Add __RUSTC_CWD within remap-path-prefix.

compiler/rustc_session/src/config.rs

@@ -1161,7 +1161,10 @@ pub fn rustc_optgroups() -> Vec<RustcOptGroup> {
         opt::multi_s(
             "",
             "remap-path-prefix",
-            "Remap source names in all output (compiler messages and output files)",
+            "Remap source names in all output (compiler messages and output files).
+            The metasyntactic variable __RUSTC_CWD can be used to represent the current
+            working directory witin the FROM pattern.
+            ",
             "FROM=TO",
         ),
     ]);
@@ -1935,11 +1938,15 @@ fn parse_remap_path_prefix(
                 error_format,
                 "--remap-path-prefix must contain '=' between FROM and TO",
             ),
-            Some((from, to)) => (PathBuf::from(from), PathBuf::from(to)),
+            Some((from, to)) => (pathbuf_understanding_pwd(from), PathBuf::from(to)),
         })
         .collect()
 }
 
+fn pathbuf_understanding_pwd(path: &str) -> PathBuf {
+    PathBuf::from(std::str::replace(path, "__RUSTC_CWD", std::env::current_dir().unwrap()))
+}
+
 pub fn build_session_options(matches: &getopts::Matches) -> Options {
     let color = parse_color(matches);
 

src/doc/rustc/src/command-line-arguments.md

@@ -345,6 +345,10 @@ replacement is purely textual, with no consideration of the current system's
 pathname syntax. For example `--remap-path-prefix foo=bar` will match
 `foo/lib.rs` but not `./foo/lib.rs`.
 
+Within the `FROM` pattern, the special string `__RUSTC_CWD` is replaced with
+the current working directory. This can be useful to ensure that the `rustc`
+command line is identical on different machines.
+
 <a id="option-json"></a>
 ## `--json`: configure json messages printed by the compiler