Auto merge of #83129 - LeSeulArtichaut:thir-unsafeck, r=nikomatsakis

Introduce the beginning of a THIR unsafety checker

This poses the foundations for the THIR unsafety checker, so that it can be implemented incrementally:
- implements a rudimentary `Visitor` for the THIR (which will definitely need some tweaking in the future)
- introduces a new `-Zthir-unsafeck` flag which tells the compiler to use THIR unsafeck instead of MIR unsafeck
- implements detection of unsafe functions
- adds revisions to the UI tests to test THIR unsafeck alongside MIR unsafeck

This uses a very simple query design, where bodies are unsafety-checked on a body per body basis. This however has some big flaws:
- the unsafety-checker builds the THIR itself, which means a lot of work is duplicated with MIR building constructing its own copy of the THIR
- unsafety-checking closures is currently completely wrong: closures should take into account the "safety context" in which they are created, here we are considering that closures are always a safe context

I had intended to fix these problems in follow-up PRs since they are always gated under the `-Zthir-unsafeck` flag (which is explicitely noted to be unsound).

r? `@nikomatsakis`
cc rust-lang/project-thir-unsafeck#3 rust-lang/project-thir-unsafeck#7

Author: bors

compiler/rustc_interface/src/passes.rs

@@ -876,7 +876,11 @@ fn analysis(tcx: TyCtxt<'_>, cnum: CrateNum) -> Result<()> {
 
     sess.time("MIR_effect_checking", || {
         for def_id in tcx.body_owners() {
-            mir::transform::check_unsafety::check_unsafety(tcx, def_id);
+            if tcx.sess.opts.debugging_opts.thir_unsafeck {
+                tcx.ensure().thir_check_unsafety(def_id);
+            } else {
+                mir::transform::check_unsafety::check_unsafety(tcx, def_id);
+            }
 
             if tcx.hir().body_const_context(def_id).is_some() {
                 tcx.ensure()

compiler/rustc_interface/src/tests.rs

@@ -736,6 +736,7 @@ fn test_debugging_options_tracking_hash() {
     tracked!(symbol_mangling_version, Some(SymbolManglingVersion::V0));
     tracked!(teach, true);
     tracked!(thinlto, Some(true));
+    tracked!(thir_unsafeck, true);
     tracked!(tune_cpu, Some(String::from("abc")));
     tracked!(tls_model, Some(TlsModel::GeneralDynamic));
     tracked!(trap_unreachable, Some(false));

compiler/rustc_middle/src/query/mod.rs

@@ -611,6 +611,19 @@ rustc_queries! {
         }
     }
 
+    /// Unsafety-check this `LocalDefId` with THIR unsafeck. This should be
+    /// used with `-Zthir-unsafeck`.
+    query thir_check_unsafety(key: LocalDefId) {
+        desc { |tcx| "unsafety-checking `{}`", tcx.def_path_str(key.to_def_id()) }
+        cache_on_disk_if { true }
+    }
+    query thir_check_unsafety_for_const_arg(key: (LocalDefId, DefId)) {
+        desc {
+            |tcx| "unsafety-checking the const argument `{}`",
+            tcx.def_path_str(key.0.to_def_id())
+        }
+    }
+
     /// HACK: when evaluated, this reports a "unsafe derive on repr(packed)" error.
     ///
     /// Unsafety checking is executed for each method separately, but we only want

