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/completion/src/completions/qualified_path.rs

@@ -590,8 +590,7 @@ fn main() { let _ = crate::$0 }
         "#,
             expect![[r##"
                 fn main()  fn main()
-                ma foo!(…) #[macro_export]
-                macro_rules! foo
+                ma foo!(…) #[macro_export] macro_rules! foo
             "##]],
         );
     }

crates/completion/src/completions/unqualified_path.rs

@@ -540,8 +540,7 @@ mod macros {
 "#,
             expect![[r##"
                 fn f()        fn f()
-                ma concat!(…) #[macro_export]
-                macro_rules! concat
+                ma concat!(…) #[macro_export] macro_rules! concat
                 md std
             "##]],
         );
@@ -597,8 +596,7 @@ fn main() { let v = $0 }
 "#,
             expect![[r##"
                 md m1
-                ma baz!(…) #[macro_export]
-                macro_rules! baz
+                ma baz!(…) #[macro_export] macro_rules! baz
                 fn main()  fn main()
                 md m2
                 ma bar!(…) macro_rules! bar

crates/completion/src/item.rs

@@ -7,6 +7,7 @@ use ide_db::helpers::{
     insert_use::{self, ImportScope, MergeBehavior},
     mod_path_to_ast, SnippetCap,
 };
+use stdx::assert_never;
 use syntax::{algo, TextRange};
 use text_edit::TextEdit;
 
@@ -396,6 +397,9 @@ impl Builder {
     }
     pub(crate) fn set_detail(mut self, detail: Option<impl Into<String>>) -> Builder {
         self.detail = detail.map(Into::into);
+        if let Some(detail) = &self.detail {
+            assert_never!(detail.contains('\n'), "multiline detail: {}", detail);
+        }
         self
     }
     #[allow(unused)]