Replace the llvm::Bool typedef with a proper newtype

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -13,7 +13,7 @@
 #![allow(non_camel_case_types)]
 #![allow(non_upper_case_globals)]
 
-use std::fmt::Debug;
+use std::fmt::{self, Debug};
 use std::marker::PhantomData;
 use std::num::NonZero;
 use std::ptr;
@@ -33,10 +33,59 @@ use crate::llvm;
 
 /// In the LLVM-C API, boolean values are passed as `typedef int LLVMBool`,
 /// which has a different ABI from Rust or C++ `bool`.
-pub(crate) type Bool = c_int;
+///
+/// This wrapper does not implement `PartialEq`.
+/// To test the underlying boolean value, use [`Self::is_true`].
+#[derive(Clone, Copy)]
+#[repr(transparent)]
+pub(crate) struct Bool {
+    value: c_int,
+}
 
-pub(crate) const True: Bool = 1 as Bool;
-pub(crate) const False: Bool = 0 as Bool;
+pub(crate) const True: Bool = Bool::TRUE;
+pub(crate) const False: Bool = Bool::FALSE;
+
+impl Bool {
+    pub(crate) const TRUE: Self = Self { value: 1 };
+    pub(crate) const FALSE: Self = Self { value: 0 };
+
+    pub(crate) const fn from_bool(rust_bool: bool) -> Self {
+        if rust_bool { Self::TRUE } else { Self::FALSE }
+    }
+
+    /// Converts this LLVM-C boolean to a Rust `bool`
+    pub(crate) fn is_true(self) -> bool {
+        // Since we're interacting with a C API, follow the C convention of
+        // treating any nonzero value as true.
+        self.value != Self::FALSE.value
+    }
+}
+
+impl Debug for Bool {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self.value {
+            0 => f.write_str("FALSE"),
+            1 => f.write_str("TRUE"),
+            // As with `Self::is_true`, treat any nonzero value as true.
+            v => write!(f, "TRUE ({v})"),
+        }
+    }
+}
+
+/// Convenience trait to convert `bool` to `llvm::Bool` with an explicit method call.
+///
+/// Being able to write `b.to_llvm_bool()` is less noisy than `llvm::Bool::from(b)`,
+/// while being more explicit and less mistake-prone than something like `b.into()`.
+pub(crate) trait ToLlvmBool: Copy {
+    fn to_llvm_bool(self) -> llvm::Bool;
+}
+
+impl ToLlvmBool for bool {
+    #[inline(always)]
+    fn to_llvm_bool(self) -> llvm::Bool {
+        llvm::Bool::from_bool(self)
+    }
+}
 
 /// Wrapper for a raw enum value returned from LLVM's C APIs.
 ///

compiler/rustc_codegen_llvm/src/llvm/mod.rs

@@ -215,7 +215,7 @@ pub(crate) fn set_initializer(llglobal: &Value, constant_val: &Value) {
 }
 
 pub(crate) fn set_global_constant(llglobal: &Value, is_constant: bool) {
-    LLVMSetGlobalConstant(llglobal, if is_constant { ffi::True } else { ffi::False });
+    LLVMSetGlobalConstant(llglobal, is_constant.to_llvm_bool());
 }
 
 pub(crate) fn get_linkage(llglobal: &Value) -> Linkage {
@@ -229,7 +229,7 @@ pub(crate) fn set_linkage(llglobal: &Value, linkage: Linkage) {
 }
 
 pub(crate) fn is_declaration(llglobal: &Value) -> bool {
-    unsafe { LLVMIsDeclaration(llglobal) == ffi::True }
+    unsafe { LLVMIsDeclaration(llglobal) }.is_true()
 }
 
 pub(crate) fn get_visibility(llglobal: &Value) -> Visibility {