From cd159fd7f9f8eec5792e6fd7f1b347ea3e028301 Mon Sep 17 00:00:00 2001 From: Michael Howell Date: Wed, 19 Aug 2020 03:05:44 -0700 Subject: [PATCH] Uplift drop-bounds lint from clippy --- compiler/rustc_lint/src/lib.rs | 3 + compiler/rustc_lint/src/traits.rs | 79 +++++++++++++++++++ library/core/src/mem/mod.rs | 1 + .../ui/drop-bounds/drop-bounds-impl-drop.rs | 14 ++++ src/test/ui/drop-bounds/drop-bounds.rs | 19 +++++ src/test/ui/drop-bounds/drop-bounds.stderr | 50 ++++++++++++ 6 files changed, 166 insertions(+) create mode 100644 compiler/rustc_lint/src/traits.rs create mode 100644 src/test/ui/drop-bounds/drop-bounds-impl-drop.rs create mode 100644 src/test/ui/drop-bounds/drop-bounds.rs create mode 100644 src/test/ui/drop-bounds/drop-bounds.stderr diff --git a/compiler/rustc_lint/src/lib.rs b/compiler/rustc_lint/src/lib.rs index 33caedfc19826..49e80f9d8a531 100644 --- a/compiler/rustc_lint/src/lib.rs +++ b/compiler/rustc_lint/src/lib.rs @@ -53,6 +53,7 @@ mod non_ascii_idents; mod nonstandard_style; mod passes; mod redundant_semicolon; +mod traits; mod types; mod unused; @@ -75,6 +76,7 @@ use internal::*; use non_ascii_idents::*; use nonstandard_style::*; use redundant_semicolon::*; +use traits::*; use types::*; use unused::*; @@ -157,6 +159,7 @@ macro_rules! late_lint_passes { MissingDebugImplementations: MissingDebugImplementations::default(), ArrayIntoIter: ArrayIntoIter, ClashingExternDeclarations: ClashingExternDeclarations::new(), + DropTraitConstraints: DropTraitConstraints, ] ); }; diff --git a/compiler/rustc_lint/src/traits.rs b/compiler/rustc_lint/src/traits.rs new file mode 100644 index 0000000000000..d4f79036e5a18 --- /dev/null +++ b/compiler/rustc_lint/src/traits.rs @@ -0,0 +1,79 @@ +use crate::LateContext; +use crate::LateLintPass; +use crate::LintContext; +use rustc_hir as hir; +use rustc_span::symbol::sym; + +declare_lint! { + /// The `drop_bounds` lint checks for generics with `std::ops::Drop` as + /// bounds. + /// + /// ### Example + /// + /// ```rust + /// fn foo() {} + /// ``` + /// + /// {{produces}} + /// + /// ### Explanation + /// + /// `Drop` bounds do not really accomplish anything. A type may have + /// compiler-generated drop glue without implementing the `Drop` trait + /// itself. The `Drop` trait also only has one method, `Drop::drop`, and + /// that function is by fiat not callable in user code. So there is really + /// no use case for using `Drop` in trait bounds. + /// + /// The most likely use case of a drop bound is to distinguish between + /// types that have destructors and types that don't. Combined with + /// specialization, a naive coder would write an implementation that + /// assumed a type could be trivially dropped, then write a specialization + /// for `T: Drop` that actually calls the destructor. Except that doing so + /// is not correct; String, for example, doesn't actually implement Drop, + /// but because String contains a Vec, assuming it can be trivially dropped + /// will leak memory. + pub DROP_BOUNDS, + Warn, + "bounds of the form `T: Drop` are useless" +} + +declare_lint_pass!