compiler/rustc_mir_build/src/check_unsafety.rs

@@ -0,0 +1,334 @@
+use crate::thir::visit::{self, Visitor};
+use crate::thir::*;
+
+use rustc_errors::struct_span_err;
+use rustc_hir as hir;
+use rustc_middle::ty::{self, TyCtxt};
+use rustc_session::lint::builtin::{UNSAFE_OP_IN_UNSAFE_FN, UNUSED_UNSAFE};
+use rustc_session::lint::Level;
+use rustc_span::def_id::{DefId, LocalDefId};
+use rustc_span::Span;
+
+struct UnsafetyVisitor<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    /// The `HirId` of the current scope, which would be the `HirId`
+    /// of the current HIR node, modulo adjustments. Used for lint levels.
+    hir_context: hir::HirId,
+    /// The current "safety context". This notably tracks whether we are in an
+    /// `unsafe` block, and whether it has been used.
+    safety_context: SafetyContext,
+    body_unsafety: BodyUnsafety,
+}
+
+impl<'tcx> UnsafetyVisitor<'tcx> {
+    fn in_safety_context<R>(
+        &mut self,
+        safety_context: SafetyContext,
+        f: impl FnOnce(&mut Self) -> R,
+    ) {
+        if let (
+            SafetyContext::UnsafeBlock { span: enclosing_span, .. },
+            SafetyContext::UnsafeBlock { span: block_span, hir_id, .. },
+        ) = (self.safety_context, safety_context)
+        {
+            self.warn_unused_unsafe(
+                hir_id,
+                block_span,
+                Some(self.tcx.sess.source_map().guess_head_span(enclosing_span)),
+            );
+            f(self);
+        } else {
+            let prev_context = self.safety_context;
+            self.safety_context = safety_context;
+
+            f(self);
+
+            if let SafetyContext::UnsafeBlock { used: false, span, hir_id } = self.safety_context {
+                self.warn_unused_unsafe(hir_id, span, self.body_unsafety.unsafe_fn_sig_span());
+            }
+            self.safety_context = prev_context;
+            return;
+        }
+    }
+
+    fn requires_unsafe(&mut self, span: Span, kind: UnsafeOpKind) {
+        let (description, note) = kind.description_and_note();
+        let unsafe_op_in_unsafe_fn_allowed = self.unsafe_op_in_unsafe_fn_allowed();
+        match self.safety_context {
+            SafetyContext::UnsafeBlock { ref mut used, .. } => {
+                if !self.body_unsafety.is_unsafe() || !unsafe_op_in_unsafe_fn_allowed {
+                    // Mark this block as useful
+                    *used = true;
+                }
+            }
+            SafetyContext::UnsafeFn if unsafe_op_in_unsafe_fn_allowed => {}
+            SafetyContext::UnsafeFn => {
+                // unsafe_op_in_unsafe_fn is disallowed
+                if kind == BorrowOfPackedField {
+                    // FIXME handle borrows of packed fields
+                } else {
+                    struct_span_err!(
+                        self.tcx.sess,
+                        span,
+                        E0133,
+                        "{} is unsafe and requires unsafe block",
+                        description,
+                    )
+                    .span_label(span, description)
+                    .note(note)
+                    .emit();
+                }
+            }
+            SafetyContext::Safe => {
+                if kind == BorrowOfPackedField {
+                    // FIXME handle borrows of packed fields
+                } else {
+                    let fn_sugg = if unsafe_op_in_unsafe_fn_allowed { " function or" } else { "" };
+                    struct_span_err!(
+                        self.tcx.sess,
+                        span,
+                        E0133,
+                        "{} is unsafe and requires unsafe{} block",
+                        description,
+                        fn_sugg,
+                    )
+                    .span_label(span, description)
+                    .note(note)
+                    .emit();
+                }
+            }
+        }
+    }
+
+    fn warn_unused_unsafe(
+        &self,
+        hir_id: hir::HirId,
+        block_span: Span,
+        enclosing_span: Option<Span>,
+    ) {
+        let block_span = self.tcx.sess.source_map().guess_head_span(block_span);
+        self.tcx.struct_span_lint_hir(UNUSED_UNSAFE, hir_id, block_span, |lint| {
+            let msg = "unnecessary `unsafe` block";
+            let mut db = lint.build(msg);
+            db.span_label(block_span, msg);
+            if let Some(enclosing_span) = enclosing_span {
+                db.span_label(
+                    enclosing_span,
+                    format!("because it's nested under this `unsafe` block"),
