Auto merge of #93678 - steffahn:better_unsafe_diagnostics, r=nagisa

Improve `unused_unsafe` lint

I’m going to add some motivation and explanation below, particularly pointing the changes in behavior from this PR.

_Edit:_ Looking for existing issues, looks like this PR fixes #88260.

_Edit2:_ Now also contains code that closes #90776.

compiler/rustc_codegen_llvm/src/abi.rs

@@ -599,13 +599,11 @@ impl<'ll, 'tcx> FnAbiLlvmExt<'ll, 'tcx> for FnAbi<'tcx, Ty<'tcx>> {
         if self.conv == Conv::CCmseNonSecureCall {
             // This will probably get ignored on all targets but those supporting the TrustZone-M
             // extension (thumbv8m targets).
-            unsafe {
+            llvm::AddCallSiteAttrString(
-                llvm::AddCallSiteAttrString(
+                callsite,
-                    callsite,
+                llvm::AttributePlace::Function,
-                    llvm::AttributePlace::Function,
+                cstr::cstr!("cmse_nonsecure_call"),
-                    cstr::cstr!("cmse_nonsecure_call"),
+            );
-                );
-            }
         }
     }
 }

compiler/rustc_middle/src/hir/nested_filter.rs

@@ -3,6 +3,10 @@ use rustc_hir::intravisit::nested_filter::NestedFilter;
 /// Do not visit nested item-like things, but visit nested things
 /// that are inside of an item-like.
 ///
+/// Notably, possible occurrences of bodies in non-item-like things
+/// include: closures/generators, inline `const {}` blocks, and
+/// constant arguments of types, e.g. in `let _: [(); /* HERE */];`.
+///
 /// **This is the most common choice.** A very common pattern is
 /// to use `visit_all_item_likes()` as an outer loop,
 /// and to have the visitor that visits the contents of each item

compiler/rustc_middle/src/lint.rs

@@ -202,6 +202,77 @@ impl<'a> LintDiagnosticBuilder<'a> {
     }
 }
 
+pub fn explain_lint_level_source<'s>(
+    sess: &'s Session,
+    lint: &'static Lint,
+    level: Level,
+    src: LintLevelSource,
+    err: &mut DiagnosticBuilder<'s>,
+) {
+    let name = lint.name_lower();
+    match src {
+        LintLevelSource::Default => {
+            sess.diag_note_once(
+                err,
+                DiagnosticMessageId::from(lint),
+                &format!("`#[{}({})]` on by default", level.as_str(), name),
+            );
+        }
+        LintLevelSource::CommandLine(lint_flag_val, orig_level) => {
+            let flag = match orig_level {
+                Level::Warn => "-W",
+                Level::Deny => "-D",
+                Level::Forbid => "-F",
+                Level::Allow => "-A",
+                Level::ForceWarn => "--force-warn",
+            };
+            let hyphen_case_lint_name = name.replace('_', "-");
+            if lint_flag_val.as_str() == name {
+                sess.diag_note_once(
+                    err,
+                    DiagnosticMessageId::from(lint),
+                    &format!(
+                        "requested on the command line with `{} {}`",
+                        flag, hyphen_case_lint_name
+                    ),
+                );
+            } else {
+                let hyphen_case_flag_val = lint_flag_val.as_str().replace('_', "-");
+                sess.diag_note_once(
+                    err,
+                    DiagnosticMessageId::from(lint),
+                    &format!(
+                        "`{} {}` implied by `{} {}`",
+                        flag, hyphen_case_lint_name, flag, hyphen_case_flag_val
+                    ),
+                );
+            }
+        }
+        LintLevelSource::Node(lint_attr_name, src, reason) => {
+            if let Some(rationale) = reason {
+                err.note(rationale.as_str());
+            }
+            sess.diag_span_note_once(
+                err,
+                DiagnosticMessageId::from(lint),
+                src,
+                "the lint level is defined here",
+            );
+            if lint_attr_name.as_str() != name {
+                let level_str = level.as_str();
+                sess.diag_note_once(
+                    err,
+                    DiagnosticMessageId::from(lint),
+                    &format!(
+                        "`#[{}({})]` implied by `#[{}({})]`",
+                        level_str, name, level_str, lint_attr_name
+                    ),
+                );
+            }
+        }
+    }
+}
+
 pub fn struct_lint_level<'s, 'd>(
     sess: &'s Session,
     lint: &'static Lint,
@@ -277,69 +348,9 @@ pub fn struct_lint_level<'s, 'd>(
             }
         }
 
