diff --git a/Cargo.lock b/Cargo.lock
index 0e12e81904c..88bce36d9a5 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -553,6 +553,12 @@ version = "0.6.12"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "11a7e20d1cce64ef2fed88b66d347f88bd9babb82845b2b858f3edbf59a4f716"
 
+[[package]]
+name = "relative-path"
+version = "1.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a479d53d7eed831f3c92ca79c61002d5987e21417d528296832f802bca532380"
+
 [[package]]
 name = "rust-argon2"
 version = "0.5.1"
@@ -603,6 +609,7 @@ dependencies = [
  "lazy_static",
  "log",
  "regex",
+ "relative-path",
  "rustc-workspace-hack",
  "rustfmt-config_proc_macro",
  "serde",
diff --git a/Cargo.toml b/Cargo.toml
index 24b3b79343b..19a35c62067 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -58,6 +58,7 @@ rustfmt-config_proc_macro = { version = "0.2", path = "config_proc_macro" }
 lazy_static = "1.0.0"
 anyhow = "1.0"
 thiserror = "1.0"
+relative-path = "1.4.0"
 
 # A noop dependency that changes in the Rust repository, it's a bit of a hack.
 # See the `src/tools/rustc-workspace-hack/README.md` file in `rust-lang/rust`
diff --git a/rust-toolchain b/rust-toolchain
index b7d71e79b1b..d0654ca157b 100644
--- a/rust-toolchain
+++ b/rust-toolchain
@@ -1,3 +1,3 @@
 [toolchain]
-channel = "nightly-2021-05-04"
+channel = "nightly-2021-05-01"
 components = ["rustc-dev"]
diff --git a/src/syntux/parser.rs b/src/syntux/parser.rs
index 0b94749f3c6..a0ba8814d4d 100644
--- a/src/syntux/parser.rs
+++ b/src/syntux/parser.rs
@@ -1,6 +1,7 @@
 use std::panic::{catch_unwind, AssertUnwindSafe};
 use std::path::{Path, PathBuf};
 
+use relative_path::RelativePath;
 use rustc_ast::token::{DelimToken, TokenKind};
 use rustc_ast::{ast, ptr};
 use rustc_errors::Diagnostic;
@@ -94,16 +95,21 @@ pub(crate) enum ParserError {
 }
 
 impl<'a> Parser<'a> {
+    // On windows, if the base path is specified with the windows path separator
+    // replace it with the expected '//' so that path traversal works as
+    // expected.
+    #[cfg(windows)]
     pub(crate) fn submod_path_from_attr(attrs: &[ast::Attribute], path: &Path) -> Option<PathBuf> {
-        let path_string = first_attr_value_str_by_name(attrs, sym::path)?.as_str();
-        // On windows, the base path might have the form
-        // `\\?\foo\bar` in which case it does not tolerate
-        // mixed `/` and `\` separators, so canonicalize
-        // `/` to `\`.
-        #[cfg(windows)]
-        let path_string = path_string.replace("/", "\\");
-
-        Some(path.join(&*path_string))
+        let mod_path = first_attr_value_str_by_name(attrs, sym::path)?
+            .as_str()
+            .replace(std::path::MAIN_SEPARATOR, "/");
+        Some(RelativePath::new(&mod_path).to_logical_path(&path))
+    }
+
+    #[cfg(not(windows))]
+    pub(crate) fn submod_path_from_attr(attrs: &[ast::Attribute], path: &Path) -> Option<PathBuf> {
+        let mod_path = first_attr_value_str_by_name(attrs, sym::path)?.as_str();
+        Some(RelativePath::new(&mod_path).to_logical_path(&path))
     }
 
     pub(crate) fn parse_file_as_module(