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Auto merge of rust-lang#5680 - ebroto:3792_let_return, r=Manishearth
let_and_return: avoid "does not live long enough" errors EDIT: Add rust-lang#3324 to the list of fixes <details> <summary>Description of old impl</summary> <br> Avoid suggesting turning the RHS expression of the last statement into the block tail expression if a temporary borrows from a local that would be destroyed before. This is my first incursion into MIR so there's probably room for improvement! </details> Avoid linting if the return type of some method or function called in the last statement has a lifetime parameter. changelog: Fix false positive in [`let_and_return`] Fixes rust-lang#3792 Fixes rust-lang#3324
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,141 @@ | ||
| use if_chain::if_chain; | ||
| use rustc_errors::Applicability; | ||
| use rustc_hir::def_id::DefId; | ||
| use rustc_hir::intravisit::{walk_expr, NestedVisitorMap, Visitor}; | ||
| use rustc_hir::{Block, Expr, ExprKind, 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}; | ||
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|
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| use crate::utils::{in_macro, match_qpath, snippet_opt, span_lint_and_then}; | ||
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| 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" | ||
| } | ||
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| declare_lint_pass!(LetReturn => [LET_AND_RETURN]); | ||
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| impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LetReturn { | ||
| fn check_block(&mut self, cx: &LateContext<'a, '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 local.attrs.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"); | ||
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|
||
| if let Some(snippet) = snippet_opt(cx, initexpr.span) { | ||
| 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"); | ||
| } | ||
| }, | ||
| ); | ||
| } | ||
| } | ||
| } | ||
| } | ||
|
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| 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 | ||
| } | ||
|
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| struct BorrowVisitor<'a, 'tcx> { | ||
| cx: &'a LateContext<'a, 'tcx>, | ||
| borrows: bool, | ||
| } | ||
|
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| impl BorrowVisitor<'_, '_> { | ||
| fn fn_def_id(&self, expr: &Expr<'_>) -> Option<DefId> { | ||
| match &expr.kind { | ||
| ExprKind::MethodCall(..) => self.cx.tables.type_dependent_def_id(expr.hir_id), | ||
| ExprKind::Call( | ||
| Expr { | ||
| kind: ExprKind::Path(qpath), | ||
| .. | ||
| }, | ||
| .., | ||
| ) => self.cx.tables.qpath_res(qpath, expr.hir_id).opt_def_id(), | ||
| _ => None, | ||
| } | ||
| } | ||
| } | ||
|
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| impl<'tcx> Visitor<'tcx> for BorrowVisitor<'_, 'tcx> { | ||
| type Map = Map<'tcx>; | ||
|
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| fn visit_expr(&mut self, expr: &'tcx Expr<'_>) { | ||
| if self.borrows { | ||
| return; | ||
| } | ||
|
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| if let Some(def_id) = self.fn_def_id(expr) { | ||
| self.borrows = self | ||
| .cx | ||
| .tcx | ||
| .fn_sig(def_id) | ||
| .output() | ||
| .skip_binder() | ||
| .walk() | ||
| .any(|arg| matches!(arg.unpack(), GenericArgKind::Lifetime(_))); | ||
| } | ||
|
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| walk_expr(self, expr); | ||
| } | ||
|
|
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| fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> { | ||
| NestedVisitorMap::None | ||
| } | ||
| } |
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