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use clippy_utils::diagnostics::span_lint_and_sugg;
use clippy_utils::source::{snippet, snippet_with_macro_callsite};
use clippy_utils::ty::is_type_diagnostic_item;
use clippy_utils::{differing_macro_contexts, get_parent_expr, in_macro, is_lang_ctor, match_def_path, paths};
use if_chain::if_chain;
use rustc_errors::Applicability;
use rustc_hir::LangItem::ResultErr;
use rustc_hir::{Expr, ExprKind, LangItem, MatchSource, QPath};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::lint::in_external_macro;
use rustc_middle::ty::{self, Ty};
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::sym;
declare_clippy_lint! {
/// ### What it does
/// Checks for usages of `Err(x)?`.
///
/// ### Why is this bad?
/// The `?` operator is designed to allow calls that
/// can fail to be easily chained. For example, `foo()?.bar()` or
/// `foo(bar()?)`. Because `Err(x)?` can't be used that way (it will
/// always return), it is more clear to write `return Err(x)`.
///
/// ### Example
/// ```rust
/// fn foo(fail: bool) -> Result<i32, String> {
/// if fail {
/// Err("failed")?;
/// }
/// Ok(0)
/// }
/// ```
/// Could be written:
///
/// ```rust
/// fn foo(fail: bool) -> Result<i32, String> {
/// if fail {
/// return Err("failed".into());
/// }
/// Ok(0)
/// }
/// ```
pub TRY_ERR,
style,
"return errors explicitly rather than hiding them behind a `?`"
}
declare_lint_pass!(TryErr => [TRY_ERR]);
impl<'tcx> LateLintPass<'tcx> for TryErr {
fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
// Looks for a structure like this:
// match ::std::ops::Try::into_result(Err(5)) {
// ::std::result::Result::Err(err) =>
// #[allow(unreachable_code)]
// return ::std::ops::Try::from_error(::std::convert::From::from(err)),
// ::std::result::Result::Ok(val) =>
// #[allow(unreachable_code)]
// val,
// };
if_chain! {
if !in_external_macro(cx.tcx.sess, expr.span);
if let ExprKind::Match(match_arg, _, MatchSource::TryDesugar) = expr.kind;
if let ExprKind::Call(match_fun, try_args) = match_arg.kind;
if let ExprKind::Path(ref match_fun_path) = match_fun.kind;
if matches!(match_fun_path, QPath::LangItem(LangItem::TryTraitBranch, _));
if let Some(try_arg) = try_args.get(0);
if let ExprKind::Call(err_fun, err_args) = try_arg.kind;
if let Some(err_arg) = err_args.get(0);
if let ExprKind::Path(ref err_fun_path) = err_fun.kind;
if is_lang_ctor(cx, err_fun_path, ResultErr);
if let Some(return_ty) = find_return_type(cx, &expr.kind);
then {
let prefix;
let suffix;
let err_ty;
if let Some(ty) = result_error_type(cx, return_ty) {
prefix = "Err(";
suffix = ")";
err_ty = ty;
} else if let Some(ty) = poll_result_error_type(cx, return_ty) {
prefix = "Poll::Ready(Err(";
suffix = "))";
err_ty = ty;
} else if let Some(ty) = poll_option_result_error_type(cx, return_ty) {
prefix = "Poll::Ready(Some(Err(";
suffix = ")))";
err_ty = ty;
} else {
return;
};
let expr_err_ty = cx.typeck_results().expr_ty(err_arg);
let differing_contexts = differing_macro_contexts(expr.span, err_arg.span);
let origin_snippet = if in_macro(expr.span) && in_macro(err_arg.span) && differing_contexts {
snippet(cx, err_arg.span.ctxt().outer_expn_data().call_site, "_")
} else if err_arg.span.from_expansion() && !in_macro(expr.span) {
snippet_with_macro_callsite(cx, err_arg.span, "_")
} else {
snippet(cx, err_arg.span, "_")
};
let ret_prefix = if get_parent_expr(cx, expr).map_or(false, |e| matches!(e.kind, ExprKind::Ret(_))) {
"" // already returns
} else {
"return "
};
let suggestion = if err_ty == expr_err_ty {
format!("{}{}{}{}", ret_prefix, prefix, origin_snippet, suffix)
} else {
format!("{}{}{}.into(){}", ret_prefix, prefix, origin_snippet, suffix)
};
span_lint_and_sugg(
cx,
TRY_ERR,
expr.span,
"returning an `Err(_)` with the `?` operator",
"try this",
suggestion,
Applicability::MachineApplicable
);
}
}
}
}
/// Finds function return type by examining return expressions in match arms.
fn find_return_type<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx ExprKind<'_>) -> Option<Ty<'tcx>> {
if let ExprKind::Match(_, arms, MatchSource::TryDesugar) = expr {
for arm in arms.iter() {
if let ExprKind::Ret(Some(ret)) = arm.body.kind {
return Some(cx.typeck_results().expr_ty(ret));
}
}
}
None
}
/// Extracts the error type from Result<T, E>.
fn result_error_type<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
if_chain! {
if let ty::Adt(_, subst) = ty.kind();
if is_type_diagnostic_item(cx, ty, sym::result_type);
then {
Some(subst.type_at(1))
} else {
None
}
}
}
/// Extracts the error type from Poll<Result<T, E>>.
fn poll_result_error_type<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
if_chain! {
if let ty::Adt(def, subst) = ty.kind();
if match_def_path(cx, def.did, &paths::POLL);
let ready_ty = subst.type_at(0);
if let ty::Adt(ready_def, ready_subst) = ready_ty.kind();
if cx.tcx.is_diagnostic_item(sym::result_type, ready_def.did);
then {
Some(ready_subst.type_at(1))
} else {
None
}
}
}
/// Extracts the error type from Poll<Option<Result<T, E>>>.
fn poll_option_result_error_type<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
if_chain! {
if let ty::Adt(def, subst) = ty.kind();
if match_def_path(cx, def.did, &paths::POLL);
let ready_ty = subst.type_at(0);
if let ty::Adt(ready_def, ready_subst) = ready_ty.kind();
if cx.tcx.is_diagnostic_item(sym::option_type, ready_def.did);
let some_ty = ready_subst.type_at(0);
if let ty::Adt(some_def, some_subst) = some_ty.kind();
if cx.tcx.is_diagnostic_item(sym::result_type, some_def.did);
then {
Some(some_subst.type_at(1))
} else {
None
}
}
}
|
