Split fuzzy_provenance_casts into lossy and fuzzy, feature gate and test it

* split `fuzzy_provenance_casts` into a ptr2int and a int2ptr lint
* feature gate both lints
* update documentation to be more realistic short term
* add tests for these lints

Author: niluxv

compiler/rustc_feature/src/active.rs

@@ -505,6 +505,8 @@ declare_features! (
     (active, static_nobundle, "1.16.0", Some(37403), None),
     /// Allows attributes on expressions and non-item statements.
     (active, stmt_expr_attributes, "1.6.0", Some(15701), None),
+    /// Allows lints part of the strict provenance effort.
+    (active, strict_provenance, "1.61.0", Some(95228), None),
     /// Allows the use of `#[target_feature]` on safe functions.
     (active, target_feature_11, "1.45.0", Some(69098), None),
     /// Allows using `#[thread_local]` on `static` items.

compiler/rustc_lint_defs/src/builtin.rs

@@ -2654,33 +2654,88 @@ declare_lint! {
     ///
     /// ### Example
     ///
+    /// ```rust
+    /// #![feature(strict_provenance)]
+    /// #![warn(fuzzy_provenance_casts)]
+    ///
     /// fn main() {
-    ///     let my_ref = &0;
-    ///     let my_addr = my_ref as usize;
+    ///     let _dangling = 16_usize as *const u8;
     /// }
     /// ```
     ///
     /// {{produces}}
     ///
     /// ### Explanation
     ///
-    /// Casting a pointer to an integer or an integer to a pointer is a lossy operation,
-    /// because beyond just an *address* a pointer may be associated with a particular
-    /// *provenance* and *segment*. This information is required by both the compiler
-    /// and the hardware to correctly execute your code. If you need to do this kind
-    /// of operation, use ptr::addr and ptr::with_addr.
+    /// This lint is part of the strict provenance effort, see [issue #95228].
+    /// Casting an integer to a pointer is considered bad style, as a pointer
+    /// contains, besides the *address* also a *provenance*, indicating what
+    /// memory the pointer is allowed to read/write. Casting an integer, which
+    /// doesn't have provenance, to a pointer requires the compiler to assign
+    /// (guess) provenance. The compiler assigns "all exposed valid" (see the
+    /// docs of [`ptr::from_exposed_addr`] for more information about this
+    /// "exposing"). This penalizes the optimiser and is not well suited for
+    /// dynamic analysis/dynamic program verification (e.g. Miri or CHERI
+    /// platforms).
     ///
-    /// This is a [future-incompatible] lint to transition this to a hard error
-    /// in the future. See [issue #9999999] for more details.
+    /// It is much better to use [`ptr::with_addr`] instead to specify the
+    /// provenance you want. If using this function is not possible because the
+    /// code relies on exposed provenance then there is as an escape hatch
+    /// [`ptr::from_exposed_addr`].
     ///
-    /// [future-incompatible]: ../index.md#future-incompatible-lints
-    /// [issue #9999999]: https://github.com/rust-lang/rust/issues/9999999
+    /// [issue #95228]: https://github.com/rust-lang/rust/issues/95228
+    /// [`ptr::with_addr`]: https://doc.rust-lang.org/core/ptr/fn.with_addr
+    /// [`ptr::from_exposed_addr`]: https://doc.rust-lang.org/core/ptr/fn.from_exposed_addr
     pub FUZZY_PROVENANCE_CASTS,
-    Warn,
-    "A lossy pointer-integer integer cast is used",
-    @future_incompatible = FutureIncompatibleInfo {
-        reference: "issue #9999999 <https://github.com/rust-lang/rust/issues/9999999>",
-    };
+    Allow,
+    "a fuzzy integer to pointer cast is used",
+    @feature_gate = sym::strict_provenance;
+}
+
+declare_lint! {
+    /// The `lossy_provenance_casts` lint detects an `as` cast between a pointer
+    /// and an integer.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// #![feature(strict_provenance)]
+    /// #![warn(lossy_provenance_casts)]
+    ///
+    /// fn main() {
+    ///     let x: u8 = 37;
+    ///     let _addr: usize = &x as *const u8 as usize;
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// This lint is part of the strict provenance effort, see [issue #95228].
+    /// Casting a pointer to an integer is a lossy operation, because beyond
+    /// just an *address* a pointer may be associated with a particular
+    /// *provenance*. This information is used by the optimiser and for dynamic
+    /// analysis/dynamic program verification (e.g. Miri or CHERI platforms).
+    ///
+    /// Since this cast is lossy, it is considered good style to use the
+    /// [`ptr::addr`] method instead, which has a similar effect, but doesn't
+    /// "expose" the pointer provenance. This improves optimisation potential.
+    /// See the docs of [`ptr::addr`] and [`ptr::expose_addr`] for more information
+    /// about exposing pointer provenance.
+    ///
+    /// If your code can't comply with strict provenance and needs to expose
+    /// the provenance, then there is [`ptr::expose_addr`] as an escape hatch,
+    /// which preserves the behaviour of `as usize` casts while being explicit
+    /// about the semantics.
+    ///
+    /// [issue #95228]: https://github.com/rust-lang/rust/issues/95228
+    /// [`ptr::addr`]: https://doc.rust-lang.org/core/ptr/fn.addr
+    /// [`ptr::expose_addr`]: https://doc.rust-lang.org/core/ptr/fn.expose_addr
+    pub LOSSY_PROVENANCE_CASTS,
+    Allow,
+    "a lossy pointer to integer cast is used",
+    @feature_gate = sym::strict_provenance;
 }
 
