Fix --pretty=expanded with --remap-path-prefix

Per https://github.com/rust-lang/rust/issues/80832, using
--pretty=expanded and --remap-path-prefix results in an ICE.

This is becasue the session source files table is stored in remapped
form, whereas --pretty-expanded looks up unremapped files. This remaps
the path prefixes before lookup.

compiler/rustc_driver/src/pretty.rs

@@ -363,8 +363,15 @@ impl<'tcx> pprust_hir::PpAnn for TypedAnnotation<'tcx> {
 
 fn get_source(input: &Input, sess: &Session) -> (String, FileName) {
     let src_name = input.source_name();
-    let src =
+    let src = String::clone(
-        String::clone(&sess.source_map().get_source_file(&src_name).unwrap().src.as_ref().unwrap());
+        &sess
+            .source_map()
+            .get_source_file(&src_name)
+            .expect("get_source_file")
+            .src
+            .as_ref()
+            .expect("src"),
+    );
     (src, src_name)
 }
 

compiler/rustc_span/src/source_map.rs

@@ -870,8 +870,10 @@ impl SourceMap {
     }
 
     pub fn get_source_file(&self, filename: &FileName) -> Option<Lrc<SourceFile>> {
+        // Remap filename before lookup
+        let filename = self.path_mapping().map_filename_prefix(filename).0;
         for sf in self.files.borrow().source_files.iter() {
-            if *filename == sf.name {
+            if filename == sf.name {
                 return Some(sf.clone());
             }
         }
@@ -1039,4 +1041,15 @@ impl FilePathMapping {
 
         (path, false)
     }
+
+    fn map_filename_prefix(&self, file: &FileName) -> (FileName, bool) {
+        match file {
+            FileName::Real(realfile) => {
+                let path = realfile.local_path();
+                let (path, mapped) = self.map_prefix(path.to_path_buf());
+                (FileName::Real(RealFileName::Named(path)), mapped)
+            }
+            other => (other.clone(), false),
+        }
+    }
 }