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| 1 | +use crate::{LateContext, LateLintPass, LintContext}; |
| 2 | + |
| 3 | +use hir::{Expr, Pat}; |
| 4 | +use rustc_errors::{Applicability, DelayDm}; |
| 5 | +use rustc_hir as hir; |
| 6 | +use rustc_infer::traits::TraitEngine; |
| 7 | +use rustc_infer::{infer::TyCtxtInferExt, traits::ObligationCause}; |
| 8 | +use rustc_middle::ty::{self, List}; |
| 9 | +use rustc_span::{sym, Span}; |
| 10 | +use rustc_trait_selection::traits::TraitEngineExt; |
| 11 | + |
| 12 | +declare_lint! { |
| 13 | + /// The `for_loops_over_fallibles` lint checks for `for` loops over `Option` or `Result` values. |
| 14 | + /// |
| 15 | + /// ### Example |
| 16 | + /// |
| 17 | + /// ```rust |
| 18 | + /// let opt = Some(1); |
| 19 | + /// for x in opt { /* ... */} |
| 20 | + /// ``` |
| 21 | + /// |
| 22 | + /// {{produces}} |
| 23 | + /// |
| 24 | + /// ### Explanation |
| 25 | + /// |
| 26 | + /// Both `Option` and `Result` implement `IntoIterator` trait, which allows using them in a `for` loop. |
| 27 | + /// `for` loop over `Option` or `Result` will iterate either 0 (if the value is `None`/`Err(_)`) |
| 28 | + /// or 1 time (if the value is `Some(_)`/`Ok(_)`). This is not very useful and is more clearly expressed |
| 29 | + /// via `if let`. |
| 30 | + /// |
| 31 | + /// `for` loop can also be accidentally written with the intention to call a function multiple times, |
| 32 | + /// while the function returns `Some(_)`, in these cases `while let` loop should be used instead. |
| 33 | + /// |
| 34 | + /// The "intended" use of `IntoIterator` implementations for `Option` and `Result` is passing them to |
| 35 | + /// generic code that expects something implementing `IntoIterator`. For example using `.chain(option)` |
| 36 | + /// to optionally add a value to an iterator. |
| 37 | + pub FOR_LOOPS_OVER_FALLIBLES, |
| 38 | + Warn, |
| 39 | + "for-looping over an `Option` or a `Result`, which is more clearly expressed as an `if let`" |
| 40 | +} |
| 41 | + |
| 42 | +declare_lint_pass!(ForLoopsOverFallibles => [FOR_LOOPS_OVER_FALLIBLES]); |
| 43 | + |
| 44 | +impl<'tcx> LateLintPass<'tcx> for ForLoopsOverFallibles { |
| 45 | + fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) { |
| 46 | + let Some((pat, arg)) = extract_for_loop(expr) else { return }; |
| 47 | + |
| 48 | + let ty = cx.typeck_results().expr_ty(arg); |
| 49 | + |
| 50 | + let &ty::Adt(adt, substs) = ty.kind() else { return }; |
| 51 | + |
| 52 | + let (article, ty, var) = match adt.did() { |
| 53 | + did if cx.tcx.is_diagnostic_item(sym::Option, did) => ("an", "Option", "Some"), |
| 54 | + did if cx.tcx.is_diagnostic_item(sym::Result, did) => ("a", "Result", "Ok"), |
| 55 | + _ => return, |
| 56 | + }; |
| 57 | + |
| 58 | + let msg = DelayDm(|| { |
| 59 | + format!( |
| 60 | + "for loop over {article} `{ty}`. This is more readably written as an `if let` statement", |
| 61 | + ) |
| 62 | + }); |
| 63 | + |
| 64 | + cx.struct_span_lint(FOR_LOOPS_OVER_FALLIBLES, arg.span, msg, |lint| { |
| 65 | + if let Some(recv) = extract_iterator_next_call(cx, arg) |
| 66 | + && let Ok(recv_snip) = cx.sess().source_map().span_to_snippet(recv.span) |
| 67 | + { |
| 68 | + lint.span_suggestion( |
| 69 | + recv.span.between(arg.span.shrink_to_hi()), |
| 70 | + format!("to iterate over `{recv_snip}` remove the call to `next`"), |
| 71 | + ".by_ref()", |
| 72 | + Applicability::MaybeIncorrect |
| 73 | + ); |
| 74 | + } else { |
| 75 | + lint.multipart_suggestion_verbose( |
| 76 | + format!("to check pattern in a loop use `while let`"), |
| 77 | + vec![ |
| 78 | + // NB can't use `until` here because `expr.span` and `pat.span` have different syntax contexts |
| 79 | + (expr.span.with_hi(pat.span.lo()), format!("while let {var}(")), |
| 80 | + (pat.span.between(arg.span), format!(") = ")), |
| 81 | + ], |
| 82 | + Applicability::MaybeIncorrect |
| 83 | + ); |
| 84 | + } |
| 85 | + |
