Fix macros std pollution in feature resolution

Cargo.toml

@@ -3,14 +3,30 @@ resolver = "2"
 
 members = [
     "sdk",
-    "macros",
     "examples/add_i32",
     "examples/add_i64",
     "examples/udt",
     "examples/contract_data",
     "examples/create_contract",
 ]
 
+# Exclude proc-macro crates from the workspace so that when building the
+# workspace their dependencies do not pollute the feature selection of other
+# crates. Ordinarily when you build a proc-macro crate as a dependency of
+# another crate, the proc-macro crate's dependencies are decoupled from the
+# dependencies of other crates. This has an important side-effect of not
+# unifiying the features selected in shared dependencies. When a proc-macro
+# crate is part of a workspace and the workspace as a whole is built the
+# proc-macro is seen as a first-class crate and its dependencies, including
+# their feature selection, affects the overall dependency graph used to build
+# all crates. This has an unfortunate side-effect for proc-macro crates, like
+# 'macros', that depend on std features when other crates, like 'sdk',
+# explicitly must not depend on std. Hopefully one day this will be fixed in:
+# https://github.com/rust-lang/cargo/issues/10827
+exclude = [
+    "macros"
+]
+
 [profile.dev]
 overflow-checks = true
 panic = "abort"

macros/Cargo.toml

@@ -9,26 +9,7 @@ edition = "2021"
 proc-macro = true
 
 [dependencies]
-
-# NB: For the time being (until we find a better solution, or abandon `no-std`
-# in the guest) we need to specify the stellar-xdr dependency for the the env
-# crate (pulled in transitively from the SDK crate) and the macros (this crate)
-# with _different versions_ of the stellar-xdr crate.
-#
-# To repeat (this is weird): ensure the rev="..." specified in this file is a
-# _different_ rev than the one specified in
-# https://github.com/stellar/rs-stellar-contract-env/blob/main/stellar-contract-env-common/Cargo.toml
-#
-# This is because the macros activate the `std` feature (as visible here) and
-# the guest crate when compiled to wasm requires _not_ activating the `std`
-# feature, to include a custom panic handler.
-#
-# If cargo resolves both dependencies to a single version of stellar-xdr, it
-# will enable the `std` feature in that "shared" version. If on the other hand
-# (as a total hack) we depend on two _different_ versions of stellar-xdr, we can
-# configure them with separate features. Which is what we're doing (for now).
-
-stellar-xdr = { git = "https://github.com/stellar/rs-stellar-xdr", rev = "277d41c", features = ["next", "std"] }
+stellar-xdr = { git = "https://github.com/stellar/rs-stellar-xdr", rev = "0b8fe26", features = ["next", "std"] }
 syn = {version="1.0",features=["full"]}
 quote = "1.0"
 proc-macro2 = "1.0"