diff --git a/clippy_lints/src/strlen_on_c_strings.rs b/clippy_lints/src/strlen_on_c_strings.rs
index dc28bb97400..fee01fb0bd1 100644
--- a/clippy_lints/src/strlen_on_c_strings.rs
+++ b/clippy_lints/src/strlen_on_c_strings.rs
@@ -1,10 +1,11 @@
 use clippy_utils::diagnostics::span_lint_and_sugg;
-use clippy_utils::match_libc_symbol;
 use clippy_utils::source::snippet_with_context;
 use clippy_utils::ty::is_type_diagnostic_item;
+use clippy_utils::visitors::is_expr_unsafe;
+use clippy_utils::{get_parent_node, match_libc_symbol};
 use if_chain::if_chain;
 use rustc_errors::Applicability;
-use rustc_hir as hir;
+use rustc_hir::{Block, BlockCheckMode, Expr, ExprKind, Node, UnsafeSource};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::{declare_lint_pass, declare_tool_lint};
 use rustc_span::symbol::sym;
@@ -39,20 +40,31 @@
 declare_lint_pass!(StrlenOnCStrings => [STRLEN_ON_C_STRINGS]);
 
 impl LateLintPass<'tcx> for StrlenOnCStrings {
-    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'_>) {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
         if_chain! {
             if !expr.span.from_expansion();
-            if let hir::ExprKind::Call(func, [recv]) = expr.kind;
-            if let hir::ExprKind::Path(path) = &func.kind;
+            if let ExprKind::Call(func, [recv]) = expr.kind;
+            if let ExprKind::Path(path) = &func.kind;
             if let Some(did) = cx.qpath_res(path, func.hir_id).opt_def_id();
             if match_libc_symbol(cx, did, "strlen");
-            if let hir::ExprKind::MethodCall(path, _, [self_arg], _) = recv.kind;
+            if let ExprKind::MethodCall(path, _, [self_arg], _) = recv.kind;
             if !recv.span.from_expansion();
             if path.ident.name == sym::as_ptr;
             then {
+                let ctxt = expr.span.ctxt();
+                let span = match get_parent_node(cx.tcx, expr.hir_id) {
+                    Some(Node::Block(&Block {
+                        rules: BlockCheckMode::UnsafeBlock(UnsafeSource::UserProvided), span, ..
+                    }))
+                    if span.ctxt() == ctxt && !is_expr_unsafe(cx, self_arg) => {
+                        span
+                    }
+                    _ => expr.span,
+                };
+
                 let ty = cx.typeck_results().expr_ty(self_arg).peel_refs();
-                let mut app = Applicability::Unspecified;
-                let val_name = snippet_with_context(cx, self_arg.span, expr.span.ctxt(), "..", &mut app).0;
+                let mut app = Applicability::MachineApplicable;
+                let val_name = snippet_with_context(cx, self_arg.span, ctxt, "..", &mut app).0;
                 let method_name = if is_type_diagnostic_item(cx, ty, sym::cstring_type) {
                     "as_bytes"
                 } else if is_type_diagnostic_item(cx, ty, sym::CStr) {
@@ -64,11 +76,11 @@ fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'_>) {
                 span_lint_and_sugg(
                     cx,
                     STRLEN_ON_C_STRINGS,
-                    expr.span,
+                    span,
                     "using `libc::strlen` on a `CString` or `CStr` value",
-                    "try this (you might also need to get rid of `unsafe` block in some cases):",
+                    "try this",
                     format!("{}.{}().len()", val_name, method_name),
-                    Applicability::Unspecified // Sometimes unnecessary `unsafe` block
+                    app,
                 );
             }
         }
diff --git a/clippy_utils/src/visitors.rs b/clippy_utils/src/visitors.rs
