Add a new lint that warns for pointers to stack memory

This adds a new lint with level `Warn` to check for code like:

```rust
fn foo() -> *const i32 {
  let x = 42;
  &x
}
```

and produce a warning like:
```text
error: returning a pointer to stack memory associated with a local variable
  --> <source>:12:5
   |
 LL|  &x
   |  ^^
```

Author: 1c3t3a

compiler/rustc_lint/messages.ftl

@@ -138,6 +138,8 @@ lint_builtin_overridden_symbol_name =
 lint_builtin_overridden_symbol_section =
     the program's behavior with overridden link sections on items is unpredictable and Rust cannot provide guarantees when you manually override them
 
+lint_builtin_returning_pointers_to_local_variables = returning a pointer to stack memory associated with a local variable
+
 lint_builtin_special_module_name_used_lib = found module declaration for lib.rs
     .note = lib.rs is the root of this crate's library target
     .help = to refer to it from other targets, use the library's name as the path

compiler/rustc_lint/src/builtin.rs

@@ -55,10 +55,10 @@ use crate::lints::{
     BuiltinIncompleteFeatures, BuiltinIncompleteFeaturesHelp, BuiltinInternalFeatures,
     BuiltinKeywordIdents, BuiltinMissingCopyImpl, BuiltinMissingDebugImpl, BuiltinMissingDoc,
     BuiltinMutablesTransmutes, BuiltinNoMangleGeneric, BuiltinNonShorthandFieldPatterns,
-    BuiltinSpecialModuleNameUsed, BuiltinTrivialBounds, BuiltinTypeAliasBounds,
-    BuiltinUngatedAsyncFnTrackCaller, BuiltinUnpermittedTypeInit, BuiltinUnpermittedTypeInitSub,
-    BuiltinUnreachablePub, BuiltinUnsafe, BuiltinUnstableFeatures, BuiltinUnusedDocComment,
-    BuiltinUnusedDocCommentSub, BuiltinWhileTrue, InvalidAsmLabel,
+    BuiltinReturningPointersToLocalVariables, BuiltinSpecialModuleNameUsed, BuiltinTrivialBounds,
+    BuiltinTypeAliasBounds, BuiltinUngatedAsyncFnTrackCaller, BuiltinUnpermittedTypeInit,
+    BuiltinUnpermittedTypeInitSub, BuiltinUnreachablePub, BuiltinUnsafe, BuiltinUnstableFeatures,
+    BuiltinUnusedDocComment, BuiltinUnusedDocCommentSub, BuiltinWhileTrue, InvalidAsmLabel,
 };
 use crate::nonstandard_style::{MethodLateContext, method_context};
 use crate::{
@@ -3027,6 +3027,157 @@ impl<'tcx> LateLintPass<'tcx> for AsmLabels {
     }
 }
 
