internal: Use basic NonEmptyVec in mbe::syntax_bridge

crates/stdx/src/non_empty_vec.rs

@@ -0,0 +1,45 @@
+//! A [`Vec`] that is guaranteed to at least contain one element.
+
+pub struct NonEmptyVec<T>(Vec<T>);
+
+impl<T> NonEmptyVec<T> {
+    #[inline]
+    pub fn new(initial: T) -> Self {
+        NonEmptyVec(vec![initial])
+    }
+
+    #[inline]
+    pub fn last_mut(&mut self) -> &mut T {
+        match self.0.last_mut() {
+            Some(it) => it,
+            None => unreachable!(),
+        }
+    }
+
+    #[inline]
+    pub fn pop(&mut self) -> Option<T> {
+        if self.0.len() <= 1 {
+            None
+        } else {
+            self.0.pop()
+        }
+    }
+
+    #[inline]
+    pub fn push(&mut self, value: T) {
+        self.0.push(value)
+    }
+
+    #[inline]
+    pub fn len(&self) -> usize {
+        self.0.len()
+    }
+
+    #[inline]
+    pub fn into_first(mut self) -> T {
+        match self.0.pop() {
+            Some(it) => it,
+            None => unreachable!(),
+        }
+    }
+}