-        let name = lint.name_lower();
+        explain_lint_level_source(sess, lint, level, src, &mut err);
-        match src {
-            LintLevelSource::Default => {
-                sess.diag_note_once(
-                    &mut err,
-                    DiagnosticMessageId::from(lint),
-                    &format!("`#[{}({})]` on by default", level.as_str(), name),
-                );
-            }
-            LintLevelSource::CommandLine(lint_flag_val, orig_level) => {
-                let flag = match orig_level {
-                    Level::Warn => "-W",
-                    Level::Deny => "-D",
-                    Level::Forbid => "-F",
-                    Level::Allow => "-A",
-                    Level::ForceWarn => "--force-warn",
-                };
-                let hyphen_case_lint_name = name.replace('_', "-");
-                if lint_flag_val.as_str() == name {
-                    sess.diag_note_once(
-                        &mut err,
-                        DiagnosticMessageId::from(lint),
-                        &format!(
-                            "requested on the command line with `{} {}`",
-                            flag, hyphen_case_lint_name
-                        ),
-                    );
-                } else {
-                    let hyphen_case_flag_val = lint_flag_val.as_str().replace('_', "-");
-                    sess.diag_note_once(
-                        &mut err,
-                        DiagnosticMessageId::from(lint),
-                        &format!(
-                            "`{} {}` implied by `{} {}`",
-                            flag, hyphen_case_lint_name, flag, hyphen_case_flag_val
-                        ),
-                    );
-                }
-            }
-            LintLevelSource::Node(lint_attr_name, src, reason) => {
-                if let Some(rationale) = reason {
-                    err.note(rationale.as_str());
-                }
-                sess.diag_span_note_once(
-                    &mut err,
-                    DiagnosticMessageId::from(lint),
-                    src,
-                    "the lint level is defined here",
-                );
-                if lint_attr_name.as_str() != name {
-                    let level_str = level.as_str();
-                    sess.diag_note_once(
-                        &mut err,
-                        DiagnosticMessageId::from(lint),
-                        &format!(
-                            "`#[{}({})]` implied by `#[{}({})]`",
-                            level_str, name, level_str, lint_attr_name
-                        ),
-                    );
-                }
-            }
-        }
 
+        let name = lint.name_lower();
         let is_force_warn = matches!(level, Level::ForceWarn);
         err.code(DiagnosticId::Lint { name, has_future_breakage, is_force_warn });
 

compiler/rustc_middle/src/mir/query.rs

@@ -2,7 +2,7 @@
 
 use crate::mir::{Body, Promoted};
 use crate::ty::{self, Ty, TyCtxt};
-use rustc_data_structures::sync::Lrc;
+use rustc_data_structures::stable_map::FxHashMap;
 use rustc_data_structures::vec_map::VecMap;
 use rustc_errors::ErrorReported;
 use rustc_hir as hir;
@@ -114,13 +114,44 @@ pub struct UnsafetyViolation {
     pub details: UnsafetyViolationDetails,
 }
 
-#[derive(Clone, TyEncodable, TyDecodable, HashStable, Debug)]
+#[derive(Copy, Clone, PartialEq, TyEncodable, TyDecodable, HashStable, Debug)]
+pub enum UnusedUnsafe {
+    /// `unsafe` block contains no unsafe operations
+    /// > ``unnecessary `unsafe` block``
+    Unused,
+    /// `unsafe` block nested under another (used) `unsafe` block
+    /// > ``… because it's nested under this `unsafe` block``
+    InUnsafeBlock(hir::HirId),
+    /// `unsafe` block nested under `unsafe fn`
+    /// > ``… because it's nested under this `unsafe fn` ``
+    ///
+    /// the second HirId here indicates the first usage of the `unsafe` block,
+    /// which allows retrival of the LintLevelSource for why that operation would
+    /// have been permitted without the block
+    InUnsafeFn(hir::HirId, hir::HirId),
+}
+
+#[derive(Copy, Clone, PartialEq, TyEncodable, TyDecodable, HashStable, Debug)]
+pub enum UsedUnsafeBlockData {
+    SomeDisallowedInUnsafeFn,
+    // the HirId here indicates the first usage of the `unsafe` block
+    // (i.e. the one that's first encountered in the MIR traversal of the unsafety check)
+    AllAllowedInUnsafeFn(hir::HirId),
+}
+
+#[derive(TyEncodable, TyDecodable, HashStable, Debug)]
 pub struct UnsafetyCheckResult {
     /// Violations that are propagated *upwards* from this function.
-    pub violations: Lrc<[UnsafetyViolation]>,
+    pub violations: Vec<UnsafetyViolation>,
-    /// `unsafe` blocks in this function, along with whether they are used. This is
+
-    /// used for the "unused_unsafe" lint.
+    /// Used `unsafe` blocks in this function. This is used for the "unused_unsafe" lint.
-    pub unsafe_blocks: Lrc<[(hir::HirId, bool)]>,
+    ///
+    /// The keys are the used `unsafe` blocks, the UnusedUnsafeKind indicates whether
+    /// or not any of the usages happen at a place that doesn't allow `unsafe_op_in_unsafe_fn`.
+    pub used_unsafe_blocks: FxHashMap<hir::HirId, UsedUnsafeBlockData>,
+
+    /// This is `Some` iff the item is not a closure.
+    pub unused_unsafes: Option<Vec<(hir::HirId, UnusedUnsafe)>>,
 }
 
