src/tools/clippy/clippy_lints/src/strlen_on_c_strings.rs

use clippy_utils::diagnostics::span_lint_and_sugg;
use clippy_utils::source::snippet_with_context;
use clippy_utils::ty::{is_type_diagnostic_item, is_type_lang_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::{Block, BlockCheckMode, Expr, ExprKind, LangItem, Node, UnsafeSource};
use rustc_lint::{LateContext, LateLintPass};
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::symbol::sym;

declare_clippy_lint! {
    /// ### What it does
    /// Checks for usage of `libc::strlen` on a `CString` or `CStr` value,
    /// and suggest calling `as_bytes().len()` or `to_bytes().len()` respectively instead.
    ///
    /// ### Why is this bad?
    /// This avoids calling an unsafe `libc` function.
    /// Currently, it also avoids calculating the length.
    ///
    /// ### Example
    /// ```rust, ignore
    /// use std::ffi::CString;
    /// let cstring = CString::new("foo").expect("CString::new failed");
    /// let len = unsafe { libc::strlen(cstring.as_ptr()) };
    /// ```
    /// Use instead:
    /// ```rust, no_run
    /// use std::ffi::CString;
    /// let cstring = CString::new("foo").expect("CString::new failed");
    /// let len = cstring.as_bytes().len();
    /// ```
    #[clippy::version = "1.55.0"]
    pub STRLEN_ON_C_STRINGS,
    complexity,
    "using `libc::strlen` on a `CString` or `CStr` value, while `as_bytes().len()` or `to_bytes().len()` respectively can be used instead"
}

declare_lint_pass!(StrlenOnCStrings => [STRLEN_ON_C_STRINGS]);

impl<'tcx> LateLintPass<'tcx> for StrlenOnCStrings {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
        if_chain! {
            if !expr.span.from_expansion();
            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 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::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_lang_item(cx, ty, LangItem::CStr) {
                    "to_bytes"
                } else {
                    return;
                };

                span_lint_and_sugg(
                    cx,
                    STRLEN_ON_C_STRINGS,
                    span,
                    "using `libc::strlen` on a `CString` or `CStr` value",
                    "try",
                    format!("{val_name}.{method_name}().len()"),
                    app,
                );
            }
        }
    }
}