 declare_lint! {
@@ -3137,6 +3192,7 @@ declare_lint_pass! {
         INCOMPLETE_INCLUDE,
         CENUM_IMPL_DROP_CAST,
         FUZZY_PROVENANCE_CASTS,
+        LOSSY_PROVENANCE_CASTS,
         CONST_EVALUATABLE_UNCHECKED,
         INEFFECTIVE_UNSTABLE_TRAIT_IMPL,
         MUST_NOT_SUSPEND,

compiler/rustc_span/src/symbol.rs

@@ -1348,6 +1348,7 @@ symbols! {
         str_trim,
         str_trim_end,
         str_trim_start,
+        strict_provenance,
         stringify,
         stringify_macro,
         struct_field_attributes,

compiler/rustc_typeck/src/check/cast.rs

@@ -809,12 +809,12 @@ impl<'a, 'tcx> CastCheck<'tcx> {
             (Ptr(m_e), Ptr(m_c)) => self.check_ptr_ptr_cast(fcx, m_e, m_c), // ptr-ptr-cast
 
             // ptr-addr-cast
-            (Ptr(m_expr), Int(_)) => {
-                self.fuzzy_provenance_ptr2int_lint(fcx, t_from);
+            (Ptr(m_expr), Int(t_c)) => {
+                self.lossy_provenance_ptr2int_lint(fcx, t_c);
                 self.check_ptr_addr_cast(fcx, m_expr)
             }
             (FnPtr, Int(_)) => {
-                self.fuzzy_provenance_ptr2int_lint(fcx, t_from);
+                // FIXME(#95489): there should eventually be a lint for these casts
                 Ok(CastKind::FnPtrAddrCast)
             }
             // addr-ptr-cast
@@ -945,7 +945,6 @@ impl<'a, 'tcx> CastCheck<'tcx> {
         fcx: &FnCtxt<'a, 'tcx>,
         m_cast: TypeAndMut<'tcx>,
     ) -> Result<CastKind, CastError> {
-        self.fuzzy_provenance_int2ptr_lint(fcx);
         // ptr-addr cast. pointer must be thin.
         match fcx.pointer_kind(m_cast.ty, self.span)? {
             None => Err(CastError::UnknownCastPtrKind),
@@ -986,25 +985,36 @@ impl<'a, 'tcx> CastCheck<'tcx> {
         }
     }
 
-    fn fuzzy_provenance_ptr2int_lint(&self, fcx: &FnCtxt<'a, 'tcx>, t_from: CastTy<'tcx>) {
+    fn lossy_provenance_ptr2int_lint(&self, fcx: &FnCtxt<'a, 'tcx>, t_c: ty::cast::IntTy) {
         fcx.tcx.struct_span_lint_hir(
-            lint::builtin::FUZZY_PROVENANCE_CASTS,
+            lint::builtin::LOSSY_PROVENANCE_CASTS,
             self.expr.hir_id,
             self.span,
             |err| {
                 let mut err = err.build(&format!(
-                    "strict provenance disallows casting pointer `{}` to integer `{}`",
+                    "under strict provenance it is considered bad style to cast pointer `{}` to integer `{}`",
                     self.expr_ty, self.cast_ty
                 ));
 
