Work around out-of-tree testing with a shim crate

Out-of-tree testing is broken with the most recent update from
rust-lang/rust because it makes `compiler-builtins` depend on `core` by
path, which isn't usually available. In order to enable testing outside
of rust-lang/rust, add a new crate `builtins-shim` that uses the same
source as `compiler-builtins` but drops the `core` dependency. This has
replaced `compiler-builtins` as the workspace member and entrypoint for
tests.

library/compiler-builtins/Cargo.toml

@@ -1,8 +1,8 @@
 [workspace]
 resolver = "2"
 members = [
+    "builtins-shim",
     "builtins-test",
-    "compiler-builtins",
     "crates/josh-sync",
     "crates/libm-macros",
     "crates/musl-math-sys",
@@ -14,8 +14,8 @@ members = [
 ]
 
 default-members = [
+    "builtins-shim",
     "builtins-test",
-    "compiler-builtins",
     "crates/libm-macros",
     "libm",
     "libm-test",
@@ -26,6 +26,10 @@ exclude = [
     # and `mangled-names` disabled, which is the opposite of what is needed for
     # other tests, so it makes sense to keep it out of the workspace.
     "builtins-test-intrinsics",
+    # We test via the `builtins-shim` crate, so exclude the `compiler-builtins`
+    # that has a dependency on `core`. See `builtins-shim/Cargo.toml` for more
+    # details.
+    "compiler-builtins",
 ]
 
 [profile.release]

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

@@ -0,0 +1,63 @@
+# NOTE: Must be kept in sync with `../compiler-builtins/Cargo.toml`.
+#
+# The manifest at `../compiler-builtins` is what actually gets used in the
+# rust-lang/rust tree; however, we can't build it out of tree because it
+# depends on `core` by path, and even optional Cargo dependencies need to be
+# available at build time. So, we work around this by having this "shim"
+# manifest that is identical except for the `core` dependency and forwards
+# to the same sources, which acts as the `compiler-builtins` Cargo entrypoint
+# for out of tree testing
+
+[package]
+name = "compiler_builtins"
+version = "0.1.160"
+authors = ["Jorge Aparicio <japaricious@gmail.com>"]
+description = "Compiler intrinsics used by the Rust compiler."
+repository = "https://github.com/rust-lang/compiler-builtins"
+license = "MIT AND Apache-2.0 WITH LLVM-exception AND (MIT OR Apache-2.0)"
+edition = "2024"
+publish = false
+links = "compiler-rt"
+
+build = "../compiler-builtins/build.rs"
+
+[lib]
+path = "../compiler-builtins/src/lib.rs"
+bench = false
+doctest = false
+test = false
+
+[build-dependencies]
+cc = { optional = true, version = "1.2" }
+
+[features]
+default = ["compiler-builtins"]
+
+# Enable compilation of C code in compiler-rt, filling in some more optimized
+# implementations and also filling in unimplemented intrinsics
+c = ["dep:cc"]
+
+# Workaround for the Cranelift codegen backend. Disables any implementations
+# which use inline assembly and fall back to pure Rust versions (if available).
+no-asm = []
+
+# Workaround for codegen backends which haven't yet implemented `f16` and
+# `f128` support. Disabled any intrinsics which use those types.
+no-f16-f128 = []
+
+# Flag this library as the unstable compiler-builtins lib
+compiler-builtins = []
+
+# Generate memory-related intrinsics like memcpy
+mem = []
+
+# Mangle all names so this can be linked in with other versions or other
+# 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/builtins-test-intrinsics/Cargo.toml

@@ -6,7 +6,7 @@ publish = false
 license = "MIT OR Apache-2.0"
 
 [dependencies]
-compiler_builtins = { path = "../compiler-builtins", features = ["compiler-builtins"] }
+compiler_builtins = { path = "../builtins-shim", features = ["compiler-builtins"] }
 panic-handler = { path = "../crates/panic-handler" }
 
 [features]

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

@@ -17,7 +17,7 @@ rustc_apfloat = "0.2.2"
 iai-callgrind = { version = "0.14.1", optional = true }
 
 [dependencies.compiler_builtins]
-path = "../compiler-builtins"
+path = "../builtins-shim"
 default-features = false
 features = ["unstable-public-internals"]
 

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

@@ -1,3 +1,9 @@
+# NOTE: Must be kept in sync with `../builtins-shim/Cargo.toml`.
+#
+# This manifest is actually used in-tree by rust-lang/rust,
+# `../builtins-shim/Cargo.toml` is used by out-of-tree testing. See the other
+# manifest for further details.
+
 [package]
 name = "compiler_builtins"
 version = "0.1.160"