 rustc_index::newtype_index! {

compiler/rustc_mir_build/src/build/block.rs

@@ -3,8 +3,6 @@ use crate::build::ForGuard::OutsideGuard;
 use crate::build::{BlockAnd, BlockAndExtension, BlockFrame, Builder};
 use rustc_middle::mir::*;
 use rustc_middle::thir::*;
-use rustc_session::lint::builtin::UNSAFE_OP_IN_UNSAFE_FN;
-use rustc_session::lint::Level;
 use rustc_span::Span;
 
 impl<'a, 'tcx> Builder<'a, 'tcx> {
@@ -209,28 +207,18 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         block.unit()
     }
 
-    /// If we are changing the safety mode, create a new source scope
+    /// If we are entering an unsafe block, create a new source scope
     fn update_source_scope_for_safety_mode(&mut self, span: Span, safety_mode: BlockSafety) {
         debug!("update_source_scope_for({:?}, {:?})", span, safety_mode);
         let new_unsafety = match safety_mode {
-            BlockSafety::Safe => None,
+            BlockSafety::Safe => return,
-            BlockSafety::BuiltinUnsafe => Some(Safety::BuiltinUnsafe),
+            BlockSafety::BuiltinUnsafe => Safety::BuiltinUnsafe,
             BlockSafety::ExplicitUnsafe(hir_id) => {
-                match self.in_scope_unsafe {
-                    Safety::Safe => {}
-                    // no longer treat `unsafe fn`s as `unsafe` contexts (see RFC #2585)
-                    Safety::FnUnsafe
-                        if self.tcx.lint_level_at_node(UNSAFE_OP_IN_UNSAFE_FN, hir_id).0
-                            != Level::Allow => {}
-                    _ => return,
-                }
                 self.in_scope_unsafe = Safety::ExplicitUnsafe(hir_id);
-                Some(Safety::ExplicitUnsafe(hir_id))
+                Safety::ExplicitUnsafe(hir_id)
             }
         };
 
-        if let Some(unsafety) = new_unsafety {
+        self.source_scope = self.new_source_scope(span, LintLevel::Inherited, Some(new_unsafety));
-            self.source_scope = self.new_source_scope(span, LintLevel::Inherited, Some(unsafety));
-        }
     }
 }

compiler/rustc_mir_transform/src/check_unsafety.rs

@@ -1,17 +1,17 @@
-use rustc_data_structures::fx::FxHashSet;
+use rustc_data_structures::fx::FxHashMap;
 use rustc_errors::struct_span_err;
 use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::hir_id::HirId;
 use rustc_hir::intravisit;
-use rustc_hir::Node;
 use rustc_middle::mir::visit::{MutatingUseContext, PlaceContext, Visitor};
-use rustc_middle::mir::*;
 use rustc_middle::ty::query::Providers;
 use rustc_middle::ty::{self, TyCtxt};
+use rustc_middle::{lint, mir::*};
 use rustc_session::lint::builtin::{UNSAFE_OP_IN_UNSAFE_FN, UNUSED_UNSAFE};
 use rustc_session::lint::Level;
 
+use std::collections::hash_map;
 use std::ops::Bound;
 
 pub struct UnsafetyChecker<'a, 'tcx> {
@@ -21,9 +21,12 @@ pub struct UnsafetyChecker<'a, 'tcx> {
     source_info: SourceInfo,
     tcx: TyCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
-    /// Mark an `unsafe` block as used, so we don't lint it.
+
-    used_unsafe: FxHashSet<hir::HirId>,
+    /// Used `unsafe` blocks in this function. This is used for the "unused_unsafe" lint.
-    inherited_blocks: Vec<(hir::HirId, bool)>,
+    ///
+    /// The keys are the used `unsafe` blocks, the UnusedUnsafeKind indicates whether
+    /// or not any of the usages happen at a place that doesn't allow `unsafe_op_in_unsafe_fn`.
+    used_unsafe_blocks: FxHashMap<HirId, UsedUnsafeBlockData>,
 }
 
