Unrolled build for #141470

Rollup merge of #141470 - GuillaumeGomez:function_casts_as_integer, r=urgau

Add new `function_casts_as_integer` lint

The `function_casts_as_integer` lint detects cases where users cast a function pointer into an integer.

*warn-by-default*

### Example

```rust
fn foo() {}
let x = foo as usize;
```

```
warning: casting a function into an integer implicitly
  --> $DIR/function_casts_as_integer.rs:9:17
   |
LL |     let x = foo as usize;
   |                 ^^^^^^^^
   |
help: add `fn() as usize`
   |
LL |     let x = foo as fn() as usize;
   |                 +++++++
```

### Explanation

You should never cast a function directly into an integer but go through a cast as `fn` first to make it obvious what's going on. It also allows to prevent confusion with (associated) constants.

Related to #81686 and https://stackoverflow.com/questions/68701177/whats-the-meaning-of-casting-a-rust-enum-variant-to-a-numeric-data-type

r? ````@urgau````

Author: rust-timer

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

@@ -265,6 +265,9 @@ 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 pointer `as *const ()`
+
 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 +1,63 @@
+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 cast
+    /// to an integer.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// fn foo() {}
+    /// let x = foo as usize;
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// When casting a function item to an integer, it implicitly creates a
+    /// function pointer that will in turn be cast to 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.
+        //
+        // Normally, only casts to integers is possible, but if it ever changed, this condition
+        // will likely need to be updated.
+        if matches!(cast_to_ty.kind(), ty::FnDef(..) | ty::FnPtr(..) | ty::RawPtr(..)) {
+            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(),
+                        cast_to_ty,
+                    },
+                },
+            );
+        }
+    }
+}

compiler/rustc_lint/src/lib.rs

@@ -45,6 +45,7 @@ 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;
@@ -89,6 +90,7 @@ 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::*;
@@ -241,6 +243,7 @@ late_lint_methods!(
             IfLetRescope: IfLetRescope::default(),
             StaticMutRefs: StaticMutRefs,
             UnqualifiedLocalImports: UnqualifiedLocalImports,
+            FunctionCastsAsInteger: FunctionCastsAsInteger,
             CheckTransmutes: CheckTransmutes,
             LifetimeSyntax: LifetimeSyntax,
         ]

compiler/rustc_lint/src/lints.rs

@@ -3019,6 +3019,26 @@ 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 *const ()",
+    applicability = "machine-applicable",
+    style = "verbose"
+)]
+pub(crate) struct FunctionCastsAsIntegerSugg<'tcx> {
+    #[primary_span]
+    pub suggestion: Span,
+    pub cast_to_ty: Ty<'tcx>,
+}
+
 #[derive(Debug)]
 pub(crate) struct MismatchedLifetimeSyntaxes {
     pub inputs: LifetimeSyntaxCategories<Vec<Span>>,

compiler/rustc_session/src/filesearch.rs

@@ -72,7 +72,7 @@ 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());
@@ -151,7 +151,10 @@ 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

@@ -622,7 +622,7 @@ 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

@@ -565,6 +565,7 @@ 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

@@ -336,6 +336,7 @@ unsafe fn throw_exception(data: Option<Box<dyn Any + Send>>) -> ! {
     // 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::<_, { AtomicOrdering::SeqCst }>(
             (&raw mut THROW_INFO.pmfnUnwind).cast(),
             ptr_t::new(exception_cleanup as *mut u8).raw(),
@@ -352,6 +353,7 @@ unsafe fn throw_exception(data: Option<Box<dyn Any + Send>>) -> ! {
             (&raw mut CATCHABLE_TYPE.pType).cast(),
             ptr_t::new((&raw mut TYPE_DESCRIPTOR).cast()).raw(),
         );
+        #[allow(function_casts_as_integer)]
         atomic_store::<_, { AtomicOrdering::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