Merge #6601

6601: add let and letm postfix to turn expressions into variables r=matklad a=bnjjj

Partially resolve #6426 

Co-authored-by: Benjamin Coenen <5719034+bnjjj@users.noreply.github.com>

crates/completion/src/lib.rs

@@ -44,6 +44,8 @@ pub use crate::{
 // - `expr.while` -> `while expr {}` or `while let ... {}` for `Option` or `Result`
 // - `expr.ref` -> `&expr`
 // - `expr.refm` -> `&mut expr`
+// - `expr.let` -> `let <|> = expr;`
+// - `expr.letm` -> `let mut <|> = expr;`
 // - `expr.not` -> `!expr`
 // - `expr.dbg` -> `dbg!(expr)`
 // - `expr.dbgr` -> `dbg!(&expr)`