+declare_lint! {
+    /// The `returning_pointers_to_local_variables` lint detects when pointer
+    /// to stack memory associated with a local variable is returned. That
+    /// pointer is immediately dangling.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,no_run
+    /// fn foo() -> *const i32 {
+    ///   let x = 42;
+    ///   &x
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Returning a pointer to memory refering to a local variable will always
+    /// end up in a dangling pointer after returning.
+    pub RETURNING_POINTERS_TO_LOCAL_VARIABLES,
+    Warn,
+    "returning a pointer to stack memory associated with a local variable",
+}
+
+declare_lint_pass!(ReturningPointersToLocalVariables => [RETURNING_POINTERS_TO_LOCAL_VARIABLES]);
+
+impl<'tcx> LateLintPass<'tcx> for ReturningPointersToLocalVariables {
+    fn check_fn(
+        &mut self,
+        cx: &LateContext<'tcx>,
+        _: HirFnKind<'tcx>,
+        fn_decl: &'tcx FnDecl<'tcx>,
+        body: &'tcx Body<'tcx>,
+        _: Span,
+        _: LocalDefId,
+    ) {
+        if !matches!(
+            fn_decl.output,
+            hir::FnRetTy::Return(&hir::Ty { kind: hir::TyKind::Ptr(_), .. }),
+        ) {
+            return;
+        }
+
+        // Check the block of the function that we're looking at.
+        if let Some(block) = Self::get_enclosing_block(cx, body.value.hir_id) {
+            match block {
+                hir::Block {
+                    stmts:
+                        [
+                            ..,
+                            hir::Stmt {
+                                kind:
+                                    hir::StmtKind::Semi(&hir::Expr {
+                                        kind: hir::ExprKind::Ret(Some(return_expr)),
+                                        ..
+                                    }),
+                                ..
+                            },
+                        ],
+                    ..
+                } => {
+                    Self::maybe_lint_return_expr(cx, return_expr, fn_decl.inputs);
+                }
+                hir::Block { expr: Some(return_expr), .. } => {
+                    Self::maybe_lint_return_expr(cx, return_expr, fn_decl.inputs);
+                }
+                _ => return,
+            }
+        }
+    }
+}
+
+impl ReturningPointersToLocalVariables {
+    /// Evaluates the return expression of a function and emits a lint if it
+    /// returns a pointer to a local variable.
+    fn maybe_lint_return_expr<'tcx>(
+        cx: &LateContext<'tcx>,
+        return_expr: &hir::Expr<'tcx>,
+        params: &'tcx [hir::Ty<'tcx>],
+    ) {
+        // Early exit if we see that this is a pointer to an input parameter.
+        if Self::expr_is_param(cx.typeck_results(), return_expr, params) {
+            return;
+        }
+
+        match return_expr {
+            hir::Expr { kind: hir::ExprKind::AddrOf(_, _, addr_expr), .. } => {
+                Self::maybe_lint_return_expr(cx, addr_expr, params)
+            }
+            hir::Expr {
+                kind:
+                    hir::ExprKind::Cast(
+                        hir::Expr { kind: hir::ExprKind::AddrOf(_, _, addr_expr), .. },
+                        _,
+                    ),
+                ..
+            } => Self::maybe_lint_return_expr(cx, addr_expr, params),
+            hir::Expr { kind: hir::ExprKind::Cast(expr, _), .. } => {
+                Self::maybe_lint_return_expr(cx, expr, params)
+            }
+            hir::Expr {
+                kind:
+                    hir::ExprKind::Path(
+                        hir::QPath::Resolved(_, hir::Path { res: hir::def::Res::Local(_), .. }),
+                        ..,
+                    ),
+                ..
+            } => cx.emit_span_lint(
+                RETURNING_POINTERS_TO_LOCAL_VARIABLES,
+                return_expr.span,
+                BuiltinReturningPointersToLocalVariables,
+            ),
+            _ => (),
+        }
+    }
+
+    fn expr_is_param<'tcx>(
+        typeck_results: &ty::TypeckResults<'tcx>,
+        expr: &hir::Expr<'tcx>,
+        params: &'tcx [hir::Ty<'tcx>],
+    ) -> bool {
+        params
+            .iter()
+            .map(|param| typeck_results.type_dependent_def_id(param.hir_id))
+            .collect::<Vec<_>>()
+            .contains(&typeck_results.type_dependent_def_id(expr.hir_id))
+    }
+
+    /// Returns the enclosing block for a [hir::HirId], if available.
+    fn get_enclosing_block<'tcx>(
+        cx: &LateContext<'tcx>,
+        hir_id: hir::HirId,
+    ) -> Option<&'tcx hir::Block<'tcx>> {
+        let map = &cx.tcx.hir();
+        let enclosing_node =
+            map.get_enclosing_scope(hir_id).map(|enclosing_id| cx.tcx.hir_node(enclosing_id));
+        enclosing_node.and_then(|node| match node {
+            hir::Node::Block(block) => Some(block),
+            hir::Node::Item(&hir::Item { kind: hir::ItemKind::Fn { body: eid, .. }, .. })
+            | hir::Node::ImplItem(&hir::ImplItem { kind: hir::ImplItemKind::Fn(_, eid), .. }) => {
+                match cx.tcx.hir().body(eid).value.kind {
+                    hir::ExprKind::Block(block, _) => Some(block),
+                    _ => None,
+                }
+            }
+            _ => None,
+        })
+    }
+}
+
 declare_lint! {
     /// The `special_module_name` lint detects module
     /// declarations for files that have a special meaning.

