bootstrap: Add and use -Z absolute-file-paths

It's unfortunate that this hack is necessary. A short summary of the background here:
- Rust-Analyzer uses `x check --json-output` to show red underlines in the editor.
- There are several different Cargo workspaces in rust-lang/rust, including src/bootstrap and src/tools/miri.
- Cargo runs each invocation of rustc relative to the *workspace*, not to the current working directory of cargo itself.
- Rustc prints file paths relative to its current working directory. As a result, it would print things like `config.rs:43:14` instead of `src/bootstrap/config.rs`.

This adds a new flag to rustc to print the files as an absolute path instead of a relative path. I went with this approach instead of one of the wrapping tools for the following reasons:
1. Cargo considers changing the working directory to be a breaking change:
   rust-lang/cargo#11007 (comment). They have open feature
   requests for adding a flag to print absolute paths, but none have been implemented.
2. Bootstrap would potentially be able to parse and rewrite the paths that cargo emits, but I don't
   want to hard-code Cargo's JSON output format in both bootstrap.py and rustbuild; unlike rustbuild,
   bootstrap.py can't use `cargo_metadata` as a library.
3. Rust-Analyzer could potentially rewrite this output, but that wouldn't fix ctrl+click on the relative path when someone runs `cargo check`.

In addition to adding and using the new flag, this also adds `local_rebuild` detection to bootstrap.py, so that I could test the change.

Before:
```
error[E0425]: cannot find value `MIRI_DEFAULT_ARGS` in crate `miri`
   --> src/bin/miri.rs:269:33
    |
269 |         args.splice(1..1, miri::MIRI_DEFAULT_ARGS.iter().map(ToString::to_string));
    |                                 ^^^^^^^^^^^^^^^^^ help: a constant with a similar name exists: `MIRI_DEFLT_ARGS`
    |
   ::: src/lib.rs:128:1
    |
128 | pub const MIRI_DEFLT_ARGS: &[&str] = &[
    | ---------------------------------- similarly named constant `MIRI_DEFLT_ARGS` defined here
```

After:
```
error[E0425]: cannot find value `MIRI_DEFAULT_ARGS` in crate `miri`
   --> /home/jyn/src/rust/src/tools/miri/src/bin/miri.rs:269:33
    |
269 |         args.splice(1..1, miri::MIRI_DEFAULT_ARGS.iter().map(ToString::to_string));
    |                                 ^^^^^^^^^^^^^^^^^ help: a constant with a similar name exists: `MIRI_DEFLT_ARGS`
    |
   ::: /home/jyn/src/rust/src/tools/miri/src/lib.rs:128:1
    |
128 | pub const MIRI_DEFLT_ARGS: &[&str] = &[
    | ---------------------------------- similarly named constant `MIRI_DEFLT_ARGS` defined here
```

Author: jyn514

compiler/rustc_driver_impl/src/lib.rs

@@ -5,6 +5,7 @@
 //! This API is completely unstable and subject to change.
 
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
+#![feature(absolute_path)]
 #![feature(lazy_cell)]
 #![feature(decl_macro)]
 #![recursion_limit = "256"]
@@ -303,7 +304,11 @@ fn run_compiler(
         registry: diagnostics_registry(),
     };
 
-    match make_input(config.opts.error_format, &matches.free) {
+    match make_input(
+        config.opts.error_format,
+        &matches.free,
+        config.opts.unstable_opts.absolute_file_paths,
+    ) {
         Err(reported) => return Err(reported),
         Ok(Some(input)) => {
             config.input = input;
@@ -476,6 +481,7 @@ fn make_output(matches: &getopts::Matches) -> (Option<PathBuf>, Option<OutFileNa
 fn make_input(
     error_format: ErrorOutputType,
     free_matches: &[String],
+    absolute_file_paths: bool,
 ) -> Result<Option<Input>, ErrorGuaranteed> {
     if free_matches.len() == 1 {
         let ifile = &free_matches[0];
@@ -503,7 +509,19 @@ fn make_input(
                 Ok(Some(Input::Str { name: FileName::anon_source_code(&src), input: src }))
             }
         } else {
-            Ok(Some(Input::File(PathBuf::from(ifile))))
+            let mut path = PathBuf::from(ifile);
+            if absolute_file_paths {
+                match std::path::absolute(path) {
+                    Ok(p) => path = p,
+                    Err(e) => {
+                        return Err(early_error_no_abort(
+                            error_format,
+                            format!("failed to convert {ifile} to an absolute path: {e}"),
+                        ));
+                    }
+                }
+            }
+            Ok(Some(Input::File(path)))
         }
     } else {
         Ok(None)

