Don't ICE when getting an input file name's stem fails

compiler/rustc_session/src/config.rs

@@ -838,10 +838,14 @@ pub enum Input {
 
 impl Input {
     pub fn filestem(&self) -> &str {
-        match *self {
-            Input::File(ref ifile) => ifile.file_stem().unwrap().to_str().unwrap(),
-            Input::Str { .. } => "rust_out",
+        if let Input::File(ifile) = self {
+            // If for some reason getting the file stem as a UTF-8 string fails,
+            // then fallback to a fixed name.
+            if let Some(name) = ifile.file_stem().and_then(OsStr::to_str) {
+                return name;
+            }
         }
+        "rust_out"
     }
 
     pub fn source_name(&self) -> FileName {

tests/run-make/dos-device-input/rmake.rs

@@ -0,0 +1,13 @@
+//@ only-windows
+// Reason: dos devices are a Windows thing
+
+use std::path::Path;
+
+use run_make_support::{rustc, static_lib_name};
+
+fn main() {
+    rustc().input(r"\\.\NUL").crate_type("staticlib").run();
+    rustc().input(r"\\?\NUL").crate_type("staticlib").run();
+
+    assert!(Path::new(&static_lib_name("rust_out")).exists());
+}