-                if let CastTy::FnPtr = t_from {
-                    err.help(
-                        "use `(... as *const u8).addr()` to obtain \
-                         the address of a function pointer",
+                let msg = "use `.addr()` to obtain the address of a pointer";
+                if let Ok(snippet) = fcx.tcx.sess.source_map().span_to_snippet(self.expr.span) {
+                    let scalar_cast = match t_c {
+                        ty::cast::IntTy::U(ty::UintTy::Usize) => String::new(),
+                        _ => format!(" as {}", self.cast_ty),
+                    };
+                    err.span_suggestion(
+                        self.span,
+                        msg,
+                        format!("({}).addr(){}", snippet, scalar_cast),
+                        Applicability::MaybeIncorrect
                     );
                 } else {
-                    err.help("use `.addr()` to obtain the address of a pointer");
+                    err.help(msg);
                 }
+                err.help(
+                    "if you can't comply with strict provenance and need to expose the pointer\
+                    provenance you can use `.expose_addr()` instead"
+                );
 
                 err.emit();
             },
@@ -1017,15 +1027,28 @@ impl<'a, 'tcx> CastCheck<'tcx> {
             self.expr.hir_id,
             self.span,
             |err| {
-                err.build(&format!(
+
+                let mut err = err.build(&format!(
                     "strict provenance disallows casting integer `{}` to pointer `{}`",
                     self.expr_ty, self.cast_ty
-                ))
-                .help(
-                    "use `.with_addr(...)` to adjust a valid pointer \
-                     in the same allocation, to this address",
-                )
-                .emit();
+                ));
+                let msg = "use `.with_addr()` to adjust a valid pointer in the same allocation, to this address";
+                if let Ok(snippet) = fcx.tcx.sess.source_map().span_to_snippet(self.expr.span) {
+                    err.span_suggestion(
+                        self.span,
+                        msg,
+                        format!("(...).with_addr({})", snippet),
+                        Applicability::HasPlaceholders,
+                    );
+                } else {
+                    err.help(msg);
+                }
+                err.help(
+                    "if you can't comply with strict provenance and don't have a pointer with \
+                    the correct provenance you can use `std::ptr::from_exposed_addr()` instead"
+                 );
+
+                err.emit();
             },
         );
     }

src/doc/unstable-book/src/language-features/strict-provenance.md

@@ -0,0 +1,22 @@
+# `strict_provenance`
+
+The tracking issue for this feature is: [#95228]
+
+[#95228]: https://github.com/rust-lang/rust/issues/95228
+-----
+
+The `strict_provenance` feature allows to enable the `fuzzy_provenance_casts` and `lossy_provenance_casts` lints.
+These lint on casts between integers and pointers, that are recommended against or invalid in the strict provenance model.
+The same feature gate is also used for the experimental strict provenance API in `std` (actually `core`).
+
+## Example
+
+```rust
+#![feature(strict_provenance)]
+#![warn(fuzzy_provenance_casts)]
+
+fn main() {
+    let _dangling = 16_usize as *const u8;
+    //~^ WARNING: strict provenance disallows casting integer `usize` to pointer `*const u8`
+}
+```

src/test/ui/feature-gates/feature-gate-strict_provenance.rs

@@ -0,0 +1,19 @@
+// check-pass
+
+#![deny(fuzzy_provenance_casts)]
+//~^ WARNING unknown lint: `fuzzy_provenance_casts`
+//~| WARNING unknown lint: `fuzzy_provenance_casts`
+//~| WARNING unknown lint: `fuzzy_provenance_casts`
+#![deny(lossy_provenance_casts)]
+//~^ WARNING unknown lint: `lossy_provenance_casts`
+//~| WARNING unknown lint: `lossy_provenance_casts`
+//~| WARNING unknown lint: `lossy_provenance_casts`
+
+fn main() {
+    // no warnings emitted since the lints are not activated
+
+    let _dangling = 16_usize as *const u8;
+
+    let x: u8 = 37;
+    let _addr: usize = &x as *const u8 as usize;
+}

