Correctly handle remapping from path containing the current directory with trailing paths

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -497,11 +497,20 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
                                 let working_dir = &self.tcx.sess.working_dir;
                                 match working_dir {
                                     RealFileName::LocalPath(absolute) => {
-                                        // If working_dir has not been remapped, then we emit a
-                                        // LocalPath variant as it's likely to be a valid path
-                                        RealFileName::LocalPath(
-                                            Path::new(absolute).join(path_to_file),
-                                        )
+                                        // Although neither working_dir or the file name were subject
+                                        // to path remapping, the concatenation between the two may
+                                        // be. Hence we need to do a remapping here.
+                                        let joined = Path::new(absolute).join(path_to_file);
+                                        let (joined, remapped) =
+                                            source_map.path_mapping().map_prefix(joined);
+                                        if remapped {
+                                            RealFileName::Remapped {
+                                                local_path: None,
+                                                virtual_name: joined,
+                                            }
+                                        } else {
+                                            RealFileName::LocalPath(joined)
+                                        }
                                     }
                                     RealFileName::Remapped { local_path: _, virtual_name } => {
                                         // If working_dir has been remapped, then we emit

src/test/run-make-fulldeps/remap-path-prefix/Makefile

@@ -0,0 +1,9 @@
+-include ../tools.mk
+
+# ignore-windows
+
+# Checks if remapping works if the remap-from string contains path to the working directory plus more
+all:
+	$(RUSTC) --remap-path-prefix $$PWD/auxiliary=/the/aux --crate-type=lib --emit=metadata auxiliary/lib.rs
+	grep "/the/aux/lib.rs" $(TMPDIR)/liblib.rmeta || exit 1
+	! grep "$$PWD/auxiliary" $(TMPDIR)/liblib.rmeta || exit 1

src/test/run-make-fulldeps/remap-path-prefix/auxiliary/lib.rs

@@ -0,0 +1,3 @@
+pub fn lib() {
+    panic!("calm");
+}