Detect extra space in keyword for better hint

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -605,6 +605,22 @@ impl<'a> Parser<'a> {
             }
         }
 
+        if let TokenKind::Ident(prev, _) = &self.prev_token.kind
+          && let TokenKind::Ident(cur, _) = &self.token.kind
+        {
+                let concat = Symbol::intern(&format!("{}{}", prev, cur));
+                let ident = Ident::new(concat, DUMMY_SP);
+                if ident.is_used_keyword() || ident.is_reserved() || ident.is_raw_guess() {
+                    let span = self.prev_token.span.to(self.token.span);
+                    err.span_suggestion_verbose(
+                        span,
+                        format!("consider removing the space to spell keyword `{}`", concat),
+                        concat,
+                        Applicability::MachineApplicable,
+                    );
+                }
+        }
+
         // `pub` may be used for an item or `pub(crate)`
         if self.prev_token.is_ident_named(sym::public)
             && (self.token.can_begin_item()

tests/ui/suggestions/issue-89640.rs

@@ -0,0 +1,3 @@
+fn main() {
+    le t x: i32 = 3; //~ ERROR expected one of
+}

tests/ui/suggestions/issue-89640.stderr

@@ -0,0 +1,13 @@
+error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `t`
+  --> $DIR/issue-89640.rs:2:8
+   |
+LL |     le t x: i32 = 3;
+   |        ^ expected one of 8 possible tokens
+   |
+help: consider removing the space to spell keyword `let`
+   |
+LL |     let x: i32 = 3;
+   |     ~~~
+
+error: aborting due to previous error
+