src/test/ui/feature-gates/feature-gate-strict_provenance.stderr

@@ -0,0 +1,63 @@
+warning: unknown lint: `fuzzy_provenance_casts`
+  --> $DIR/feature-gate-strict_provenance.rs:3:1
+   |
+LL | #![deny(fuzzy_provenance_casts)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(unknown_lints)]` on by default
+   = note: the `fuzzy_provenance_casts` lint is unstable
+   = note: see issue #95228 <https://github.com/rust-lang/rust/issues/95228> for more information
+   = help: add `#![feature(strict_provenance)]` to the crate attributes to enable
+
+warning: unknown lint: `lossy_provenance_casts`
+  --> $DIR/feature-gate-strict_provenance.rs:7:1
+   |
+LL | #![deny(lossy_provenance_casts)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the `lossy_provenance_casts` lint is unstable
+   = note: see issue #95228 <https://github.com/rust-lang/rust/issues/95228> for more information
+   = help: add `#![feature(strict_provenance)]` to the crate attributes to enable
+
+warning: unknown lint: `fuzzy_provenance_casts`
+  --> $DIR/feature-gate-strict_provenance.rs:3:1
+   |
+LL | #![deny(fuzzy_provenance_casts)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the `fuzzy_provenance_casts` lint is unstable
+   = note: see issue #95228 <https://github.com/rust-lang/rust/issues/95228> for more information
+   = help: add `#![feature(strict_provenance)]` to the crate attributes to enable
+
+warning: unknown lint: `lossy_provenance_casts`
+  --> $DIR/feature-gate-strict_provenance.rs:7:1
+   |
+LL | #![deny(lossy_provenance_casts)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the `lossy_provenance_casts` lint is unstable
+   = note: see issue #95228 <https://github.com/rust-lang/rust/issues/95228> for more information
+   = help: add `#![feature(strict_provenance)]` to the crate attributes to enable
+
+warning: unknown lint: `fuzzy_provenance_casts`
+  --> $DIR/feature-gate-strict_provenance.rs:3:1
+   |
+LL | #![deny(fuzzy_provenance_casts)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the `fuzzy_provenance_casts` lint is unstable
+   = note: see issue #95228 <https://github.com/rust-lang/rust/issues/95228> for more information
+   = help: add `#![feature(strict_provenance)]` to the crate attributes to enable
+
+warning: unknown lint: `lossy_provenance_casts`
+  --> $DIR/feature-gate-strict_provenance.rs:7:1
+   |
+LL | #![deny(lossy_provenance_casts)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the `lossy_provenance_casts` lint is unstable
+   = note: see issue #95228 <https://github.com/rust-lang/rust/issues/95228> for more information
+   = help: add `#![feature(strict_provenance)]` to the crate attributes to enable
+
+warning: 6 warnings emitted
+

src/test/ui/lint/lint-strict-provenance-fuzzy-casts.rs

@@ -0,0 +1,7 @@
+#![feature(strict_provenance)]
+#![deny(fuzzy_provenance_casts)]
+
+fn main() {
+    let dangling = 16_usize as *const u8;
+    //~^ ERROR strict provenance disallows casting integer `usize` to pointer `*const u8`
+}

src/test/ui/lint/lint-strict-provenance-fuzzy-casts.stderr

@@ -0,0 +1,19 @@
+error: strict provenance disallows casting integer `usize` to pointer `*const u8`
+  --> $DIR/lint-strict-provenance-fuzzy-casts.rs:5:20
+   |
+LL |     let dangling = 16_usize as *const u8;
+   |                    ^^^^^^^^^^^^^^^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/lint-strict-provenance-fuzzy-casts.rs:2:9
+   |
+LL | #![deny(fuzzy_provenance_casts)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^
+   = help: if you can't comply with strict provenance and don't have a pointer with the correct provenance you can use `std::ptr::from_exposed_addr()` instead
+help: use `.with_addr()` to adjust a valid pointer in the same allocation, to this address
+   |
+LL |     let dangling = (...).with_addr(16_usize);
+   |                    ~~~~~~~~~~~~~~~~~~~~~~~~~
+
+error: aborting due to previous error
+

src/test/ui/lint/lint-strict-provenance-lossy-casts.rs

@@ -0,0 +1,11 @@
+#![feature(strict_provenance)]
+#![deny(lossy_provenance_casts)]
+
+fn main() {
+    let x: u8 = 37;
+    let addr: usize = &x as *const u8 as usize;
+    //~^ ERROR under strict provenance it is considered bad style to cast pointer `*const u8` to integer `usize`
+
+    let addr_32bit = &x as *const u8 as u32;
+    //~^ ERROR under strict provenance it is considered bad style to cast pointer `*const u8` to integer `u32`
+}