Adjust tidy edition lint to force 2021

This has a few exceptions today (library crates, a few submodules), but is
mostly accurate.

Author: camelid

src/tools/tidy/src/edition.rs

@@ -1,10 +1,15 @@
-//! Tidy check to ensure that crate `edition` is '2018'
+//! Tidy check to ensure that crate `edition` is '2018' or '2021'.
 
 use std::path::Path;
 
 fn is_edition_2018(mut line: &str) -> bool {
     line = line.trim();
-    line == "edition = \"2018\"" || line == "edition = \'2018\'"
+    line == "edition = \"2018\""
+}
+
+fn is_edition_2021(mut line: &str) -> bool {
+    line = line.trim();
+    line == "edition = \"2021\""
 }
 
 pub fn check(path: &Path, bad: &mut bool) {
@@ -13,17 +18,38 @@ pub fn check(path: &Path, bad: &mut bool) {
         &mut |path| super::filter_dirs(path) || path.ends_with("src/test"),
         &mut |entry, contents| {
             let file = entry.path();
+            let filestr = file.to_string_lossy().replace("\\", "/");
             let filename = file.file_name().unwrap();
             if filename != "Cargo.toml" {
                 return;
             }
-            let has_edition = contents.lines().any(is_edition_2018);
-            if !has_edition {
-                tidy_error!(
-                    bad,
-                    "{} doesn't have `edition = \"2018\"` on a separate line",
-                    file.display()
-                );
+
+            // Library crates are not yet ready to migrate to 2021.
+            //
+            // The reference and rustc-dev-guide are submodules, so are left at
+            // 2018 for now. They should be removed from this exception list
+            // when bumped.
+            if path.components().any(|c| c.as_os_str() == "library")
+                || filestr.contains("src/doc/reference/style-check/Cargo.toml")
+                || filestr.contains("src/doc/rustc-dev-guide/ci/date-check/Cargo.toml")
+            {
+                let has = contents.lines().any(is_edition_2018);
+                if !has {
+                    tidy_error!(
+                        bad,
+                        "{} doesn't have `edition = \"2018\"` on a separate line",
+                        file.display()
+                    );
+                }
+            } else {
+                let is_2021 = contents.lines().any(is_edition_2021);
+                if !is_2021 {
+                    tidy_error!(
+                        bad,
+                        "{} doesn't have `edition = \"2021\"` on a separate line",
+                        file.display()
+                    );
+                }
             }
         },
     );