--- /dev/null
+use if_chain::if_chain;
+use rustc_ast::ast::Attribute;
+use rustc_errors::Applicability;
+use rustc_hir::intravisit::{walk_expr, FnKind, NestedVisitorMap, Visitor};
+use rustc_hir::{Block, Body, Expr, ExprKind, FnDecl, HirId, MatchSource, PatKind, StmtKind};
+use rustc_lint::{LateContext, LateLintPass, LintContext};
+use rustc_middle::hir::map::Map;
+use rustc_middle::lint::in_external_macro;
+use rustc_middle::ty::subst::GenericArgKind;
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::source_map::Span;
+use rustc_span::sym;
+
+use crate::utils::{fn_def_id, in_macro, match_qpath, snippet_opt, span_lint_and_sugg, span_lint_and_then};
+
+declare_clippy_lint! {
+ /// **What it does:** Checks for `let`-bindings, which are subsequently
+ /// returned.
+ ///
+ /// **Why is this bad?** It is just extraneous code. Remove it to make your code
+ /// more rusty.
+ ///
+ /// **Known problems:** None.
+ ///
+ /// **Example:**
+ /// ```rust
+ /// fn foo() -> String {
+ /// let x = String::new();
+ /// x
+ /// }
+ /// ```
+ /// instead, use
+ /// ```
+ /// fn foo() -> String {
+ /// String::new()
+ /// }
+ /// ```
+ pub LET_AND_RETURN,
+ style,
+ "creating a let-binding and then immediately returning it like `let x = expr; x` at the end of a block"
+}
+
+declare_clippy_lint! {
+ /// **What it does:** Checks for return statements at the end of a block.
+ ///
+ /// **Why is this bad?** Removing the `return` and semicolon will make the code
+ /// more rusty.
+ ///
+ /// **Known problems:** None.
+ ///
+ /// **Example:**
+ /// ```rust
+ /// fn foo(x: usize) -> usize {
+ /// return x;
+ /// }
+ /// ```
+ /// simplify to
+ /// ```rust
+ /// fn foo(x: usize) -> usize {
+ /// x
+ /// }
+ /// ```
+ pub NEEDLESS_RETURN,
+ style,
+ "using a return statement like `return expr;` where an expression would suffice"
+}
+
+#[derive(PartialEq, Eq, Copy, Clone)]
+enum RetReplacement {
+ Empty,
+ Block,
+}
+
+declare_lint_pass!(Return => [LET_AND_RETURN, NEEDLESS_RETURN]);
+
+impl<'tcx> LateLintPass<'tcx> for Return {
+ fn check_block(&mut self, cx: &LateContext<'tcx>, block: &'tcx Block<'_>) {
+ // we need both a let-binding stmt and an expr
+ if_chain! {
+ if let Some(retexpr) = block.expr;
+ if let Some(stmt) = block.stmts.iter().last();
+ if let StmtKind::Local(local) = &stmt.kind;
+ if local.ty.is_none();
+ if cx.tcx.hir().attrs(local.hir_id).is_empty();
+ if let Some(initexpr) = &local.init;
+ if let PatKind::Binding(.., ident, _) = local.pat.kind;
+ if let ExprKind::Path(qpath) = &retexpr.kind;
+ if match_qpath(qpath, &[&*ident.name.as_str()]);
+ if !last_statement_borrows(cx, initexpr);
+ if !in_external_macro(cx.sess(), initexpr.span);
+ if !in_external_macro(cx.sess(), retexpr.span);
+ if !in_external_macro(cx.sess(), local.span);
+ if !in_macro(local.span);
+ then {
+ span_lint_and_then(
+ cx,
+ LET_AND_RETURN,
+ retexpr.span,
+ "returning the result of a `let` binding from a block",
+ |err| {
+ err.span_label(local.span, "unnecessary `let` binding");
+
+ if let Some(mut snippet) = snippet_opt(cx, initexpr.span) {
+ if !cx.typeck_results().expr_adjustments(&retexpr).is_empty() {
+ snippet.push_str(" as _");
+ }
+ err.multipart_suggestion(
+ "return the expression directly",
+ vec![
+ (local.span, String::new()),
+ (retexpr.span, snippet),
+ ],
+ Applicability::MachineApplicable,
+ );
+ } else {
+ err.span_help(initexpr.span, "this expression can be directly returned");
+ }
+ },
+ );
+ }
+ }
+ }
+
+ fn check_fn(
+ &mut self,
+ cx: &LateContext<'tcx>,
+ kind: FnKind<'tcx>,
+ _: &'tcx FnDecl<'tcx>,
+ body: &'tcx Body<'tcx>,
+ _: Span,
+ _: HirId,
+ ) {
+ match kind {
+ FnKind::Closure => {
+ // when returning without value in closure, replace this `return`
+ // with an empty block to prevent invalid suggestion (see #6501)
+ let replacement = if let ExprKind::Ret(None) = &body.value.kind {
+ RetReplacement::Block
+ } else {
+ RetReplacement::Empty
+ };
+ check_final_expr(cx, &body.value, Some(body.value.span), replacement)
+ },
+ FnKind::ItemFn(..) | FnKind::Method(..) => {
+ if let ExprKind::Block(ref block, _) = body.value.kind {
