Move examples/intrinsics.rs to its own crate

Currently there is an interesting situation with the way features get
enabled; `testcrate` enables `mangled-names`, but the `intrinsics.rs`
example requires this feature be disabled (otherwise the test fails with
missing symbols, as expected). This is also the reason that `testcrate`
is not a default workspace member, meaning `cargo test` doesn't actually
run `testcrate`'s tests; making it a default member would mean that
`compiler-builtins/mangled-names` gets enabled when
`examples/intrinsics.rs` gets built, due to the way features get
unified.

Simplify the situation by making moving the example to its own crate as
`builtins-test-intrinsics`. This also means `testcrate` can become a
default member so it is included in `cargo check` or `cargo test` when
run at the workspace root.

`testcrate` and `builtins-test-intrinsics` still can't be built at the
same time since there isn't a straightforward way to have Cargo build
`compiler-builtins` twice with different features. This is a side effect
of us using non-additive features, but there isn't really a better
option since enabling both mangled and unmangled names would render
`builtins-test-intrinsics` useless.

library/compiler-builtins/Cargo.toml

@@ -71,13 +71,20 @@ rustc-dep-of-std = ['compiler-builtins', 'core']
 # are not normally public but are required by the `testcrate`
 public-test-deps = []
 
-[[example]]
-name = "intrinsics"
-required-features = ["compiler-builtins"]
-
 [workspace]
 resolver = "2"
-members = ["testcrate"]
+members = [
+    # Note that builtins-test-intrinsics cannot be  a default member because it
+    # needs the `mangled-names` feature disabled, while `testcrate` needs it
+    # enabled.
+    "builtins-test-intrinsics",
+    "testcrate",
+]
+
+default-members = [
+    ".",
+    "testcrate",
+]
 
 [profile.release]
 panic = 'abort'

library/compiler-builtins/README.md

@@ -16,8 +16,8 @@ It is distributed as part of Rust's sysroot.
    [C implementation][2] to Rust.
 4. Add a test to compare the behavior of the ported intrinsic(s) with their
    implementation on the testing host.
-5. Add the intrinsic to `examples/intrinsics.rs` to verify it can be linked on
-   all targets.
+5. Add the intrinsic to `builtins-test-intrinsics/src/main.rs` to verify it
+   can be linked on all targets.
 6. Send a Pull Request (PR).
 7. Once the PR passes our extensive testing infrastructure, we'll merge it!
 8. Celebrate :tada:

library/compiler-builtins/build.rs

@@ -2,7 +2,7 @@ use std::{collections::BTreeMap, env, path::PathBuf, sync::atomic::Ordering};
 
 mod configure;
 
-use configure::{configure_f16_f128, Target};
+use configure::{configure_aliases, configure_f16_f128, Target};
 
 fn main() {
     println!("cargo::rerun-if-changed=build.rs");
@@ -13,6 +13,7 @@ fn main() {
 
     configure_check_cfg();
     configure_f16_f128(&target);
+    configure_aliases(&target);
 
     configure_libm(&target);
 
@@ -71,20 +72,6 @@ fn main() {
         }
     }
 
-    // To compile intrinsics.rs for thumb targets, where there is no libc
-    println!("cargo::rustc-check-cfg=cfg(thumb)");
-    if llvm_target[0].starts_with("thumb") {
-        println!("cargo:rustc-cfg=thumb")
-    }
-
-    // compiler-rt `cfg`s away some intrinsics for thumbv6m and thumbv8m.base because
-    // these targets do not have full Thumb-2 support but only original Thumb-1.
-    // We have to cfg our code accordingly.
-    println!("cargo::rustc-check-cfg=cfg(thumb_1)");
-    if llvm_target[0] == "thumbv6m" || llvm_target[0] == "thumbv8m.base" {
-        println!("cargo:rustc-cfg=thumb_1")
-    }
-
     // Only emit the ARM Linux atomic emulation on pre-ARMv6 architectures. This
     // includes the old androideabi. It is deprecated but it is available as a
     // rustc target (arm-linux-androideabi).