+                );
+            }
+            db.emit();
+        });
+    }
+
+    /// Whether the `unsafe_op_in_unsafe_fn` lint is `allow`ed at the current HIR node.
+    fn unsafe_op_in_unsafe_fn_allowed(&self) -> bool {
+        self.tcx.lint_level_at_node(UNSAFE_OP_IN_UNSAFE_FN, self.hir_context).0 == Level::Allow
+    }
+}
+
+impl<'thir, 'tcx> Visitor<'thir, 'tcx> for UnsafetyVisitor<'tcx> {
+    fn visit_block(&mut self, block: &Block<'thir, 'tcx>) {
+        if let BlockSafety::ExplicitUnsafe(hir_id) = block.safety_mode {
+            self.in_safety_context(
+                SafetyContext::UnsafeBlock { span: block.span, hir_id, used: false },
+                |this| visit::walk_block(this, block),
+            );
+        } else {
+            visit::walk_block(self, block);
+        }
+    }
+
+    fn visit_expr(&mut self, expr: &'thir Expr<'thir, 'tcx>) {
+        match expr.kind {
+            ExprKind::Scope { value, lint_level: LintLevel::Explicit(hir_id), region_scope: _ } => {
+                let prev_id = self.hir_context;
+                self.hir_context = hir_id;
+                self.visit_expr(value);
+                self.hir_context = prev_id;
+                return;
+            }
+            ExprKind::Call { fun, ty: _, args: _, from_hir_call: _, fn_span: _ } => {
+                if fun.ty.fn_sig(self.tcx).unsafety() == hir::Unsafety::Unsafe {
+                    self.requires_unsafe(expr.span, CallToUnsafeFunction);
+                }
+            }
+            _ => {}
+        }
+
+        visit::walk_expr(self, expr);
+    }
+}
+
+#[derive(Clone, Copy)]
+enum SafetyContext {
+    Safe,
+    UnsafeFn,
+    UnsafeBlock { span: Span, hir_id: hir::HirId, used: bool },
+}
+
+#[derive(Clone, Copy)]
+enum BodyUnsafety {
+    /// The body is not unsafe.
+    Safe,
+    /// The body is an unsafe function. The span points to
+    /// the signature of the function.
+    Unsafe(Span),
+}
+
+impl BodyUnsafety {
+    /// Returns whether the body is unsafe.
+    fn is_unsafe(&self) -> bool {
+        matches!(self, BodyUnsafety::Unsafe(_))
+    }
+
+    /// If the body is unsafe, returns the `Span` of its signature.
+    fn unsafe_fn_sig_span(self) -> Option<Span> {
+        match self {
+            BodyUnsafety::Unsafe(span) => Some(span),
+            BodyUnsafety::Safe => None,
+        }
+    }
+}
+
+#[derive(Clone, Copy, PartialEq)]
+enum UnsafeOpKind {
+    CallToUnsafeFunction,
+    #[allow(dead_code)] // FIXME
+    UseOfInlineAssembly,
+    #[allow(dead_code)] // FIXME
+    InitializingTypeWith,
+    #[allow(dead_code)] // FIXME
+    CastOfPointerToInt,
+    #[allow(dead_code)] // FIXME
+    BorrowOfPackedField,
+    #[allow(dead_code)] // FIXME
+    UseOfMutableStatic,
+    #[allow(dead_code)] // FIXME
+    UseOfExternStatic,
+    #[allow(dead_code)] // FIXME
+    DerefOfRawPointer,
+    #[allow(dead_code)] // FIXME
+    AssignToDroppingUnionField,
+    #[allow(dead_code)] // FIXME
+    AccessToUnionField,
+    #[allow(dead_code)] // FIXME
+    MutationOfLayoutConstrainedField,
+    #[allow(dead_code)] // FIXME
+    BorrowOfLayoutConstrainedField,
+    #[allow(dead_code)] // FIXME
+    CallToFunctionWith,
+}
+
+use UnsafeOpKind::*;
+
+impl UnsafeOpKind {
+    pub fn description_and_note(&self) -> (&'static str, &'static str) {
+        match self {
+            CallToUnsafeFunction => (
+                "call to unsafe function",
+                "consult the function's documentation for information on how to avoid undefined \
+                 behavior",
+            ),
+            UseOfInlineAssembly => (
+                "use of inline assembly",
+                "inline assembly is entirely unchecked and can cause undefined behavior",
+            ),
+            InitializingTypeWith => (
+                "initializing type with `rustc_layout_scalar_valid_range` attr",
+                "initializing a layout restricted type's field with a value outside the valid \
+                 range is undefined behavior",
+            ),
+            CastOfPointerToInt => {
+                ("cast of pointer to int", "casting pointers to integers in constants")
+            }
+            BorrowOfPackedField => (
+                "borrow of packed field",
+                "fields of packed structs might be misaligned: dereferencing a misaligned pointer \
+                 or even just creating a misaligned reference is undefined behavior",
+            ),
+            UseOfMutableStatic => (
+                "use of mutable static",