| 86 | + if suggest_question_mark(cx, adt, substs, expr.span) { |
| 87 | + lint.span_suggestion( |
| 88 | + arg.span.shrink_to_hi(), |
| 89 | + "consider unwrapping the `Result` with `?` to iterate over its contents", |
| 90 | + "?", |
| 91 | + Applicability::MaybeIncorrect, |
| 92 | + ); |
| 93 | + } |
| 94 | + |
| 95 | + lint.multipart_suggestion_verbose( |
| 96 | + "consider using `if let` to clear intent", |
| 97 | + vec![ |
| 98 | + // NB can't use `until` here because `expr.span` and `pat.span` have different syntax contexts |
| 99 | + (expr.span.with_hi(pat.span.lo()), format!("if let {var}(")), |
| 100 | + (pat.span.between(arg.span), format!(") = ")), |
| 101 | + ], |
| 102 | + Applicability::MaybeIncorrect, |
| 103 | + ) |
| 104 | + }) |
| 105 | + } |
| 106 | +} |
| 107 | + |
| 108 | +fn extract_for_loop<'tcx>(expr: &Expr<'tcx>) -> Option<(&'tcx Pat<'tcx>, &'tcx Expr<'tcx>)> { |
| 109 | + if let hir::ExprKind::DropTemps(e) = expr.kind |
| 110 | + && let hir::ExprKind::Match(iterexpr, [arm], hir::MatchSource::ForLoopDesugar) = e.kind |
| 111 | + && let hir::ExprKind::Call(_, [arg]) = iterexpr.kind |
| 112 | + && let hir::ExprKind::Loop(block, ..) = arm.body.kind |
| 113 | + && let [stmt] = block.stmts |
| 114 | + && let hir::StmtKind::Expr(e) = stmt.kind |
| 115 | + && let hir::ExprKind::Match(_, [_, some_arm], _) = e.kind |
| 116 | + && let hir::PatKind::Struct(_, [field], _) = some_arm.pat.kind |
| 117 | + { |
| 118 | + Some((field.pat, arg)) |
| 119 | + } else { |
| 120 | + None |
| 121 | + } |
| 122 | +} |
| 123 | + |
| 124 | +fn extract_iterator_next_call<'tcx>( |
| 125 | + cx: &LateContext<'_>, |
| 126 | + expr: &Expr<'tcx>, |
| 127 | +) -> Option<&'tcx Expr<'tcx>> { |
| 128 | + // This won't work for `Iterator::next(iter)`, is this an issue? |
| 129 | + if let hir::ExprKind::MethodCall(_, recv, _, _) = expr.kind |
| 130 | + && cx.typeck_results().type_dependent_def_id(expr.hir_id) == cx.tcx.lang_items().next_fn() |
| 131 | + { |
| 132 | + Some(recv) |
| 133 | + } else { |
| 134 | + return None |
| 135 | + } |
| 136 | +} |
| 137 | + |
| 138 | +fn suggest_question_mark<'tcx>( |
| 139 | + cx: &LateContext<'tcx>, |
| 140 | + adt: ty::AdtDef<'tcx>, |
| 141 | + substs: &List<ty::GenericArg<'tcx>>, |
| 142 | + span: Span, |
| 143 | +) -> bool { |
| 144 | + let Some(body_id) = cx.enclosing_body else { return false }; |
| 145 | + let Some(into_iterator_did) = cx.tcx.get_diagnostic_item(sym::IntoIterator) else { return false }; |
| 146 | + |
| 147 | + if !cx.tcx.is_diagnostic_item(sym::Result, adt.did()) { |
| 148 | + return false; |
| 149 | + } |
| 150 | + |
| 151 | + // Check that the function/closure/constant we are in has a `Result` type. |
| 152 | + // Otherwise suggesting using `?` may not be a good idea. |
| 153 | + { |
| 154 | + let ty = cx.typeck_results().expr_ty(&cx.tcx.hir().body(body_id).value); |
| 155 | + let ty::Adt(ret_adt, ..) = ty.kind() else { return false }; |
| 156 | + if !cx.tcx.is_diagnostic_item(sym::Result, ret_adt.did()) { |
| 157 | + return false; |
| 158 | + } |
| 159 | + } |
| 160 | + |
| 161 | + let ty = substs.type_at(0); |
| 162 | + let infcx = cx.tcx.infer_ctxt().build(); |
| 163 | + let mut fulfill_cx = <dyn TraitEngine<'_>>::new(infcx.tcx); |
| 164 | + |
| 165 | + let cause = ObligationCause::new( |
| 166 | + span, |
| 167 | + body_id.hir_id, |
| 168 | + rustc_infer::traits::ObligationCauseCode::MiscObligation, |
| 169 | + ); |
| 170 | + fulfill_cx.register_bound( |
| 171 | + &infcx, |
| 172 | + ty::ParamEnv::empty(), |
| 173 | + // Erase any region vids from the type, which may not be resolved |
| 174 | + infcx.tcx.erase_regions(ty), |
| 175 | + into_iterator_did, |
| 176 | + cause, |
| 177 | + ); |
| 178 | + |
| 179 | + // Select all, including ambiguous predicates |
| 180 | + let errors = fulfill_cx.select_all_or_error(&infcx); |
| 181 | + |
| 182 | + errors.is_empty() |
| 183 | +} |
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