Allow the customization of sysroot_src in the generated rust-project.… (#3306)

When specifying a local rust toolchain with an unconventional rust-src
location; this attribute will allow you to influence what the
sysroot_src path in the resulting rust-project.json.

In my particular case I'm working in a conda environment which I
configure as a new_local_repo in my project.

There I define a rust toolchain and my rustc sources live in
`$CONDA_PREFIX/lib/rustlib/src/rust/library`. By default, if I'm unable
to provide this path, the resulting sysroot_src in my project ends up
resolving to the equivalent of `$CONDA_PREFIX/library`.

---------

Co-authored-by: Daniel Wagner-Hall <dawagner@gmail.com>

Author: photex

rust/private/rust_analyzer.bzl

@@ -298,6 +298,7 @@ def _rust_analyzer_toolchain_impl(ctx):
         proc_macro_srv = ctx.executable.proc_macro_srv,
         rustc = ctx.executable.rustc,
         rustc_srcs = ctx.attr.rustc_srcs,
+        rustc_srcs_path = ctx.attr.rustc_srcs_path,
     )
 
     return [toolchain]
@@ -323,6 +324,10 @@ rust_analyzer_toolchain = rule(
             doc = "The source code of rustc.",
             mandatory = True,
         ),
+        "rustc_srcs_path": attr.string(
+            doc = "The direct path to rustc srcs relative to rustc_srcs package root.",
+            default = "library",
+        ),
     },
 )
 
@@ -336,8 +341,8 @@ def _rust_analyzer_detect_sysroot_impl(ctx):
         )
 
     rustc_srcs = rust_analyzer_toolchain.rustc_srcs
+    sysroot_src = rustc_srcs.label.package + "/" + rust_analyzer_toolchain.rustc_srcs_path
 
-    sysroot_src = rustc_srcs.label.package + "/library"
     if rustc_srcs.label.workspace_root:
         sysroot_src = _OUTPUT_BASE_TEMPLATE + rustc_srcs.label.workspace_root + "/" + sysroot_src
     else: