Add new function_casts_as_integer lint

compiler/rustc_driver_impl/src/signal_handler.rs

@@ -152,7 +152,8 @@ pub(super) fn install() {
         libc::sigaltstack(&alt_stack, ptr::null_mut());
 
         let mut sa: libc::sigaction = mem::zeroed();
-        sa.sa_sigaction = print_stack_trace as libc::sighandler_t;
+        sa.sa_sigaction =
+            print_stack_trace as unsafe extern "C" fn(libc::c_int) as libc::sighandler_t;
         sa.sa_flags = libc::SA_NODEFER | libc::SA_RESETHAND | libc::SA_ONSTACK;
         libc::sigemptyset(&mut sa.sa_mask);
         for (signum, _signame) in KILL_SIGNALS {

compiler/rustc_lint/messages.ftl

@@ -285,6 +285,9 @@ lint_forgetting_copy_types = calls to `std::mem::forget` with a value that imple
 lint_forgetting_references = calls to `std::mem::forget` with a reference instead of an owned value does nothing
     .label = argument has type `{$arg_ty}`
 
+lint_function_casts_as_integer = direct cast of function item into an integer
+    .cast_as_fn = first cast to a function pointer `{$cast_from_ty}`
+
 lint_hidden_glob_reexport = private item shadows public glob re-export
     .note_glob_reexport = the name `{$name}` in the {$namespace} namespace is supposed to be publicly re-exported here
     .note_private_item = but the private item here shadows it

compiler/rustc_lint/src/function_cast_as_integer.rs

@@ -0,0 +1,62 @@
+use rustc_hir as hir;
+use rustc_middle::ty;
+use rustc_session::{declare_lint, declare_lint_pass};
+use rustc_span::BytePos;
+
+use crate::lints::{FunctionCastsAsIntegerDiag, FunctionCastsAsIntegerSugg};
+use crate::{LateContext, LateLintPass};
+
+declare_lint! {
+    /// The `function_casts_as_integer` lint detects cases where a function item is casted
+    /// into an integer.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// fn foo() {}
+    /// let x = foo as usize;
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// When casting a function item into an integer, it's implicitly creating a
+    /// function pointer that will in turn be casted into an integer. By making
+    /// it explicit, it improves readability of the code and prevents bugs.
+    pub FUNCTION_CASTS_AS_INTEGER,
+    Warn,
+    "casting a function into an integer",
+}
+
+declare_lint_pass!(
+    /// Lint for casts of functions into integers.
+    FunctionCastsAsInteger => [FUNCTION_CASTS_AS_INTEGER]
+);
+
+impl<'tcx> LateLintPass<'tcx> for FunctionCastsAsInteger {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'tcx>) {
+        let hir::ExprKind::Cast(cast_from_expr, cast_to_expr) = expr.kind else { return };
+        let cast_to_ty = cx.typeck_results().expr_ty(expr);
+        // Casting to a function (pointer?), so all good.
+        if matches!(cast_to_ty.kind(), ty::FnDef(..) | ty::FnPtr(..)) {
+            return;
+        }
+        let cast_from_ty = cx.typeck_results().expr_ty(cast_from_expr);
+        if matches!(cast_from_ty.kind(), ty::FnDef(..)) {
+            cx.tcx.emit_node_span_lint(
+                FUNCTION_CASTS_AS_INTEGER,
+                expr.hir_id,
+                cast_to_expr.span.with_lo(cast_from_expr.span.hi() + BytePos(1)),
+                FunctionCastsAsIntegerDiag {
+                    sugg: FunctionCastsAsIntegerSugg {
+                        suggestion: cast_from_expr.span.shrink_to_hi(),
+                        // We get the function pointer to have a nice display.
+                        cast_from_ty: cx.typeck_results().expr_ty_adjusted(cast_from_expr),
+                        cast_to_ty,
+                    },
+                },
+            );
+        }
+    }
+}

compiler/rustc_lint/src/lib.rs

@@ -48,6 +48,7 @@ mod errors;
 mod expect;
 mod for_loops_over_fallibles;
 mod foreign_modules;
+mod function_cast_as_integer;
 mod if_let_rescope;
 mod impl_trait_overcaptures;
 mod internal;
@@ -92,6 +93,7 @@ use deref_into_dyn_supertrait::*;
 use drop_forget_useless::*;
 use enum_intrinsics_non_enums::EnumIntrinsicsNonEnums;
 use for_loops_over_fallibles::*;
+use function_cast_as_integer::*;
 use if_let_rescope::IfLetRescope;
 use impl_trait_overcaptures::ImplTraitOvercaptures;
 use internal::*;
@@ -247,6 +249,7 @@ late_lint_methods!(
             IfLetRescope: IfLetRescope::default(),
             StaticMutRefs: StaticMutRefs,
             UnqualifiedLocalImports: UnqualifiedLocalImports,
+            FunctionCastsAsInteger: FunctionCastsAsInteger,
             CheckTransmutes: CheckTransmutes,
             LifetimeSyntax: LifetimeSyntax,
         ]

compiler/rustc_lint/src/lints.rs

@@ -3190,6 +3190,27 @@ pub(crate) struct ReservedMultihash {
     pub suggestion: Span,
 }
 
+#[derive(LintDiagnostic)]
+#[diag(lint_function_casts_as_integer)]
+pub(crate) struct FunctionCastsAsIntegerDiag<'tcx> {
+    #[subdiagnostic]
+    pub(crate) sugg: FunctionCastsAsIntegerSugg<'tcx>,
+}
+
+#[derive(Subdiagnostic)]
+#[suggestion(
+    lint_cast_as_fn,
+    code = " as {cast_from_ty}",
+    applicability = "machine-applicable",
+    style = "verbose"
+)]
+pub(crate) struct FunctionCastsAsIntegerSugg<'tcx> {
+    #[primary_span]
+    pub suggestion: Span,
+    pub cast_from_ty: Ty<'tcx>,
+    pub cast_to_ty: Ty<'tcx>,
+}
+
 #[derive(Debug)]
 pub(crate) struct MismatchedLifetimeSyntaxes {
     pub lifetime_name: String,

compiler/rustc_session/src/filesearch.rs

@@ -73,7 +73,7 @@ fn current_dll_path() -> Result<PathBuf, String> {
 
             #[cfg(not(target_os = "aix"))]
             unsafe {
-                let addr = current_dll_path as usize as *mut _;
+                let addr = current_dll_path as fn() -> Result<PathBuf, String> as *mut _;
                 let mut info = std::mem::zeroed();
                 if libc::dladdr(addr, &mut info) == 0 {
                     return Err("dladdr failed".into());
@@ -152,7 +152,10 @@ fn current_dll_path() -> Result<PathBuf, String> {
     unsafe {
         GetModuleHandleExW(
             GET_MODULE_HANDLE_EX_FLAG_FROM_ADDRESS,
-            PCWSTR(current_dll_path as *mut u16),
+            PCWSTR(
+                current_dll_path as fn() -> Result<std::path::PathBuf, std::string::String>
+                    as *mut u16,
+            ),
             &mut module,
         )
     }

library/core/src/intrinsics/mod.rs

@@ -1429,7 +1429,7 @@ pub const fn forget<T: ?Sized>(_: T);
 /// // Crucially, we `as`-cast to a raw pointer before `transmute`ing to a function pointer.
 /// // This avoids an integer-to-pointer `transmute`, which can be problematic.
 /// // Transmuting between raw pointers and function pointers (i.e., two pointer types) is fine.
-/// let pointer = foo as *const ();
+/// let pointer = foo as fn() -> i32 as *const ();
 /// let function = unsafe {
 ///     std::mem::transmute::<*const (), fn() -> i32>(pointer)
 /// };

library/coretests/tests/ptr.rs

@@ -557,6 +557,7 @@ fn ptr_metadata() {
 
 #[test]
 fn ptr_metadata_bounds() {
+    #[allow(unknown_lints, function_casts_as_integer)]
     fn metadata_eq_method_address<T: ?Sized>() -> usize {
         // The `Metadata` associated type has an `Ord` bound, so this is valid:
         <<T as Pointee>::Metadata as PartialEq>::eq as usize

library/panic_unwind/src/seh.rs

@@ -325,6 +325,7 @@ pub(crate) unsafe fn panic(data: Box<dyn Any + Send>) -> u32 {
     // In any case, we basically need to do something like this until we can
     // express more operations in statics (and we may never be able to).
     unsafe {
+        #[allow(function_casts_as_integer)]
         atomic_store_seqcst(
             (&raw mut THROW_INFO.pmfnUnwind).cast(),
             ptr_t::new(exception_cleanup as *mut u8).raw(),
@@ -341,6 +342,7 @@ pub(crate) unsafe fn panic(data: Box<dyn Any + Send>) -> u32 {
             (&raw mut CATCHABLE_TYPE.pType).cast(),
             ptr_t::new((&raw mut TYPE_DESCRIPTOR).cast()).raw(),
         );
+        #[allow(function_casts_as_integer)]
         atomic_store_seqcst(
             (&raw mut CATCHABLE_TYPE.copyFunction).cast(),
             ptr_t::new(exception_copy as *mut u8).raw(),

library/std/src/backtrace.rs

@@ -293,7 +293,7 @@ impl Backtrace {
         if !Backtrace::enabled() {
             return Backtrace { inner: Inner::Disabled };
         }
-        Backtrace::create(Backtrace::capture as usize)
+        Backtrace::create(Backtrace::capture as fn() -> Backtrace as usize)
     }
 
     /// Forcibly captures a full backtrace, regardless of environment variable
@@ -309,7 +309,7 @@ impl Backtrace {
     #[stable(feature = "backtrace", since = "1.65.0")]
     #[inline(never)] // want to make sure there's a frame here to remove
     pub fn force_capture() -> Backtrace {
-        Backtrace::create(Backtrace::force_capture as usize)
+        Backtrace::create(Backtrace::force_capture as fn() -> Backtrace as usize)
     }
 
     /// Forcibly captures a disabled backtrace, regardless of environment

library/std/src/sys/pal/unix/stack_overflow.rs

@@ -169,7 +169,9 @@ mod imp {
                 }
 
                 action.sa_flags = SA_SIGINFO | SA_ONSTACK;
-                action.sa_sigaction = signal_handler as sighandler_t;
+                action.sa_sigaction = signal_handler
+                    as unsafe extern "C" fn(i32, *mut libc::siginfo_t, *mut libc::c_void)
+                    as sighandler_t;
                 // SAFETY: only overriding signals if the default is set
                 unsafe { sigaction(signal, &action, ptr::null_mut()) };
             }

library/std/src/sys/pal/windows/compat.rs

@@ -155,7 +155,7 @@ macro_rules! compat_fn_with_fallback {
             /// When that is called it attempts to load the requested symbol.
             /// If it succeeds, `PTR` is set to the address of that symbol.
             /// If it fails, then `PTR` is set to `fallback`.
-            static PTR: Atomic<*mut c_void> = AtomicPtr::new(load as *mut _);
+            static PTR: Atomic<*mut c_void> = AtomicPtr::new(load as unsafe extern "system" fn($($argname: $argtype),*) -> $rettype as *mut _);
 
             unsafe extern "system" fn load($($argname: $argtype),*) -> $rettype {
                 unsafe {
@@ -171,7 +171,7 @@ macro_rules! compat_fn_with_fallback {
                         PTR.store(f.as_ptr(), Ordering::Relaxed);
                         mem::transmute(f)
                     } else {
-                        PTR.store(fallback as *mut _, Ordering::Relaxed);
+                        PTR.store(fallback as unsafe extern "system" fn($($argname: $argtype),*) -> $rettype as *mut _, Ordering::Relaxed);
                         fallback
                     }
                 }

src/tools/clippy/clippy_lints/src/disallowed_macros.rs

@@ -1,4 +1,3 @@
-
 use clippy_config::Conf;
 use clippy_config::types::{DisallowedPath, create_disallowed_map};
 use clippy_utils::diagnostics::{span_lint_and_then, span_lint_hir_and_then};

src/tools/clippy/tests/ui/cast_enum_constructor.rs

@@ -1,5 +1,5 @@
 #![warn(clippy::cast_enum_constructor)]
-#![allow(clippy::fn_to_numeric_cast)]
+#![allow(clippy::fn_to_numeric_cast, function_casts_as_integer)]
 
 fn main() {
     enum Foo {

src/tools/clippy/tests/ui/confusing_method_to_numeric_cast.fixed

@@ -1,5 +1,6 @@
 #![feature(float_minimum_maximum)]
 #![warn(clippy::confusing_method_to_numeric_cast)]
+#![allow(function_casts_as_integer)]
 
