Parse default edition from buckconfig

Summary: Ordinarily this would be controlled from a BUCK_TREE file / PACKAGE file using `write_package_value`. But our existing PACKAGE files in fbsource are very specific to internal use (with things like Citadel CI config) and are stripped by ShipIt as part of the export to GitHub. This diff reproduces the hierarchical effect of a PACKAGE file using `read_config`.

Reviewed By: JakobDegen

Differential Revision: D74375801

fbshipit-source-id: eb7d256aaf7cfb3b013b905e1b1df560982223e1

Author: David Tolnay

shim/shims.bzl

@@ -243,6 +243,7 @@ def cpp_binary(
     )
 
 def rust_library(
+        edition = None,
         rustc_flags = [],
         deps = [],
         named_deps = None,
@@ -265,6 +266,7 @@ def rust_library(
     visibility = ["PUBLIC"]
 
     prelude.rust_library(
+        edition = edition or _default_rust_edition(),
         rustc_flags = rustc_flags + [_CFG_BUCK_BUILD],
         deps = deps,
         visibility = visibility,
@@ -273,6 +275,7 @@ def rust_library(
     )
 
 def rust_binary(
+        edition = None,
         rustc_flags = [],
         deps = [],
         autocargo = None,
@@ -286,20 +289,23 @@ def rust_binary(
 
     # @lint-ignore BUCKLINT: avoid "Direct usage of native rules is not allowed."
     prelude.rust_binary(
+        edition = edition or _default_rust_edition(),
         rustc_flags = rustc_flags + [_CFG_BUCK_BUILD],
         deps = deps,
         visibility = visibility,
         **kwargs
     )
 
 def rust_unittest(
+        edition = None,
         rustc_flags = [],
         deps = [],
         visibility = ["PUBLIC"],
         **kwargs):
     deps = _fix_deps(deps)
 
     prelude.rust_test(
+        edition = edition or _default_rust_edition(),
         rustc_flags = rustc_flags + [_CFG_BUCK_BUILD],
         deps = deps,
         visibility = visibility,
@@ -436,6 +442,26 @@ def _fix_resources(resources):
 
     fail("Unexpected type {} for resources".format(type(resources)))
 
+def _default_rust_edition():
+    package = native.package_name()
+
+    # Parse buckconfig entries in the following form:
+    #
+    #     [rust]
+    #     default_edition = 2024
+    #     default_edition:buck2 = 2021
+    #     default_edition:buck2/dice = 2024
+    #
+    if package:
+        split = package.split("/")
+        for i in range(len(split)):
+            parent_directory = "/".join(split[:len(split) - i])
+            edition = read_config("rust", "default_edition:" + parent_directory)
+            if edition != None:
+                return edition
+
+    return read_config("rust", "default_edition")
+
 # Do a nasty conversion of e.g. ("supercaml", None, "ocaml-dev") to
 # 'fbcode//third-party-buck/platform010/build/supercaml:ocaml-dev'
 # (which will then get mapped to `shim//third-party/ocaml:ocaml-dev`).