 impl<'a, 'tcx> UnsafetyChecker<'a, 'tcx> {
@@ -40,8 +43,7 @@ impl<'a, 'tcx> UnsafetyChecker<'a, 'tcx> {
             source_info: SourceInfo::outermost(body.span),
             tcx,
             param_env,
-            used_unsafe: Default::default(),
+            used_unsafe_blocks: Default::default(),
-            inherited_blocks: vec![],
         }
     }
 }
@@ -123,9 +125,9 @@ impl<'tcx> Visitor<'tcx> for UnsafetyChecker<'_, 'tcx> {
                     }
                 }
                 &AggregateKind::Closure(def_id, _) | &AggregateKind::Generator(def_id, _, _) => {
-                    let UnsafetyCheckResult { violations, unsafe_blocks } =
+                    let UnsafetyCheckResult { violations, used_unsafe_blocks, .. } =
                         self.tcx.unsafety_check_result(def_id.expect_local());
-                    self.register_violations(&violations, &unsafe_blocks);
+                    self.register_violations(violations, used_unsafe_blocks);
                 }
             },
             _ => {}
@@ -251,61 +253,72 @@ impl<'tcx> UnsafetyChecker<'_, 'tcx> {
             .assert_crate_local()
             .lint_root;
         self.register_violations(
-            &[UnsafetyViolation { source_info, lint_root, kind, details }],
+            [&UnsafetyViolation { source_info, lint_root, kind, details }],
-            &[],
+            [],
         );
     }
 
-    fn register_violations(
+    fn register_violations<'a>(
         &mut self,
-        violations: &[UnsafetyViolation],
+        violations: impl IntoIterator<Item = &'a UnsafetyViolation>,
-        unsafe_blocks: &[(hir::HirId, bool)],
+        new_used_unsafe_blocks: impl IntoIterator<Item = (&'a HirId, &'a UsedUnsafeBlockData)>,
     ) {
+        use UsedUnsafeBlockData::{AllAllowedInUnsafeFn, SomeDisallowedInUnsafeFn};
+
+        let update_entry = |this: &mut Self, hir_id, new_usage| {
+            match this.used_unsafe_blocks.entry(hir_id) {
+                hash_map::Entry::Occupied(mut entry) => {
+                    if new_usage == SomeDisallowedInUnsafeFn {
+                        *entry.get_mut() = SomeDisallowedInUnsafeFn;
+                    }
+                }
+                hash_map::Entry::Vacant(entry) => {
+                    entry.insert(new_usage);
+                }
+            };
+        };
         let safety = self.body.source_scopes[self.source_info.scope]
             .local_data
             .as_ref()
             .assert_crate_local()
             .safety;
-        let within_unsafe = match safety {
+        match safety {
             // `unsafe` blocks are required in safe code
-            Safety::Safe => {
+            Safety::Safe => violations.into_iter().for_each(|&violation| {
-                for violation in violations {
+                match violation.kind {
-                    match violation.kind {
+                    UnsafetyViolationKind::General => {}
-                        UnsafetyViolationKind::General => {}
+                    UnsafetyViolationKind::UnsafeFn => {
-                        UnsafetyViolationKind::UnsafeFn => {
+                        bug!("`UnsafetyViolationKind::UnsafeFn` in an `Safe` context")
-                            bug!("`UnsafetyViolationKind::UnsafeFn` in an `Safe` context")
-                        }
-                    }
-                    if !self.violations.contains(violation) {
-                        self.violations.push(*violation)
                     }
                 }
-                false
+                if !self.violations.contains(&violation) {
-            }
+                    self.violations.push(violation)
+                }
+            }),
             // With the RFC 2585, no longer allow `unsafe` operations in `unsafe fn`s
-            Safety::FnUnsafe => {
+            Safety::FnUnsafe => violations.into_iter().for_each(|&(mut violation)| {
-                for violation in violations {
+                violation.kind = UnsafetyViolationKind::UnsafeFn;
-                    let mut violation = *violation;
+                if !self.violations.contains(&violation) {
-
+                    self.violations.push(violation)
-                    violation.kind = UnsafetyViolationKind::UnsafeFn;
-                    if !self.violations.contains(&violation) {
-                        self.violations.push(violation)
-                    }
                 }
-                false
+            }),
-            }
+            Safety::BuiltinUnsafe => {}
-            Safety::BuiltinUnsafe => true,
+            Safety::ExplicitUnsafe(hir_id) => violations.into_iter().for_each(|violation| {
-            Safety::ExplicitUnsafe(hir_id) => {
+                update_entry(
-                // mark unsafe block as used if there are any unsafe operations inside
+                    self,
-                if !violations.is_empty() {
+                    hir_id,
-                    self.used_unsafe.insert(hir_id);
+                    match self.tcx.lint_level_at_node(UNSAFE_OP_IN_UNSAFE_FN, violation.lint_root).0
-                }
+                    {
-                true
+                        Level::Allow => AllAllowedInUnsafeFn(violation.lint_root),
-            }
+                        _ => SomeDisallowedInUnsafeFn,
+                    },
+                )
+            }),
         };
-        self.inherited_blocks.extend(
+
-            unsafe_blocks.iter().map(|&(hir_id, is_used)| (hir_id, is_used && !within_unsafe)),
+        new_used_unsafe_blocks
-        );
+            .into_iter()
+            .for_each(|(&hir_id, &usage_data)| update_entry(self, hir_id, usage_data));
     }
     fn check_mut_borrowing_layout_constrained_field(
         &mut self,
@@ -387,17 +400,64 @@ pub(crate) fn provide(providers: &mut Providers) {
     };
 }
 