compiler/rustc_lint/src/lib.rs

@@ -245,6 +245,7 @@ late_lint_methods!(
             IfLetRescope: IfLetRescope::default(),
             StaticMutRefs: StaticMutRefs,
             UnqualifiedLocalImports: UnqualifiedLocalImports,
+            ReturningPointersToLocalVariables : ReturningPointersToLocalVariables,
         ]
     ]
 );

compiler/rustc_lint/src/lints.rs

@@ -536,6 +536,10 @@ pub(crate) enum BuiltinSpecialModuleNameUsed {
     Main,
 }
 
+#[derive(LintDiagnostic)]
+#[diag(lint_builtin_returning_pointers_to_local_variables)]
+pub(crate) struct BuiltinReturningPointersToLocalVariables;
+
 // deref_into_dyn_supertrait.rs
 #[derive(LintDiagnostic)]
 #[diag(lint_supertrait_as_deref_target)]

src/tools/miri/tests/fail/validity/dangling_ref3.rs

@@ -1,5 +1,6 @@
 // Make sure we catch this even without Stacked Borrows
 //@compile-flags: -Zmiri-disable-stacked-borrows
+#![allow(returning_pointers_to_local_variables)]
 use std::mem;
 
 fn dangling() -> *const u8 {

tests/ui/lint/lint-returning-pointers-to-local-variables.rs

@@ -0,0 +1,47 @@
+//@ check-pass
+
+#![warn(returning_pointers_to_local_variables)]
+
+fn foo() -> *const u32 {
+    let empty = 42u32;
+    return &empty as *const _;
+    //~^ WARN returning a pointer to stack memory associated with a local variable
+}
+
+fn bar() -> *const u32 {
+    let empty = 42u32;
+    &empty as *const _
+    //~^ WARN returning a pointer to stack memory associated with a local variable
+}
+
+fn baz() -> *const u32 {
+    let empty = 42u32;
+    return ∅
+    //~^ WARN returning a pointer to stack memory associated with a local variable
+}
+
+fn faa() -> *const u32 {
+    let empty = 42u32;
+    &empty
+    //~^ WARN returning a pointer to stack memory associated with a local variable
+}
+
+fn pointer_to_pointer() -> *const *mut u32 {
+    let mut empty = 42u32;
+    &(&mut empty as *mut u32) as *const _
+    //~^ WARN returning a pointer to stack memory associated with a local variable
+}
+
+fn dont_lint_param(val: u32) -> *const u32 {
+    &val
+}
+
+struct Foo {}
+
+impl Foo {
+    fn dont_lint_self_param(&self) -> *const Foo {
+        self
+    }
+}
+
+fn main() {}

tests/ui/lint/lint-returning-pointers-to-local-variables.stderr

@@ -0,0 +1,38 @@
+warning: returning a pointer to stack memory associated with a local variable
+  --> $DIR/lint-returning-pointers-to-local-variables.rs:7:13
+   |
+LL |     return &empty as *const _;
+   |             ^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/lint-returning-pointers-to-local-variables.rs:3:9
+   |
+LL | #![warn(returning_pointers_to_local_variables)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+warning: returning a pointer to stack memory associated with a local variable
+  --> $DIR/lint-returning-pointers-to-local-variables.rs:13:6
+   |
+LL |     &empty as *const _
+   |      ^^^^^
+
+warning: returning a pointer to stack memory associated with a local variable
+  --> $DIR/lint-returning-pointers-to-local-variables.rs:19:13
+   |
+LL |     return ∅
+   |             ^^^^^
+
+warning: returning a pointer to stack memory associated with a local variable
+  --> $DIR/lint-returning-pointers-to-local-variables.rs:25:6
+   |
+LL |     &empty
+   |      ^^^^^
+
+warning: returning a pointer to stack memory associated with a local variable
+  --> $DIR/lint-returning-pointers-to-local-variables.rs:31:12
+   |
+LL |     &(&mut empty as *mut u32) as *const _
+   |            ^^^^^
+
+warning: 5 warnings emitted
+