compiler/rustc_interface/src/tests.rs

@@ -657,6 +657,7 @@ fn test_unstable_options_tracking_hash() {
 
     // Make sure that changing an [UNTRACKED] option leaves the hash unchanged.
     // tidy-alphabetical-start
+    untracked!(absolute_file_paths, true);
     untracked!(assert_incr_state, Some(String::from("loaded")));
     untracked!(deduplicate_diagnostics, false);
     untracked!(dep_tasks, true);

compiler/rustc_session/src/options.rs

@@ -1352,6 +1352,8 @@ options! {
     // - src/doc/unstable-book/src/compiler-flags
 
     // tidy-alphabetical-start
+    absolute_file_paths: bool = (false, parse_bool, [UNTRACKED],
+        "use absolute file paths in diagnostics, not relative paths (default: no)"),
     allow_features: Option<Vec<String>> = (None, parse_opt_comma_list, [TRACKED],
         "only allow the listed language features to be enabled in code (comma separated)"),
     always_encode_mir: bool = (false, parse_bool, [TRACKED],

src/bootstrap/bootstrap.py

@@ -474,6 +474,7 @@ def __init__(self):
         self.verbose = False
         self.git_version = None
         self.nix_deps_dir = None
+        self.local_rebuild = False
         self._should_fix_bins_and_dylibs = None
 
     def download_toolchain(self):
@@ -894,6 +895,10 @@ def build_bootstrap(self, color, warnings, verbose_count):
         if target_linker is not None:
             env["RUSTFLAGS"] += " -C linker=" + target_linker
         env["RUSTFLAGS"] += " -Wrust_2018_idioms -Wunused_lifetimes"
+        # Make sure that rust-analyzer can map error diagnostics to the file.
+        # cfg(bootstrap)
+        if self.local_rebuild:
+            env["RUSTFLAGS"] += " -Zabsolute-file-paths"
         if warnings == "default":
             deny_warnings = self.get_toml("deny-warnings", "rust") != "false"
         else:
@@ -1054,6 +1059,17 @@ def bootstrap(args):
     if not os.path.exists(build.build_dir):
         os.makedirs(build.build_dir)
 
+    build.local_rebuild = build.get_toml("local_rebuild", "build")
+    if build.local_rebuild is None and not build.rustc().startswith(build.bin_root()):
+        with open(os.path.join(build.rust_root, "src", "version")) as f:
+            expected_version = f.read().split(".")[:2]
+        output = require([build.rustc(), "--version"]).decode(sys.getdefaultencoding())
+        actual_version = output.split(" ")[1].split(".")[:2]
+        if expected_version == actual_version:
+            if build.verbose:
+                print("auto-detected local-rebuild {}".format(actual_version))
+            build.local_rebuild = True
+
     # Fetch/build the bootstrap
     build.download_toolchain()
     sys.stdout.flush()

src/bootstrap/builder.rs

@@ -1392,6 +1392,11 @@ impl<'a> Builder<'a> {
         // cargo would implicitly add it, it was discover that sometimes bootstrap only use
         // `rustflags` without `cargo` making it required.
         rustflags.arg("-Zunstable-options");
+        // Make ctrl+click, etc. work with workspaces other than the root workspace.
+        // cfg(bootstrap)
+        if stage > 0 {
+            rustflags.arg("-Zabsolute-file-paths");
+        }
         for (restricted_mode, name, values) in EXTRA_CHECK_CFGS {
             if *restricted_mode == None || *restricted_mode == Some(mode) {
                 // Creating a string of the values by concatenating each value:

src/doc/unstable-book/src/compiler-flags/absolute-file-paths.md

@@ -0,0 +1,9 @@
+# `absolute-file-paths`
+
+This features is currently perma-unstable, with no tracking issue.
+
+------------------------
+
+This feature allows you to configure rustc to print diagnostics and other output using absolute file paths, not relative file paths.
+
+Set `-Zabsolute-file-paths` to enable this feature.