Switch to in-tree rustc dependencies with a cfg flag

crates/parser/Cargo.toml

@@ -13,7 +13,7 @@ doctest = false
 
 [dependencies]
 drop_bomb = "0.1.5"
-rustc_lexer.workspace = true
+rustc-dependencies.workspace = true
 
 limit.workspace = true
 
@@ -22,3 +22,6 @@ expect-test = "1.4.0"
 
 stdx.workspace = true
 sourcegen.workspace = true
+
+[features]
+in-rust-tree = ["rustc-dependencies/in-rust-tree"]

crates/parser/src/lexed_str.rs

@@ -8,6 +8,7 @@
 //! Note that these tokens, unlike the tokens we feed into the parser, do
 //! include info about comments and whitespace.
 
+use rustc_dependencies::lexer as rustc_lexer;
 use std::ops;
 
 use crate::{

crates/parser/src/lib.rs

@@ -19,6 +19,7 @@
 
 #![warn(rust_2018_idioms, unused_lifetimes, semicolon_in_expressions_from_macros)]
 #![allow(rustdoc::private_intra_doc_links)]
+#![cfg_attr(feature = "in-rust-tree", feature(rustc_private))]
 
 mod lexed_str;
 mod token_set;