Configure Mold in ci-config.toml, so it's used only on CI

Author: sourcefrog

.cargo/ci-config.toml

@@ -10,3 +10,15 @@
 # Here, we opted to use `[target.'cfg(all())']` instead of `[build]` because `[target.'**']` is guaranteed to be cumulative.
 [target.'cfg(all())']
 rustflags = ["-D", "warnings"]
+
+# Use Mold on Linux, because it's faster than GNU ld and LLD.
+#
+# We no longer set this in the default `config.toml` so that developers can opt in to Wild, which
+# is faster than Mold, in their own ~/.cargo/config.toml.
+[target.x86_64-unknown-linux-gnu]
+linker = "clang"
+rustflags = ["-C", "link-arg=-fuse-ld=mold"]
+
+[target.aarch64-unknown-linux-gnu]
+linker = "clang"
+rustflags = ["-C", "link-arg=-fuse-ld=mold"]

.cargo/config.toml

@@ -5,14 +5,6 @@ rustflags = ["-C", "symbol-mangling-version=v0", "--cfg", "tokio_unstable"]
 [alias]
 xtask = "run --package xtask --"
 
-[target.x86_64-unknown-linux-gnu]
-linker = "clang"
-rustflags = ["-C", "link-arg=-fuse-ld=mold"]
-
-[target.aarch64-unknown-linux-gnu]
-linker = "clang"
-rustflags = ["-C", "link-arg=-fuse-ld=mold"]
-
 [target.'cfg(target_os = "windows")']
 rustflags = [
     "--cfg",