+ check_block_return(cx, block);
+ }
+ },
+ }
+ }
+}
+
+fn attr_is_cfg(attr: &Attribute) -> bool {
+ attr.meta_item_list().is_some() && attr.has_name(sym::cfg)
+}
+
+fn check_block_return<'tcx>(cx: &LateContext<'tcx>, block: &Block<'tcx>) {
+ if let Some(expr) = block.expr {
+ check_final_expr(cx, expr, Some(expr.span), RetReplacement::Empty);
+ } else if let Some(stmt) = block.stmts.iter().last() {
+ match stmt.kind {
+ StmtKind::Expr(ref expr) | StmtKind::Semi(ref expr) => {
+ check_final_expr(cx, expr, Some(stmt.span), RetReplacement::Empty);
+ },
+ _ => (),
+ }
+ }
+}
+
+fn check_final_expr<'tcx>(
+ cx: &LateContext<'tcx>,
+ expr: &'tcx Expr<'tcx>,
+ span: Option<Span>,
+ replacement: RetReplacement,
+) {
+ match expr.kind {
+ // simple return is always "bad"
+ ExprKind::Ret(ref inner) => {
+ // allow `#[cfg(a)] return a; #[cfg(b)] return b;`
+ let attrs = cx.tcx.hir().attrs(expr.hir_id);
+ if !attrs.iter().any(attr_is_cfg) {
+ let borrows = inner.map_or(false, |inner| last_statement_borrows(cx, inner));
+ if !borrows {
+ emit_return_lint(
+ cx,
+ span.expect("`else return` is not possible"),
+ inner.as_ref().map(|i| i.span),
+ replacement,
+ );
+ }
+ }
+ },
+ // a whole block? check it!
+ ExprKind::Block(ref block, _) => {
+ check_block_return(cx, block);
+ },
+ ExprKind::If(_, then, else_clause_opt) => {
+ if let ExprKind::Block(ref ifblock, _) = then.kind {
+ check_block_return(cx, ifblock);
+ }
+ if let Some(else_clause) = else_clause_opt {
+ check_final_expr(cx, else_clause, None, RetReplacement::Empty);
+ }
+ },
+ // a match expr, check all arms
+ // an if/if let expr, check both exprs
+ // note, if without else is going to be a type checking error anyways
+ // (except for unit type functions) so we don't match it
+ ExprKind::Match(_, ref arms, source) => match source {
+ MatchSource::Normal => {
+ for arm in arms.iter() {
+ check_final_expr(cx, &arm.body, Some(arm.body.span), RetReplacement::Block);
+ }
+ },
+ MatchSource::IfLetDesugar {
+ contains_else_clause: true,
+ } => {
+ if let ExprKind::Block(ref ifblock, _) = arms[0].body.kind {
+ check_block_return(cx, ifblock);
+ }
+ check_final_expr(cx, arms[1].body, None, RetReplacement::Empty);
+ },
+ _ => (),
+ },
+ _ => (),
+ }
+}
+
+fn emit_return_lint(cx: &LateContext<'_>, ret_span: Span, inner_span: Option<Span>, replacement: RetReplacement) {
+ if ret_span.from_expansion() {
+ return;
+ }
+ match inner_span {
+ Some(inner_span) => {
+ if in_external_macro(cx.tcx.sess, inner_span) || inner_span.from_expansion() {
+ return;
+ }
+
+ span_lint_and_then(cx, NEEDLESS_RETURN, ret_span, "unneeded `return` statement", |diag| {
+ if let Some(snippet) = snippet_opt(cx, inner_span) {
+ diag.span_suggestion(ret_span, "remove `return`", snippet, Applicability::MachineApplicable);
+ }
+ })
+ },
+ None => match replacement {
+ RetReplacement::Empty => {
+ span_lint_and_sugg(
+ cx,
+ NEEDLESS_RETURN,
+ ret_span,
+ "unneeded `return` statement",
+ "remove `return`",
+ String::new(),
+ Applicability::MachineApplicable,
+ );
+ },
+ RetReplacement::Block => {
+ span_lint_and_sugg(
+ cx,
+ NEEDLESS_RETURN,
+ ret_span,
+ "unneeded `return` statement",
+ "replace `return` with an empty block",
+ "{}".to_string(),
+ Applicability::MachineApplicable,
+ );
+ },
+ },
+ }
+}
+
+fn last_statement_borrows<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) -> bool {
+ let mut visitor = BorrowVisitor { cx, borrows: false };
+ walk_expr(&mut visitor, expr);
+ visitor.borrows
+}
+
+struct BorrowVisitor<'a, 'tcx> {
+ cx: &'a LateContext<'tcx>,
+ borrows: bool,
+}
+
+impl<'tcx> Visitor<'tcx> for BorrowVisitor<'_, 'tcx> {
+ type Map = Map<'tcx>;
+
+ fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
+ if self.borrows {
+ return;
+ }
+
+ if let Some(def_id) = fn_def_id(self.cx, expr) {
+ self.borrows = self
+ .cx
+ .tcx
+ .fn_sig(def_id)
+ .output()
+ .skip_binder()
+ .walk()
+ .any(|arg| matches!(arg.unpack(), GenericArgKind::Lifetime(_)));
+ }
+
+ walk_expr(self, expr);
+ }
+
+ fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
+ NestedVisitorMap::None
+ }
+}
projects / rustc.git / blobdiff