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

@@ -0,0 +1,11 @@
+[package]
+name = "builtins-test-intrinsics"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+compiler_builtins = { path = "../", features = ["compiler-builtins"]}
+panic-handler = { path = '../crates/panic-handler' }
+
+[features]
+c = ["compiler_builtins/c"]

library/compiler-builtins/builtins-test-intrinsics/build.rs

@@ -0,0 +1,11 @@
+mod builtins_configure {
+    include!("../configure.rs");
+}
+
+fn main() {
+    println!("cargo::rerun-if-changed=../configure.rs");
+
+    let target = builtins_configure::Target::from_env();
+    builtins_configure::configure_f16_f128(&target);
+    builtins_configure::configure_aliases(&target);
+}

library/compiler-builtins/ci/run.sh

@@ -120,22 +120,22 @@ done
 
 rm -f "${rlib_paths[@]}"
 
-build_intrinsics() {
-    cargo build --target "$target" -v --example intrinsics  "$@"
+build_intrinsics_test() {
+    cargo build --target "$target" -v --package builtins-test-intrinsics "$@"
 }
 
-# Verify that we haven't drop any intrinsic/symbol
-build_intrinsics
-build_intrinsics --release
-build_intrinsics --features c
-build_intrinsics --features c --release
+# Verify that we haven't dropped any intrinsics/symbols
+build_intrinsics_test
+build_intrinsics_test --release
+build_intrinsics_test --features c
+build_intrinsics_test --features c --release
 
 # Verify that there are no undefined symbols to `panic` within our
 # implementations
 CARGO_PROFILE_DEV_LTO=true \
-    cargo build --target "$target" --example intrinsics
+    cargo build --target "$target" --package builtins-test-intrinsics
 CARGO_PROFILE_RELEASE_LTO=true \
-    cargo build --target "$target" --example intrinsics --release
+    cargo build --target "$target" --package builtins-test-intrinsics --release
 
 # Ensure no references to any symbols from core
 update_rlib_paths

library/compiler-builtins/configure.rs

@@ -6,6 +6,7 @@ use std::env;
 #[allow(dead_code)]
 pub struct Target {
     pub triple: String,
+    pub triple_split: Vec<String>,
     pub opt_level: String,
     pub cargo_features: Vec<String>,
     pub os: String,
@@ -19,6 +20,8 @@ pub struct Target {
 
 impl Target {
     pub fn from_env() -> Self {
+        let triple = env::var("TARGET").unwrap();
+        let triple_split = triple.split('-').map(ToOwned::to_owned).collect();
         let little_endian = match env::var("CARGO_CFG_TARGET_ENDIAN").unwrap().as_str() {
             "little" => true,
             "big" => false,
@@ -30,7 +33,8 @@ impl Target {
             .collect();
 
         Self {
-            triple: env::var("TARGET").unwrap(),
+            triple,
+            triple_split,
             os: env::var("CARGO_CFG_TARGET_OS").unwrap(),
             opt_level: env::var("OPT_LEVEL").unwrap(),
             cargo_features,
@@ -56,6 +60,22 @@ impl Target {
     }
 }
 
+pub fn configure_aliases(target: &Target) {
+    // To compile builtins-test-intrinsics for thumb targets, where there is no libc
+    println!("cargo::rustc-check-cfg=cfg(thumb)");
+    if target.triple_split[0].starts_with("thumb") {
+        println!("cargo:rustc-cfg=thumb")
+    }
+
+    // compiler-rt `cfg`s away some intrinsics for thumbv6m and thumbv8m.base because
+    // these targets do not have full Thumb-2 support but only original Thumb-1.
+    // We have to cfg our code accordingly.
+    println!("cargo::rustc-check-cfg=cfg(thumb_1)");
+    if target.triple_split[0] == "thumbv6m" || target.triple_split[0] == "thumbv8m.base" {
+        println!("cargo:rustc-cfg=thumb_1")
+    }
+}
+
 /// Configure whether or not `f16` and `f128` support should be enabled.
 pub fn configure_f16_f128(target: &Target) {
     // Set whether or not `f16` and `f128` are supported at a basic level by LLVM. This only means

library/compiler-builtins/testcrate/build.rs

@@ -115,5 +115,6 @@ fn main() {
         println!("cargo:rustc-cfg=feature=\"{name}\"");
     }
 
+    builtins_configure::configure_aliases(&target);
     builtins_configure::configure_f16_f128(&target);
 }