index 823df5cb751..4bfd3c64b9c 100644
--- a/clippy_utils/src/visitors.rs
+++ b/clippy_utils/src/visitors.rs
@@ -1,8 +1,10 @@
 use crate::path_to_local_id;
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
-use rustc_hir::intravisit::{self, walk_expr, NestedVisitorMap, Visitor};
-use rustc_hir::{Arm, Block, Body, BodyId, Expr, ExprKind, HirId, Stmt, UnOp};
+use rustc_hir::intravisit::{self, walk_block, walk_expr, NestedVisitorMap, Visitor};
+use rustc_hir::{
+    Arm, Block, BlockCheckMode, Body, BodyId, Expr, ExprKind, HirId, ItemId, ItemKind, Stmt, UnOp, Unsafety,
+};
 use rustc_lint::LateContext;
 use rustc_middle::hir::map::Map;
 use rustc_middle::ty;
@@ -317,3 +319,64 @@ fn visit_expr(&mut self, e: &'tcx Expr<'_>) {
     v.visit_expr(e);
     v.is_const
 }
+
+/// Checks if the given expression performs an unsafe operation outside of an unsafe block.
+pub fn is_expr_unsafe(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) -> bool {
+    struct V<'a, 'tcx> {
+        cx: &'a LateContext<'tcx>,
+        is_unsafe: bool,
+    }
+    impl<'tcx> Visitor<'tcx> for V<'_, 'tcx> {
+        type Map = Map<'tcx>;
+        fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
+            NestedVisitorMap::OnlyBodies(self.cx.tcx.hir())
+        }
+        fn visit_expr(&mut self, e: &'tcx Expr<'_>) {
+            if self.is_unsafe {
+                return;
+            }
+            match e.kind {
+                ExprKind::Unary(UnOp::Deref, e) if self.cx.typeck_results().expr_ty(e).is_unsafe_ptr() => {
+                    self.is_unsafe = true;
+                },
+                ExprKind::MethodCall(..)
+                    if self
+                        .cx
+                        .typeck_results()
+                        .type_dependent_def_id(e.hir_id)
+                        .map_or(false, |id| self.cx.tcx.fn_sig(id).unsafety() == Unsafety::Unsafe) =>
+                {
+                    self.is_unsafe = true;
+                },
+                ExprKind::Call(func, _) => match *self.cx.typeck_results().expr_ty(func).peel_refs().kind() {
+                    ty::FnDef(id, _) if self.cx.tcx.fn_sig(id).unsafety() == Unsafety::Unsafe => self.is_unsafe = true,
+                    ty::FnPtr(sig) if sig.unsafety() == Unsafety::Unsafe => self.is_unsafe = true,
+                    _ => walk_expr(self, e),
+                },
+                ExprKind::Path(ref p)
+                    if self
+                        .cx
+                        .qpath_res(p, e.hir_id)
+                        .opt_def_id()
+                        .map_or(false, |id| self.cx.tcx.is_mutable_static(id)) =>
+                {
+                    self.is_unsafe = true;
+                },
+                _ => walk_expr(self, e),
+            }
+        }
+        fn visit_block(&mut self, b: &'tcx Block<'_>) {
+            if !matches!(b.rules, BlockCheckMode::UnsafeBlock(_)) {
+                walk_block(self, b);
+            }
+        }
+        fn visit_nested_item(&mut self, id: ItemId) {
+            if let ItemKind::Impl(i) = &self.cx.tcx.hir().item(id).kind {
+                self.is_unsafe = i.unsafety == Unsafety::Unsafe;
+            }
+        }
+    }
+    let mut v = V { cx, is_unsafe: false };
+    v.visit_expr(e);
+    v.is_unsafe
+}
diff --git a/tests/ui/strlen_on_c_strings.rs b/tests/ui/strlen_on_c_strings.rs
index 121ddf1b9b1..a3451d992a0 100644
--- a/tests/ui/strlen_on_c_strings.rs
+++ b/tests/ui/strlen_on_c_strings.rs
@@ -16,4 +16,16 @@ fn main() {
     let len = unsafe { libc::strlen(cstr.as_ptr()) };
 
