Improve feature combinations for ureq/reqwest

Our features are a bit convoluted, most annoyingly we cannot build with
`--all-features`. However we can make life for users a little easier.

Explicitly we want users to be able to:

- Use async-interface/WASM without using esplora (to implement their own blockchain)
- Use esplora in an ergonomic manner

Currently using esplora requires either reqwest or ureq. Instead of
making the user add all the features manually we can add features that
add the required feature sets, this makes it easier for users to
understand what is required and also makes usage easier.

With this patch applied we can do

- `cargo check --no-default-features --features=use-esplora-reqwest`
- `cargo check --no-default-features --features=use-esplora-ureq`
- `cargo check --features=use-esplora-ureq`
- `cargo check --no-default-features --features=async-trait`

.github/workflows/cont_integration.yml

@@ -16,7 +16,7 @@ jobs:
           - default
           - minimal
           - all-keys
-          - minimal,esplora,ureq
+          - minimal,use-esplora-ureq
           - key-value-db
           - electrum
           - compact_filters
@@ -25,7 +25,7 @@ jobs:
           - rpc
           - verify
           - async-interface
-          - async-interface,esplora,reqwest
+          - use-esplora-reqwest
     steps:
       - name: checkout
         uses: actions/checkout@v2
@@ -139,7 +139,7 @@ jobs:
       - name: Update toolchain
         run: rustup update
       - name: Check
-        run: cargo check --target wasm32-unknown-unknown --features esplora,reqwest --no-default-features
+        run: cargo check --target wasm32-unknown-unknown --features use-esplora-reqwest --no-default-features
 
 
   fmt:

Cargo.toml

@@ -60,13 +60,20 @@ rpc = ["bitcoincore-rpc"]
 #
 # - Users wanting asynchronous HTTP calls should enable `async-interface` to get
 #   access to the asynchronous method implementations. Then, if Esplora is wanted,
-#   enable `esplora` AND `reqwest`.
+#   enable `esplora` AND `reqwest` (`--features=use-esplora-reqwest`).
 # - Users wanting blocking HTTP calls can use any of the other blockchain
 #   implementations (`compact_filters`, `electrum`, or `esplora`). Users wanting to
-#   use Esplora should enable `esplora` AND `ureq`.
+#   use Esplora should enable `esplora` AND `ureq`  (`--features=use-esplora-ureq`).
+#
+# WARNING: Please take care with the features below, various combinations will
+# fail to build. We cannot currently build `bdk` with `--all-features`.
 async-interface = ["async-trait"]
 electrum = ["electrum-client"]
-esplora = ["futures"]           # Requires one of: `ureq` or `reqwest`.
+# MUST ALSO USE `--no-default-features`.
+use-esplora-reqwest = ["async-interface", "esplora", "reqwest", "futures"]
+use-esplora-ureq = ["esplora", "ureq"]
+# Typical configurations will not need to use `esplora` feature directly.
+esplora = []
 
 
 # Debug/Test features

src/lib.rs

@@ -223,9 +223,6 @@ compile_error!(
     "Features async-interface and compact_filters are mutually exclusive and cannot be enabled together"
 );
 
-#[cfg(all(feature = "esplora", not(feature = "ureq"), not(feature = "reqwest")))]
-compile_error!("Feature missing: esplora requires either ureq or reqwest to be enabled");
-
 #[cfg(feature = "keys-bip39")]
 extern crate bip39;