internal: use idiomatic form of assertions

Ideally, we should just return an InvalidParameter dialog here, but that
shows error message to the end user, and we generally avoid that

crates/rust-analyzer/src/handlers.rs

@@ -7,6 +7,7 @@ use std::{
     process::{self, Stdio},
 };
 
+use always_assert::always;
 use ide::{
     AnnotationConfig, AssistKind, AssistResolveStrategy, FileId, FilePosition, FileRange,
     HoverAction, HoverGotoTypeData, Query, RangeInfo, Runnable, RunnableKind, SingleResolve,
@@ -265,10 +266,11 @@ pub(crate) fn handle_on_type_formatting(
     // `text.char_at(position) == typed_char`.
     position.offset -= TextSize::of('.');
     let char_typed = params.ch.chars().next().unwrap_or('\0');
-    assert!({
+
     let text = snap.analysis.file_text(position.file_id)?;
-        text[usize::from(position.offset)..].starts_with(char_typed)
-    });
+    if !always!(text[usize::from(position.offset)..].starts_with(char_typed)) {
+        return Ok(None);
+    }
 
     // We have an assist that inserts ` ` after typing `->` in `fn foo() ->{`,
     // but it requires precise cursor positioning to work, and one can't

docs/dev/style.md

@@ -260,7 +260,9 @@ if idx >= len {
 ## Assertions
 
 Assert liberally.
-Prefer `stdx::never!` to standard `assert!`.
+Prefer [`stdx::never!`](https://docs.rs/always-assert/0.1.2/always_assert/macro.never.html) to standard `assert!`.
+
+**Rationale:** See [cross cutting concern: error handling](https://github.com/rust-analyzer/rust-analyzer/blob/master/docs/dev/architecture.md#error-handling).
 
 ## Getters & Setters