-struct UnusedUnsafeVisitor<'a> {
+/// Context information for [`UnusedUnsafeVisitor`] traversal,
-    used_unsafe: &'a FxHashSet<hir::HirId>,
+/// saves (innermost) relevant context
-    unsafe_blocks: &'a mut Vec<(hir::HirId, bool)>,
+#[derive(Copy, Clone, Debug)]
+enum Context {
+    Safe,
+    /// in an `unsafe fn`
+    UnsafeFn(HirId),
+    /// in a *used* `unsafe` block
+    /// (i.e. a block without unused-unsafe warning)
+    UnsafeBlock(HirId),
 }
 
-impl<'tcx> intravisit::Visitor<'tcx> for UnusedUnsafeVisitor<'_> {
+struct UnusedUnsafeVisitor<'a, 'tcx> {
+    tcx: TyCtxt<'tcx>,
+    used_unsafe_blocks: &'a FxHashMap<HirId, UsedUnsafeBlockData>,
+    context: Context,
+    unused_unsafes: &'a mut Vec<(HirId, UnusedUnsafe)>,
+}
+
+impl<'tcx> intravisit::Visitor<'tcx> for UnusedUnsafeVisitor<'_, 'tcx> {
     fn visit_block(&mut self, block: &'tcx hir::Block<'tcx>) {
-        intravisit::walk_block(self, block);
+        use UsedUnsafeBlockData::{AllAllowedInUnsafeFn, SomeDisallowedInUnsafeFn};
 
         if let hir::BlockCheckMode::UnsafeBlock(hir::UnsafeSource::UserProvided) = block.rules {
-            self.unsafe_blocks.push((block.hir_id, self.used_unsafe.contains(&block.hir_id)));
+            let used = match self.tcx.lint_level_at_node(UNUSED_UNSAFE, block.hir_id) {
+                (Level::Allow, _) => Some(SomeDisallowedInUnsafeFn),
+                _ => self.used_unsafe_blocks.get(&block.hir_id).copied(),
+            };
+            let unused_unsafe = match (self.context, used) {
+                (_, None) => UnusedUnsafe::Unused,
+                (Context::Safe, Some(_))
+                | (Context::UnsafeFn(_), Some(SomeDisallowedInUnsafeFn)) => {
+                    let previous_context = self.context;
+                    self.context = Context::UnsafeBlock(block.hir_id);
+                    intravisit::walk_block(self, block);
+                    self.context = previous_context;
+                    return;
+                }
+                (Context::UnsafeFn(hir_id), Some(AllAllowedInUnsafeFn(lint_root))) => {
+                    UnusedUnsafe::InUnsafeFn(hir_id, lint_root)
+                }
+                (Context::UnsafeBlock(hir_id), Some(_)) => UnusedUnsafe::InUnsafeBlock(hir_id),
+            };
+            self.unused_unsafes.push((block.hir_id, unused_unsafe));
+        }
+        intravisit::walk_block(self, block);
+    }
+
+    fn visit_fn(
+        &mut self,
+        fk: intravisit::FnKind<'tcx>,
+        _fd: &'tcx hir::FnDecl<'tcx>,
+        b: hir::BodyId,
+        _s: rustc_span::Span,
+        _id: HirId,
+    ) {
+        if matches!(fk, intravisit::FnKind::Closure) {
+            self.visit_body(self.tcx.hir().body(b))
         }
     }
 }
@@ -405,20 +465,38 @@ impl<'tcx> intravisit::Visitor<'tcx> for UnusedUnsafeVisitor<'_> {
 fn check_unused_unsafe(
     tcx: TyCtxt<'_>,
     def_id: LocalDefId,
-    used_unsafe: &FxHashSet<hir::HirId>,
+    used_unsafe_blocks: &FxHashMap<HirId, UsedUnsafeBlockData>,
-    unsafe_blocks: &mut Vec<(hir::HirId, bool)>,
+) -> Vec<(HirId, UnusedUnsafe)> {
-) {
+    let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
-    let body_id = tcx.hir().maybe_body_owned_by(tcx.hir().local_def_id_to_hir_id(def_id));
+    let body_id = tcx.hir().maybe_body_owned_by(hir_id);
 
     let Some(body_id) = body_id else {
         debug!("check_unused_unsafe({:?}) - no body found", def_id);
-        return;
+        return vec![];
     };
     let body = tcx.hir().body(body_id);
-    debug!("check_unused_unsafe({:?}, body={:?}, used_unsafe={:?})", def_id, body, used_unsafe);
 
-    let mut visitor = UnusedUnsafeVisitor { used_unsafe, unsafe_blocks };
+    let context = match tcx.hir().fn_sig_by_hir_id(hir_id) {
+        Some(sig) if sig.header.unsafety == hir::Unsafety::Unsafe => Context::UnsafeFn(hir_id),
+        _ => Context::Safe,
+    };
+
+    debug!(
+        "check_unused_unsafe({:?}, context={:?}, body={:?}, used_unsafe_blocks={:?})",
+        def_id, body, context, used_unsafe_blocks
+    );
+
+    let mut unused_unsafes = vec![];
+
+    let mut visitor = UnusedUnsafeVisitor {
+        tcx,
+        used_unsafe_blocks,
+        context,
+        unused_unsafes: &mut unused_unsafes,
+    };
     intravisit::Visitor::visit_body(&mut visitor, body);
+
+    unused_unsafes
 }
 
 fn unsafety_check_result<'tcx>(
@@ -436,56 +514,52 @@ fn unsafety_check_result<'tcx>(
     let mut checker = UnsafetyChecker::new(body, def.did, tcx, param_env);
     checker.visit_body(&body);
 
-    check_unused_unsafe(tcx, def.did, &checker.used_unsafe, &mut checker.inherited_blocks);
+    let unused_unsafes = (!tcx.is_closure(def.did.to_def_id()))
+        .then(|| check_unused_unsafe(tcx, def.did, &checker.used_unsafe_blocks));
 
     tcx.arena.alloc(UnsafetyCheckResult {
-        violations: checker.violations.into(),
+        violations: checker.violations,
-        unsafe_blocks: checker.inherited_blocks.into(),
+        used_unsafe_blocks: checker.used_unsafe_blocks,
+        unused_unsafes,
     })
 }
 
-/// Returns the `HirId` for an enclosing scope that is also `unsafe`.
+fn report_unused_unsafe(tcx: TyCtxt<'_>, kind: UnusedUnsafe, id: HirId) {
-fn is_enclosed(
-    tcx: TyCtxt<'_>,
-    used_unsafe: &FxHashSet<hir::HirId>,
-    id: hir::HirId,
-    unsafe_op_in_unsafe_fn_allowed: bool,
-) -> Option<(&'static str, hir::HirId)> {
-    let parent_id = tcx.hir().get_parent_node(id);
-    if parent_id != id {
-        if used_unsafe.contains(&parent_id) {
-            Some(("block", parent_id))
-        } else if let Some(Node::Item(&hir::Item {
-            kind: hir::ItemKind::Fn(ref sig, _, _), ..
-        })) = tcx.hir().find(parent_id)
-        {
-            if sig.header.unsafety == hir::Unsafety::Unsafe && unsafe_op_in_unsafe_fn_allowed {
-                Some(("fn", parent_id))
-            } else {
-                None
-            }
-        } else {
-            is_enclosed(tcx, used_unsafe, parent_id, unsafe_op_in_unsafe_fn_allowed)
-        }
-    } else {
-        None
-    }
-}
-
-fn report_unused_unsafe(tcx: TyCtxt<'_>, used_unsafe: &FxHashSet<hir::HirId>, id: hir::HirId) {
     let span = tcx.sess.source_map().guess_head_span(tcx.hir().span(id));
     tcx.struct_span_lint_hir(UNUSED_UNSAFE, id, span, |lint| {
         let msg = "unnecessary `unsafe` block";
         let mut db = lint.build(msg);
         db.span_label(span, msg);
-        if let Some((kind, id)) =
+        match kind {
-            is_enclosed(tcx, used_unsafe, id, unsafe_op_in_unsafe_fn_allowed(tcx, id))
+            UnusedUnsafe::Unused => {}
-        {
+            UnusedUnsafe::InUnsafeBlock(id) => {
-            db.span_label(
+                db.span_label(
-                tcx.sess.source_map().guess_head_span(tcx.hir().span(id)),
+                    tcx.sess.source_map().guess_head_span(tcx.hir().span(id)),
-                format!("because it's nested under this `unsafe` {}", kind),
+                    format!("because it's nested under this `unsafe` block"),
-            );
+                );
+            }
+            UnusedUnsafe::InUnsafeFn(id, usage_lint_root) => {
+                db.span_label(
+                    tcx.sess.source_map().guess_head_span(tcx.hir().span(id)),
+                    format!("because it's nested under this `unsafe` fn"),
+                )
+                .note(
+                    "this `unsafe` block does contain unsafe operations, \
+                    but those are already allowed in an `unsafe fn`",
+                );
+                let (level, source) =
+                    tcx.lint_level_at_node(UNSAFE_OP_IN_UNSAFE_FN, usage_lint_root);
+                assert_eq!(level, Level::Allow);
+                lint::explain_lint_level_source(
+                    tcx.sess,
+                    UNSAFE_OP_IN_UNSAFE_FN,
+                    Level::Allow,
+                    source,
+                    &mut db,
+                );
+            }
         }
+
         db.emit();
     });
 }
@@ -498,7 +572,7 @@ pub fn check_unsafety(tcx: TyCtxt<'_>, def_id: LocalDefId) {
         return;
     }
 
