Fix verbatim paths used with include!

When using `concat!` to join paths, the Unix path separator (`/`) is often used. This breaks on Windows if the base path is a verbatim path (i.e. starts with `\\?\`).

compiler/rustc_expand/src/base.rs

@@ -1,5 +1,6 @@
 use std::default::Default;
 use std::iter;
+use std::path::Component::Prefix;
 use std::path::{Path, PathBuf};
 use std::rc::Rc;
 
@@ -1293,7 +1294,12 @@ pub fn resolve_path(sess: &Session, path: impl Into<PathBuf>, span: Span) -> PRe
         base_path.push(path);
         Ok(base_path)
     } else {
-        Ok(path)
+        // This ensures that Windows verbatim paths are fixed if mixed path separators are used,
+        // which can happen when `concat!` is used to join paths.
+        match path.components().next() {
+            Some(Prefix(prefix)) if prefix.kind().is_verbatim() => Ok(path.components().collect()),
+            _ => Ok(path),
+        }
     }
 }
 

tests/run-make/import-macro-verbatim/include/include.txt

@@ -0,0 +1 @@
+static TEST: &str = "Hello World!";

tests/run-make/import-macro-verbatim/rmake.rs

@@ -0,0 +1,8 @@
+//@ only-windows other platforms do not have Windows verbatim paths
+use run_make_support::rustc;
+fn main() {
+    // Canonicalizing the path ensures that it's verbatim (i.e. starts with `\\?\`)
+    let mut path = std::fs::canonicalize(file!()).unwrap();
+    path.pop();
+    rustc().input("verbatim.rs").env("VERBATIM_DIR", path).run();
+}

tests/run-make/import-macro-verbatim/verbatim.rs

@@ -0,0 +1,12 @@
+//! Include a file by concating the verbatim path using `/` instead of `\`
+
+include!(concat!(env!("VERBATIM_DIR"), "/include/include.txt"));
+fn main() {
+    assert_eq!(TEST, "Hello World!");
+
+    let s = include_str!(concat!(env!("VERBATIM_DIR"), "/include/include.txt"));
+    assert_eq!(s, "static TEST: &str = \"Hello World!\";\n");
+
+    let b = include_bytes!(concat!(env!("VERBATIM_DIR"), "/include/include.txt"));
+    assert_eq!(b, b"static TEST: &str = \"Hello World!\";\n");
+}