     let len = unsafe { strlen(cstr.as_ptr()) };
+
+    let pcstr: *const &CStr = &cstr;
+    let len = unsafe { strlen((*pcstr).as_ptr()) };
+
+    unsafe fn unsafe_identity<T>(x: T) -> T {
+        x
+    }
+    let len = unsafe { strlen(unsafe_identity(cstr).as_ptr()) };
+    let len = unsafe { strlen(unsafe { unsafe_identity(cstr) }.as_ptr()) };
+
+    let f: unsafe fn(_) -> _ = unsafe_identity;
+    let len = unsafe { strlen(f(cstr).as_ptr()) };
 }
diff --git a/tests/ui/strlen_on_c_strings.stderr b/tests/ui/strlen_on_c_strings.stderr
index 5576d9d2676..0dbecda62e3 100644
--- a/tests/ui/strlen_on_c_strings.stderr
+++ b/tests/ui/strlen_on_c_strings.stderr
@@ -1,36 +1,46 @@
 error: using `libc::strlen` on a `CString` or `CStr` value
-  --> $DIR/strlen_on_c_strings.rs:12:24
+  --> $DIR/strlen_on_c_strings.rs:12:15
    |
 LL |     let len = unsafe { libc::strlen(cstring.as_ptr()) };
-   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `cstring.as_bytes().len()`
    |
    = note: `-D clippy::strlen-on-c-strings` implied by `-D warnings`
-help: try this (you might also need to get rid of `unsafe` block in some cases):
-   |
-LL |     let len = unsafe { cstring.as_bytes().len() };
-   |                        ~~~~~~~~~~~~~~~~~~~~~~~~
 
 error: using `libc::strlen` on a `CString` or `CStr` value
-  --> $DIR/strlen_on_c_strings.rs:16:24
+  --> $DIR/strlen_on_c_strings.rs:16:15
    |
 LL |     let len = unsafe { libc::strlen(cstr.as_ptr()) };
-   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-help: try this (you might also need to get rid of `unsafe` block in some cases):
-   |
-LL |     let len = unsafe { cstr.to_bytes().len() };
-   |                        ~~~~~~~~~~~~~~~~~~~~~
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `cstr.to_bytes().len()`
 
 error: using `libc::strlen` on a `CString` or `CStr` value
-  --> $DIR/strlen_on_c_strings.rs:18:24
+  --> $DIR/strlen_on_c_strings.rs:18:15
    |
 LL |     let len = unsafe { strlen(cstr.as_ptr()) };
-   |                        ^^^^^^^^^^^^^^^^^^^^^
-   |
-help: try this (you might also need to get rid of `unsafe` block in some cases):
-   |
-LL |     let len = unsafe { cstr.to_bytes().len() };
-   |                        ~~~~~~~~~~~~~~~~~~~~~
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `cstr.to_bytes().len()`
 
-error: aborting due to 3 previous errors
+error: using `libc::strlen` on a `CString` or `CStr` value
+  --> $DIR/strlen_on_c_strings.rs:21:24
+   |
+LL |     let len = unsafe { strlen((*pcstr).as_ptr()) };
+   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `(*pcstr).to_bytes().len()`
+
+error: using `libc::strlen` on a `CString` or `CStr` value
+  --> $DIR/strlen_on_c_strings.rs:26:24
+   |
+LL |     let len = unsafe { strlen(unsafe_identity(cstr).as_ptr()) };
+   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `unsafe_identity(cstr).to_bytes().len()`
+
+error: using `libc::strlen` on a `CString` or `CStr` value
+  --> $DIR/strlen_on_c_strings.rs:27:15
+   |
+LL |     let len = unsafe { strlen(unsafe { unsafe_identity(cstr) }.as_ptr()) };
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `unsafe { unsafe_identity(cstr) }.to_bytes().len()`
+
+error: using `libc::strlen` on a `CString` or `CStr` value
+  --> $DIR/strlen_on_c_strings.rs:30:24
+   |
+LL |     let len = unsafe { strlen(f(cstr).as_ptr()) };
+   |                        ^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `f(cstr).to_bytes().len()`
+
+error: aborting due to 7 previous errors