Add diagnostics test for mistyped inclusive ranges

src/test/ui/parser/range-inclusive-extra-equals.rs

@@ -0,0 +1,10 @@
+// Makes sure that a helpful message is shown when someone mistypes
+// an inclusive range as `..==` rather than `..=`. This is an
+// easy mistake, because of the resemblance to`==`.
+// See #86395 for a bit of background.
+
+pub fn main() {
+    if let 1..==3 = 1 {} //~ERROR unexpected `=` after inclusive range
+                      //~|HELP use `..=` instead
+                      //~|NOTE inclusive ranges end with a single equals sign
+}

src/test/ui/parser/range-inclusive-extra-equals.stderr

@@ -0,0 +1,10 @@
+error: unexpected `=` after inclusive range
+  --> $DIR/range-inclusive-extra-equals.rs:7:13
+   |
+LL |     if let 1..==3 = 1 {}
+   |             ^^^^ help: use `..=` instead
+   |
+   = note: inclusive ranges end with a single equals sign (`..=`)
+
+error: aborting due to previous error
+