Remove the one use of P<[T]>.

A `Vec` is fine, the additional word (vector vs. boxed slice) doesn't
matter here.

Author: nnethercote

compiler/rustc_ast/src/ast.rs

@@ -1714,7 +1714,7 @@ pub enum ExprKind {
     ///
     /// Usually not written directly in user code but
     /// indirectly via the macro `core::mem::offset_of!(...)`.
-    OffsetOf(P<Ty>, P<[Ident]>),
+    OffsetOf(P<Ty>, Vec<Ident>),
 
     /// A macro invocation; pre-expansion.
     MacCall(P<MacCall>),

compiler/rustc_parse/src/parser/expr.rs

@@ -1119,7 +1119,7 @@ impl<'a> Parser<'a> {
     /// Parse the field access used in offset_of, matched by `$(e:expr)+`.
     /// Currently returns a list of idents. However, it should be possible in
     /// future to also do array indices, which might be arbitrary expressions.
-    fn parse_floating_field_access(&mut self) -> PResult<'a, P<[Ident]>> {
+    fn parse_floating_field_access(&mut self) -> PResult<'a, Vec<Ident>> {
         let mut fields = Vec::new();
         let mut trailing_dot = None;