-    let UnsafetyCheckResult { violations, unsafe_blocks } = tcx.unsafety_check_result(def_id);
+    let UnsafetyCheckResult { violations, unused_unsafes, .. } = tcx.unsafety_check_result(def_id);
 
     for &UnsafetyViolation { source_info, lint_root, kind, details } in violations.iter() {
         let (description, note) = details.description_and_note();
@@ -539,20 +613,8 @@ pub fn check_unsafety(tcx: TyCtxt<'_>, def_id: LocalDefId) {
         }
     }
 
-    let (mut unsafe_used, mut unsafe_unused): (FxHashSet<_>, Vec<_>) = Default::default();
+    for &(block_id, kind) in unused_unsafes.as_ref().unwrap() {
-    for &(block_id, is_used) in unsafe_blocks.iter() {
+        report_unused_unsafe(tcx, kind, block_id);
-        if is_used {
-            unsafe_used.insert(block_id);
-        } else {
-            unsafe_unused.push(block_id);
-        }
-    }
-    // The unused unsafe blocks might not be in source order; sort them so that the unused unsafe
-    // error messages are properly aligned and the issue-45107 and lint-unused-unsafe tests pass.
-    unsafe_unused.sort_by_cached_key(|hir_id| tcx.hir().span(*hir_id));
-
-    for &block_id in &unsafe_unused {
-        report_unused_unsafe(tcx, &unsafe_used, block_id);
     }
 }
 

src/test/ui/span/lint-unused-unsafe-thir.rs

@@ -0,0 +1,61 @@
+// FIXME: This file is tracking old lint behavior that's still unchanged in the
+// unstable -Zthir-unsafeck implementation. See lint-unused-unsafe.rs for more details.
+//
+// Exercise the unused_unsafe attribute in some positive and negative cases
+
+// compile-flags: -Zthir-unsafeck
+
+#![allow(dead_code)]
+#![deny(unused_unsafe)]
+
+
+mod foo {
+    extern "C" {
+        pub fn bar();
+    }
+}
+
+fn callback<T, F>(_f: F) -> T where F: FnOnce() -> T { panic!() }
+unsafe fn unsf() {}
+
+fn bad1() { unsafe {} }                  //~ ERROR: unnecessary `unsafe` block
+fn bad2() { unsafe { bad1() } }          //~ ERROR: unnecessary `unsafe` block
+unsafe fn bad3() { unsafe {} }           //~ ERROR: unnecessary `unsafe` block
+fn bad4() { unsafe { callback(||{}) } }  //~ ERROR: unnecessary `unsafe` block
+unsafe fn bad5() { unsafe { unsf() } }   //~ ERROR: unnecessary `unsafe` block
+fn bad6() {
+    unsafe {                             // don't put the warning here
+        unsafe {                         //~ ERROR: unnecessary `unsafe` block
+            unsf()
+        }
+    }
+}
+unsafe fn bad7() {
+    unsafe {                             //~ ERROR: unnecessary `unsafe` block
+        unsafe {                         //~ ERROR: unnecessary `unsafe` block
+            unsf()
+        }
+    }
+}
+
+unsafe fn good0() { unsf() }
+fn good1() { unsafe { unsf() } }
+fn good2() {
+    /* bug uncovered when implementing warning about unused unsafe blocks. Be
+       sure that when purity is inherited that the source of the unsafe-ness
+       is tracked correctly */
+    unsafe {
+        unsafe fn what() -> Vec<String> { panic!() }
+
+        callback(|| {
+            what();
+        });
+    }
+}
+
+unsafe fn good3() { foo::bar() }
+fn good4() { unsafe { foo::bar() } }
+
+#[allow(unused_unsafe)] fn allowed() { unsafe {} }
+
+fn main() {}

src/test/ui/span/lint-unused-unsafe-thir.stderr

@@ -1,23 +1,23 @@
 error: unnecessary `unsafe` block
-  --> $DIR/lint-unused-unsafe.rs:19:13
+  --> $DIR/lint-unused-unsafe-thir.rs:21:13
    |
 LL | fn bad1() { unsafe {} }
    |             ^^^^^^ unnecessary `unsafe` block
    |
 note: the lint level is defined here
-  --> $DIR/lint-unused-unsafe.rs:7:9
+  --> $DIR/lint-unused-unsafe-thir.rs:9:9
    |
 LL | #![deny(unused_unsafe)]
    |         ^^^^^^^^^^^^^
 
