[ty] Fix range calculation in variable binding completions

Previously the cursor position was calculated using the length
of the typed token even if the cursor position was in the
middle of the token (f<CURSOR>oo).

I haven't managed to construct a test based on autocomplete suggestions
that would fail because of this but if you test the
`is_in_variable_binding` function directly it becomes clear
that the previous calculation was not entirely correct.

Author: RasmusNygren

crates/ty_ide/src/completion.rs

@@ -1341,7 +1341,18 @@ fn is_in_definition_place(
 /// E.g. naming a parameter, type parameter, or `for` <name>).
 fn is_in_variable_binding(parsed: &ParsedModuleRef, offset: TextSize, typed: Option<&str>) -> bool {
     let range = if let Some(typed) = typed {
-        let start = offset.saturating_sub(typed.text_len());
+        let tokens = tokens_start_before(parsed.tokens(), offset);
+        // `typed` contains the entire token even if the cursor sits in the
+        // middle of it (pa<CURSOR>ram). To correctly suggest completions
+        // based on the cursor's current position, we must only consider
+        // the characters in the token that are left of the cursor.
+        // E.g. we should only count "pa" if the state is "pa<CUSROR>ram".
+        let typed_len = typed
+            .text_len()
+            .min(tokens.last().map_or(typed.text_len(), |token| {
+                offset.saturating_sub(token.start())
+            }));
+        let start = offset.saturating_sub(typed_len);
         TextRange::new(start, offset)
     } else {
         TextRange::empty(offset)