+                "mutable statics can be mutated by multiple threads: aliasing violations or data \
+                 races will cause undefined behavior",
+            ),
+            UseOfExternStatic => (
+                "use of extern static",
+                "extern statics are not controlled by the Rust type system: invalid data, \
+                 aliasing violations or data races will cause undefined behavior",
+            ),
+            DerefOfRawPointer => (
+                "dereference of raw pointer",
+                "raw pointers may be NULL, dangling or unaligned; they can violate aliasing rules \
+                 and cause data races: all of these are undefined behavior",
+            ),
+            AssignToDroppingUnionField => (
+                "assignment to union field that might need dropping",
+                "the previous content of the field will be dropped, which causes undefined \
+                 behavior if the field was not properly initialized",
+            ),
+            AccessToUnionField => (
+                "access to union field",
+                "the field may not be properly initialized: using uninitialized data will cause \
+                 undefined behavior",
+            ),
+            MutationOfLayoutConstrainedField => (
+                "mutation of layout constrained field",
+                "mutating layout constrained fields cannot statically be checked for valid values",
+            ),
+            BorrowOfLayoutConstrainedField => (
+                "borrow of layout constrained field with interior mutability",
+                "references to fields of layout constrained fields lose the constraints. Coupled \
+                 with interior mutability, the field can be changed to invalid values",
+            ),
+            CallToFunctionWith => (
+                "call to function with `#[target_feature]`",
+                "can only be called if the required target features are available",
+            ),
+        }
+    }
+}
+
+// FIXME: checking unsafety for closures should be handled by their parent body,
+// as they inherit their "safety context" from their declaration site.
+pub fn check_unsafety<'tcx>(tcx: TyCtxt<'tcx>, thir: &Expr<'_, 'tcx>, hir_id: hir::HirId) {
+    let body_unsafety = tcx.hir().fn_sig_by_hir_id(hir_id).map_or(BodyUnsafety::Safe, |fn_sig| {
+        if fn_sig.header.unsafety == hir::Unsafety::Unsafe {
+            BodyUnsafety::Unsafe(fn_sig.span)
+        } else {
+            BodyUnsafety::Safe
+        }
+    });
+    let safety_context =
+        if body_unsafety.is_unsafe() { SafetyContext::UnsafeFn } else { SafetyContext::Safe };
+    let mut visitor = UnsafetyVisitor { tcx, safety_context, hir_context: hir_id, body_unsafety };
+    visitor.visit_expr(thir);
+}
+
+crate fn thir_check_unsafety_inner<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    def: ty::WithOptConstParam<LocalDefId>,
+) {
+    let hir_id = tcx.hir().local_def_id_to_hir_id(def.did);
+    let body_id = tcx.hir().body_owned_by(hir_id);
+    let body = tcx.hir().body(body_id);
+
+    let arena = Arena::default();
+    let thir = cx::build_thir(tcx, def, &arena, &body.value);
+    check_unsafety(tcx, thir, hir_id);
+}
+
+crate fn thir_check_unsafety<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) {
+    if let Some(def) = ty::WithOptConstParam::try_lookup(def_id, tcx) {
+        tcx.thir_check_unsafety_for_const_arg(def)
+    } else {
+        thir_check_unsafety_inner(tcx, ty::WithOptConstParam::unknown(def_id))
+    }
+}
+
+crate fn thir_check_unsafety_for_const_arg<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    (did, param_did): (LocalDefId, DefId),
+) {
+    thir_check_unsafety_inner(tcx, ty::WithOptConstParam { did, const_param_did: Some(param_did) })
+}

compiler/rustc_mir_build/src/lib.rs

@@ -19,6 +19,7 @@ extern crate tracing;
 extern crate rustc_middle;
 
 mod build;
+mod check_unsafety;
 mod lints;
 pub mod thir;
 
@@ -28,4 +29,6 @@ pub fn provide(providers: &mut Providers) {
     providers.check_match = thir::pattern::check_match;
     providers.lit_to_const = thir::constant::lit_to_const;
     providers.mir_built = build::mir_built;
+    providers.thir_check_unsafety = check_unsafety::thir_check_unsafety;
+    providers.thir_check_unsafety_for_const_arg = check_unsafety::thir_check_unsafety_for_const_arg;
 }

compiler/rustc_mir_build/src/thir/mod.rs

@@ -29,6 +29,7 @@ mod arena;
 pub use arena::Arena;
 
 mod util;
+pub mod visit;
 
 #[derive(Copy, Clone, Debug)]
 pub enum LintLevel {