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use crate::utils::{match_qpath, paths, snippet, snippet_with_macro_callsite, span_lint_and_sugg};
use if_chain::if_chain;
use rustc::lint::in_external_macro;
use rustc::ty::Ty;
use rustc_errors::Applicability;
use rustc_hir::*;
use rustc_lint::{LateContext, LateLintPass};
use rustc_session::{declare_lint_pass, declare_tool_lint};
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)`.
///
/// **Known problems:** None.
///
/// **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<'a, 'tcx> LateLintPass<'a, 'tcx> for TryErr {
fn check_expr(&mut self, cx: &LateContext<'a, '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(ref match_arg, _, MatchSource::TryDesugar) = expr.kind;
if let ExprKind::Call(ref match_fun, ref try_args) = match_arg.kind;
if let ExprKind::Path(ref match_fun_path) = match_fun.kind;
if match_qpath(match_fun_path, &paths::TRY_INTO_RESULT);
if let Some(ref try_arg) = try_args.get(0);
if let ExprKind::Call(ref err_fun, ref err_args) = try_arg.kind;
if let Some(ref err_arg) = err_args.get(0);
if let ExprKind::Path(ref err_fun_path) = err_fun.kind;
if match_qpath(err_fun_path, &paths::RESULT_ERR);
if let Some(return_type) = find_err_return_type(cx, &expr.kind);
then {
let err_type = cx.tables.expr_ty(err_arg);
let origin_snippet = if err_arg.span.from_expansion() {
snippet_with_macro_callsite(cx, err_arg.span, "_")
} else {
snippet(cx, err_arg.span, "_")
};
let suggestion = if err_type == return_type {
format!("return Err({})", origin_snippet)
} else {
format!("return Err({}.into())", origin_snippet)
};
span_lint_and_sugg(
cx,
TRY_ERR,
expr.span,
"returning an `Err(_)` with the `?` operator",
"try this",
suggestion,
Applicability::MachineApplicable
);
}
}
}
}
// In order to determine whether to suggest `.into()` or not, we need to find the error type the
// function returns. To do that, we look for the From::from call (see tree above), and capture
// its output type.
fn find_err_return_type<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, expr: &'tcx ExprKind<'_>) -> Option<Ty<'tcx>> {
if let ExprKind::Match(_, ref arms, MatchSource::TryDesugar) = expr {
arms.iter().find_map(|ty| find_err_return_type_arm(cx, ty))
} else {
None
}
}
// Check for From::from in one of the match arms.
fn find_err_return_type_arm<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, arm: &'tcx Arm<'_>) -> Option<Ty<'tcx>> {
if_chain! {
if let ExprKind::Ret(Some(ref err_ret)) = arm.body.kind;
if let ExprKind::Call(ref from_error_path, ref from_error_args) = err_ret.kind;
if let ExprKind::Path(ref from_error_fn) = from_error_path.kind;
if match_qpath(from_error_fn, &paths::TRY_FROM_ERROR);
if let Some(from_error_arg) = from_error_args.get(0);
then {
Some(cx.tables.expr_ty(from_error_arg))
} else {
None
}
}
}
|