 fn main() {
     let _ = u16::MAX as usize; //~ confusing_method_to_numeric_cast

src/tools/clippy/tests/ui/confusing_method_to_numeric_cast.rs

@@ -1,5 +1,6 @@
 #![feature(float_minimum_maximum)]
 #![warn(clippy::confusing_method_to_numeric_cast)]
+#![allow(function_casts_as_integer)]
 
 fn main() {
     let _ = u16::max as usize; //~ confusing_method_to_numeric_cast

src/tools/clippy/tests/ui/confusing_method_to_numeric_cast.stderr

@@ -1,5 +1,5 @@
 error: casting function pointer `u16::max` to `usize`
-  --> tests/ui/confusing_method_to_numeric_cast.rs:5:13
+  --> tests/ui/confusing_method_to_numeric_cast.rs:6:13
    |
 LL |     let _ = u16::max as usize;
    |             ^^^^^^^^^^^^^^^^^
@@ -13,7 +13,7 @@ LL +     let _ = u16::MAX as usize;
    |
 
 error: casting function pointer `u16::min` to `usize`
-  --> tests/ui/confusing_method_to_numeric_cast.rs:6:13
+  --> tests/ui/confusing_method_to_numeric_cast.rs:7:13
    |
 LL |     let _ = u16::min as usize;
    |             ^^^^^^^^^^^^^^^^^
@@ -25,7 +25,7 @@ LL +     let _ = u16::MIN as usize;
    |
 
 error: casting function pointer `u16::max_value` to `usize`
-  --> tests/ui/confusing_method_to_numeric_cast.rs:7:13
+  --> tests/ui/confusing_method_to_numeric_cast.rs:8:13
    |
 LL |     let _ = u16::max_value as usize;
    |             ^^^^^^^^^^^^^^^^^^^^^^^
@@ -37,7 +37,7 @@ LL +     let _ = u16::MAX as usize;
    |
 
 error: casting function pointer `u16::min_value` to `usize`
-  --> tests/ui/confusing_method_to_numeric_cast.rs:8:13
+  --> tests/ui/confusing_method_to_numeric_cast.rs:9:13
    |
 LL |     let _ = u16::min_value as usize;
    |             ^^^^^^^^^^^^^^^^^^^^^^^
@@ -49,7 +49,7 @@ LL +     let _ = u16::MIN as usize;
    |
 
 error: casting function pointer `f32::maximum` to `usize`
-  --> tests/ui/confusing_method_to_numeric_cast.rs:10:13
+  --> tests/ui/confusing_method_to_numeric_cast.rs:11:13
    |
 LL |     let _ = f32::maximum as usize;
    |             ^^^^^^^^^^^^^^^^^^^^^
@@ -61,7 +61,7 @@ LL +     let _ = f32::MAX as usize;
    |
 
 error: casting function pointer `f32::max` to `usize`
-  --> tests/ui/confusing_method_to_numeric_cast.rs:11:13
+  --> tests/ui/confusing_method_to_numeric_cast.rs:12:13
    |
 LL |     let _ = f32::max as usize;
    |             ^^^^^^^^^^^^^^^^^
@@ -73,7 +73,7 @@ LL +     let _ = f32::MAX as usize;
    |
 
 error: casting function pointer `f32::minimum` to `usize`
-  --> tests/ui/confusing_method_to_numeric_cast.rs:12:13
+  --> tests/ui/confusing_method_to_numeric_cast.rs:13:13
    |
 LL |     let _ = f32::minimum as usize;
    |             ^^^^^^^^^^^^^^^^^^^^^
@@ -85,7 +85,7 @@ LL +     let _ = f32::MIN as usize;
    |
 
 error: casting function pointer `f32::min` to `usize`
-  --> tests/ui/confusing_method_to_numeric_cast.rs:13:13
+  --> tests/ui/confusing_method_to_numeric_cast.rs:14:13
    |
 LL |     let _ = f32::min as usize;
    |             ^^^^^^^^^^^^^^^^^

src/tools/clippy/tests/ui/derive.rs

@@ -9,7 +9,6 @@
 #![expect(incomplete_features)] // `unsafe_fields` is incomplete for the time being
 #![feature(unsafe_fields)] // `clone()` cannot be derived automatically on unsafe fields
 
-
 #[derive(Copy)]
 struct Qux;