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| 1 | +use crate::context::LintContext; |
| 2 | +use crate::rustc_middle::ty::TypeFoldable; |
| 3 | +use crate::LateContext; |
| 4 | +use crate::LateLintPass; |
| 5 | +use rustc_hir::def::DefKind; |
| 6 | +use rustc_hir::{Expr, ExprKind}; |
| 7 | +use rustc_middle::ty; |
| 8 | +use rustc_span::symbol::sym; |
| 9 | + |
| 10 | +declare_lint! { |
| 11 | + /// The `noop_method_call` lint detects specific calls to noop methods |
| 12 | + /// such as a calling `<&T as Clone>::clone` where `T: !Clone`. |
| 13 | + /// |
| 14 | + /// ### Example |
| 15 | + /// |
| 16 | + /// ```rust |
| 17 | + /// # #![allow(unused)] |
| 18 | + /// #![warn(noop_method_call)] |
| 19 | + /// struct Foo; |
| 20 | + /// let foo = &Foo; |
| 21 | + /// let clone: &Foo = foo.clone(); |
| 22 | + /// ``` |
| 23 | + /// |
| 24 | + /// {{produces}} |
| 25 | + /// |
| 26 | + /// ### Explanation |
| 27 | + /// |
| 28 | + /// Some method calls are noops meaning that they do nothing. Usually such methods |
| 29 | + /// are the result of blanket implementations that happen to create some method invocations |
| 30 | + /// that end up not doing anything. For instance, `Clone` is implemented on all `&T`, but |
| 31 | + /// calling `clone` on a `&T` where `T` does not implement clone, actually doesn't do anything |
| 32 | + /// as references are copy. This lint detects these calls and warns the user about them. |
| 33 | + pub NOOP_METHOD_CALL, |
| 34 | + Allow, |
| 35 | + "detects the use of well-known noop methods" |
| 36 | +} |
| 37 | + |
| 38 | +declare_lint_pass!(NoopMethodCall => [NOOP_METHOD_CALL]); |
| 39 | + |
| 40 | +impl<'tcx> LateLintPass<'tcx> for NoopMethodCall { |
| 41 | + fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) { |
| 42 | + // We only care about method calls. |
| 43 | + let (call, elements) = match expr.kind { |
| 44 | + ExprKind::MethodCall(call, _, elements, _) => (call, elements), |
| 45 | + _ => return, |
| 46 | + }; |
| 47 | + // We only care about method calls corresponding to the `Clone`, `Deref` and `Borrow` |
| 48 | + // traits and ignore any other method call. |
| 49 | + let (trait_id, did) = match cx.typeck_results().type_dependent_def(expr.hir_id) { |
| 50 | + // Verify we are dealing with a method/associated function. |
| 51 | + Some((DefKind::AssocFn, did)) => match cx.tcx.trait_of_item(did) { |
| 52 | + // Check that we're dealing with a trait method for one of the traits we care about. |
| 53 | + Some(trait_id) |
| 54 | + if [sym::Clone, sym::Deref, sym::Borrow] |
| 55 | + .iter() |
| 56 | + .any(|s| cx.tcx.is_diagnostic_item(*s, trait_id)) => |
| 57 | + { |
| 58 | + (trait_id, did) |
| 59 | + } |
| 60 | + _ => return, |
| 61 | + }, |
| 62 | + _ => return, |
| 63 | + }; |
| 64 | + let substs = cx.typeck_results().node_substs(expr.hir_id); |
| 65 | + if substs.needs_subst() { |
| 66 | + // We can't resolve on types that require monomorphization, so we don't handle them if |
| 67 | + // we need to perfom substitution. |
| 68 | + return; |
| 69 | + } |
| 70 | + let param_env = cx.tcx.param_env(trait_id); |
| 71 | + // Resolve the trait method instance. |
| 72 | + let i = match ty::Instance::resolve(cx.tcx, param_env, did, substs) { |
| 73 | + Ok(Some(i)) => i, |
| 74 | + _ => return, |
| 75 | + }; |
| 76 | + // (Re)check that it implements the noop diagnostic. |
| 77 | + for s in [sym::noop_method_clone, sym::noop_method_deref, sym::noop_method_borrow].iter() { |
| 78 | + if cx.tcx.is_diagnostic_item(*s, i.def_id()) { |
| 79 | + let method = &call.ident.name; |
| 80 | + let receiver = &elements[0]; |
| 81 | + let receiver_ty = cx.typeck_results().expr_ty(receiver); |
| 82 | + let expr_ty = cx.typeck_results().expr_ty_adjusted(expr); |
| 83 | + if receiver_ty != expr_ty { |
| 84 | + // This lint will only trigger if the receiver type and resulting expression \ |
| 85 | + // type are the same, implying that the method call is unnecessary. |
| 86 | + return; |
| 87 | + } |
| 88 | + let expr_span = expr.span; |
| 89 | + let note = format!( |
| 90 | + "the type `{:?}` which `{}` is being called on is the same as \ |
| 91 | + the type returned from `{}`, so the method call does not do \ |
| 92 | + anything and can be removed", |
| 93 | + receiver_ty, method, method, |
| 94 | + ); |
| 95 | + |
| 96 | + let span = expr_span.with_lo(receiver.span.hi()); |
| 97 | + cx.struct_span_lint(NOOP_METHOD_CALL, span, |lint| { |
| 98 | + let method = &call.ident.name; |
| 99 | + let message = format!( |
| 100 | + "call to `.{}()` on a reference in this situation does nothing", |
| 101 | + &method, |
| 102 | + ); |
| 103 | + lint.build(&message) |
| 104 | + .span_label(span, "unnecessary method call") |
| 105 | + .note(¬e) |
| 106 | + .emit() |
| 107 | + }); |
| 108 | + } |
| 109 | + } |
| 110 | + } |
| 111 | +} |
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