 error: unnecessary `unsafe` block
-  --> $DIR/lint-unused-unsafe.rs:20:13
+  --> $DIR/lint-unused-unsafe-thir.rs:22:13
    |
 LL | fn bad2() { unsafe { bad1() } }
    |             ^^^^^^ unnecessary `unsafe` block
 
 error: unnecessary `unsafe` block
-  --> $DIR/lint-unused-unsafe.rs:21:20
+  --> $DIR/lint-unused-unsafe-thir.rs:23:20
    |
 LL | unsafe fn bad3() { unsafe {} }
    | ----------------   ^^^^^^ unnecessary `unsafe` block
@@ -25,13 +25,13 @@ LL | unsafe fn bad3() { unsafe {} }
    | because it's nested under this `unsafe` fn
 
 error: unnecessary `unsafe` block
-  --> $DIR/lint-unused-unsafe.rs:22:13
+  --> $DIR/lint-unused-unsafe-thir.rs:24:13
    |
 LL | fn bad4() { unsafe { callback(||{}) } }
    |             ^^^^^^ unnecessary `unsafe` block
 
 error: unnecessary `unsafe` block
-  --> $DIR/lint-unused-unsafe.rs:23:20
+  --> $DIR/lint-unused-unsafe-thir.rs:25:20
    |
 LL | unsafe fn bad5() { unsafe { unsf() } }
    | ----------------   ^^^^^^ unnecessary `unsafe` block
@@ -39,7 +39,7 @@ LL | unsafe fn bad5() { unsafe { unsf() } }
    | because it's nested under this `unsafe` fn
 
 error: unnecessary `unsafe` block
-  --> $DIR/lint-unused-unsafe.rs:26:9
+  --> $DIR/lint-unused-unsafe-thir.rs:28:9
    |
 LL |     unsafe {                             // don't put the warning here
    |     ------ because it's nested under this `unsafe` block
@@ -47,7 +47,7 @@ LL |         unsafe {
    |         ^^^^^^ unnecessary `unsafe` block
 
 error: unnecessary `unsafe` block
-  --> $DIR/lint-unused-unsafe.rs:33:9
+  --> $DIR/lint-unused-unsafe-thir.rs:35:9
    |
 LL |     unsafe {
    |     ------ because it's nested under this `unsafe` block
@@ -55,7 +55,7 @@ LL |         unsafe {
    |         ^^^^^^ unnecessary `unsafe` block
 
 error: unnecessary `unsafe` block
-  --> $DIR/lint-unused-unsafe.rs:32:5
+  --> $DIR/lint-unused-unsafe-thir.rs:34:5
    |
 LL | unsafe fn bad7() {
    | ---------------- because it's nested under this `unsafe` fn

src/test/ui/unsafe/rfc-2585-unsafe_op_in_unsafe_fn.mir.stderr

@@ -76,12 +76,10 @@ LL |     unsafe {}
    |     ^^^^^^ unnecessary `unsafe` block
 
 error: unnecessary `unsafe` block
-  --> $DIR/rfc-2585-unsafe_op_in_unsafe_fn.rs:47:14
+  --> $DIR/rfc-2585-unsafe_op_in_unsafe_fn.rs:47:5
    |
 LL |     unsafe { unsafe { unsf() } }
-   |     ------   ^^^^^^ unnecessary `unsafe` block
+   |     ^^^^^^ unnecessary `unsafe` block
-   |     |
-   |     because it's nested under this `unsafe` block
 
 error: unnecessary `unsafe` block
   --> $DIR/rfc-2585-unsafe_op_in_unsafe_fn.rs:58:5
@@ -91,6 +89,13 @@ LL | unsafe fn allow_level() {
 ...
 LL |     unsafe { unsf() }
    |     ^^^^^^ unnecessary `unsafe` block
+   |
+   = note: this `unsafe` block does contain unsafe operations, but those are already allowed in an `unsafe fn`
+note: the lint level is defined here
+  --> $DIR/rfc-2585-unsafe_op_in_unsafe_fn.rs:51:9
+   |
+LL | #[allow(unsafe_op_in_unsafe_fn)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^
 
 error: unnecessary `unsafe` block
   --> $DIR/rfc-2585-unsafe_op_in_unsafe_fn.rs:70:9
@@ -100,6 +105,13 @@ LL | unsafe fn nested_allow_level() {
 ...
 LL |         unsafe { unsf() }
    |         ^^^^^^ unnecessary `unsafe` block
+   |
+   = note: this `unsafe` block does contain unsafe operations, but those are already allowed in an `unsafe fn`
+note: the lint level is defined here
+  --> $DIR/rfc-2585-unsafe_op_in_unsafe_fn.rs:63:13
+   |
+LL |     #[allow(unsafe_op_in_unsafe_fn)]
+   |             ^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0133]: call to unsafe function is unsafe and requires unsafe block
   --> $DIR/rfc-2585-unsafe_op_in_unsafe_fn.rs:76:5