compiler-builtins: Clean up features

Remove the `compiler-builtins` from default because it prevents testing
via the default `cargo test` command. It made more sense as a default
when `compiler-builtins` was a dependency that some crates added via
crates.io, but is no longer needed

The `rustc-dep-of-std` feature is also removed since it doesn't do
anything notable.

Author: tgross35

library/alloc/Cargo.toml

@@ -16,7 +16,7 @@ bench = false
 
 [dependencies]
 core = { path = "../core", public = true }
-compiler_builtins = { path = "../compiler-builtins/compiler-builtins", features = ["rustc-dep-of-std"] }
+compiler_builtins = { path = "../compiler-builtins/compiler-builtins", features = ["compiler-builtins"] }
 
 [features]
 compiler-builtins-mem = ['compiler_builtins/mem']

library/compiler-builtins/builtins-shim/Cargo.toml

@@ -31,7 +31,7 @@ test = false
 cc = { optional = true, version = "1.2" }
 
 [features]
-default = ["compiler-builtins"]
+default = []
 
 # Enable compilation of C code in compiler-rt, filling in some more optimized
 # implementations and also filling in unimplemented intrinsics
@@ -45,7 +45,8 @@ no-asm = []
 # `f128` support. Disabled any intrinsics which use those types.
 no-f16-f128 = []
 
-# Flag this library as the unstable compiler-builtins lib
+# Flag this library as the unstable compiler-builtins lib. This must be enabled
+# when using as `std`'s dependency.'
 compiler-builtins = []
 
 # Generate memory-related intrinsics like memcpy
@@ -55,9 +56,6 @@ mem = []
 # compiler-rt implementations. Also used for testing
 mangled-names = []
 
-# Only used in the compiler's build system
-rustc-dep-of-std = ["compiler-builtins"]
-
 # This makes certain traits and function specializations public that
 # are not normally public but are required by the `builtins-test`
 unstable-public-internals = []

library/compiler-builtins/compiler-builtins/Cargo.toml

@@ -29,7 +29,7 @@ core = { path = "../../core", optional = true }
 cc = { optional = true, version = "1.2" }
 
 [features]
-default = ["compiler-builtins"]
+default = []
 
 # Enable compilation of C code in compiler-rt, filling in some more optimized
 # implementations and also filling in unimplemented intrinsics
@@ -43,8 +43,9 @@ no-asm = []
 # `f128` support. Disabled any intrinsics which use those types.
 no-f16-f128 = []
 
-# Flag this library as the unstable compiler-builtins lib
-compiler-builtins = []
+# Flag this library as the unstable compiler-builtins lib. This must be enabled
+# when using as `std`'s dependency.'
+compiler-builtins = ["dep:core"]
 
 # Generate memory-related intrinsics like memcpy
 mem = []
@@ -53,9 +54,6 @@ mem = []
 # compiler-rt implementations. Also used for testing
 mangled-names = []
 
-# Only used in the compiler's build system
-rustc-dep-of-std = ["compiler-builtins", "dep:core"]
-
 # This makes certain traits and function specializations public that
 # are not normally public but are required by the `builtins-test`
 unstable-public-internals = []