Add a test that for_each_fn correctly lists all functions

Create a new test that checks `for_each_fn` against `ALL_FUNCTIONS`,
i.e. the manually entered function list against the automatically
collected list. If any are missing (e.g. new symbol added), then this
will produce an error.

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

@@ -13,6 +13,7 @@ test-musl-serialized = ["rand"]
 
 [dependencies]
 libm = { path = "../.." }
+libm-macros = { path = "../libm-macros" }
 
 [build-dependencies]
 rand = { version = "0.8.5", optional = true }

library/compiler-builtins/libm/crates/libm-test/tests/check_coverage.rs

@@ -0,0 +1,60 @@
+//! Ensure that `for_each_function!` isn't missing any symbols.
+
+/// Files in `src/` that do not export a testable symbol.
+const ALLOWED_SKIPS: &[&str] = &[
+    // Not a generic test function
+    "fenv",
+    // Nonpublic functions
+    "expo2",
+    "k_cos",
+    "k_cosf",
+    "k_expo2",
+    "k_expo2f",
+    "k_sin",
+    "k_sinf",
+    "k_tan",
+    "k_tanf",
+    "rem_pio2",
+    "rem_pio2_large",
+    "rem_pio2f",
+];
+
+macro_rules! callback {
+    (
+        fn_name: $name:ident,
+        CFn: $_CFn:ty,
+        CArgs: $_CArgs:ty,
+        CRet: $_CRet:ty,
+        RustFn: $_RustFn:ty,
+        RustArgs: $_RustArgs:ty,
+        RustRet: $_RustRet:ty,
+        extra: [$push_to:ident],
+    ) => {
+        $push_to.push(stringify!($name));
+    };
+}
+
+#[test]
+fn test_for_each_function_all_included() {
+    let mut included = Vec::new();
+    let mut missing = Vec::new();
+
+    libm_macros::for_each_function! {
+        callback: callback,
+        extra: [included],
+    };
+
+    for f in libm_test::ALL_FUNCTIONS {
+        if !included.contains(f) && !ALLOWED_SKIPS.contains(f) {
+            missing.push(f)
+        }
+    }
+
+    if !missing.is_empty() {
+        panic!(
+            "missing tests for the following: {missing:#?} \
+            \nmake sure any new functions are entered in \
+            `ALL_FUNCTIONS` (in `libm-macros`)."
+        );
+    }
+}