and crashing really isn't an option.
+///
+/// Shamelessly stolen from: https://www.sqlite.org/assert.html
+#[macro_export]
+macro_rules! assert_never {
+    ($cond:expr) => { $crate::assert_always!($cond, "") };
+    ($cond:expr, $($fmt:tt)*) => {{
+        let value = $cond;
+        if value {
+            $crate::on_assert_failure(
+                format_args!($($fmt)*)
+            );
+        }
+        value
+    }};
+}
+
+type AssertHook = fn(&panic::Location<'_>, fmt::Arguments<'_>);
+static HOOK: AtomicUsize = AtomicUsize::new(0);
+
+pub fn set_assert_hook(hook: AssertHook) {
+    HOOK.store(hook as usize, SeqCst);
+}
+
+#[cold]
+#[track_caller]
+pub fn on_assert_failure(args: fmt::Arguments) {
+    let hook: usize = HOOK.load(SeqCst);
+    if hook == 0 {
+        panic!("\n  assertion failed: {}\n", args);
+    }
+
+    let hook: AssertHook = unsafe { mem::transmute::<usize, AssertHook>(hook) };
+    hook(panic::Location::caller(), args)
+}