( + /// Lint for bounds of the form `T: Drop`, which usually + /// indicate an attempt to emulate `std::mem::needs_drop`. + DropTraitConstraints => [DROP_BOUNDS] +); + +impl<'tcx> LateLintPass<'tcx> for DropTraitConstraints { + fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::Item<'tcx>) { + use rustc_middle::ty::PredicateAtom::*; + + let def_id = cx.tcx.hir().local_def_id(item.hir_id); + let predicates = cx.tcx.explicit_predicates_of(def_id); + for &(predicate, span) in predicates.predicates { + let trait_predicate = match predicate.skip_binders() { + Trait(trait_predicate, _constness) => trait_predicate, + _ => continue, + }; + let def_id = trait_predicate.trait_ref.def_id; + if cx.tcx.lang_items().drop_trait() == Some(def_id) { + // Explicitly allow `impl Drop`, a drop-guards-as-Voldemort-type pattern. + if trait_predicate.trait_ref.self_ty().is_impl_trait() { + continue; + } + cx.struct_span_lint(DROP_BOUNDS, span, |lint| { + let needs_drop = match cx.tcx.get_diagnostic_item(sym::needs_drop) { + Some(needs_drop) => needs_drop, + None => return, + }; + let msg = format!( + "bounds on `{}` are useless, consider instead \ + using `{}` to detect if a type has a destructor", + predicate, + cx.tcx.def_path_str(needs_drop) + ); + lint.build(&msg).emit() + }); + } + } + } +} diff --git a/library/core/src/mem/mod.rs b/library/core/src/mem/mod.rs index aa1b5529df222..a2c7da6e6958e 100644 --- a/library/core/src/mem/mod.rs +++ b/library/core/src/mem/mod.rs @@ -568,6 +568,7 @@ pub unsafe fn align_of_val_raw(val: *const T) -> usize { #[inline] #[stable(feature = "needs_drop", since = "1.21.0")] #[rustc_const_stable(feature = "const_needs_drop", since = "1.36.0")] +#[rustc_diagnostic_item = "needs_drop"] pub const fn needs_drop() -> bool { intrinsics::needs_drop::() } diff --git a/src/test/ui/drop-bounds/drop-bounds-impl-drop.rs b/src/test/ui/drop-bounds/drop-bounds-impl-drop.rs new file mode 100644 index 0000000000000..063efc7b31abd --- /dev/null +++ b/src/test/ui/drop-bounds/drop-bounds-impl-drop.rs @@ -0,0 +1,14 @@ +// run-pass +#![deny(drop_bounds)] +// As a special exemption, `impl Drop` in the return position raises no error. +// This allows a convenient way to return an unnamed drop guard. +fn voldemort_type() -> impl Drop { + struct Voldemort; + impl Drop for Voldemort { + fn drop(&mut self) {} + } + Voldemort +} +fn main() { + let _ = voldemort_type(); +} diff --git a/src/test/ui/drop-bounds/drop-bounds.rs b/src/test/ui/drop-bounds/drop-bounds.rs new file mode 100644 index 0000000000000..c73538278d3be --- /dev/null +++ b/src/test/ui/drop-bounds/drop-bounds.rs @@ -0,0 +1,19 @@ +#![deny(drop_bounds)] +fn foo() {} //~ ERROR +fn bar() +where + U: Drop, //~ ERROR +{ +} +fn baz(_x: impl Drop) {} //~ ERROR +struct Foo { //~ ERROR + _x: T +} +struct Bar where U: Drop { //~ ERROR + _x: U +} +trait Baz: Drop { //~ ERROR +} +impl Baz for T { //~ ERROR +} +fn main() {} diff --git a/src/test/ui/drop-bounds/drop-bounds.stderr b/src/test/ui/drop-bounds/drop-bounds.stderr new file mode 100644 index 0000000000000..15ba4c9a64989 --- /dev/null +++ b/src/test/ui/drop-bounds/drop-bounds.stderr @@ -0,0 +1,50 @@ +error: bounds on `T: Drop` are useless, consider instead using `std::mem::needs_drop` to detect if a type has a destructor + --> $DIR/drop-bounds.rs:2:11 + | +LL | fn foo() {} + | ^^^^ + | +note: the lint level is defined here + --> $DIR/drop-bounds.rs:1:9 + | +LL | #![deny(drop_bounds)] + | ^^^^^^^^^^^ + +error: bounds on `U: Drop` are useless, consider instead using `std::mem::needs_drop` to detect if a type has a destructor + --> $DIR/drop-bounds.rs:5:8 + | +LL | U: Drop, + | ^^^^ + +error: bounds on `impl Drop: Drop` are useless, consider instead using `std::mem::needs_drop` to detect if a type has a destructor + --> $DIR/drop-bounds.rs:8:17 + | +LL | fn baz(_x: impl Drop) {} + | ^^^^ + +error: bounds on `T: Drop` are useless, consider instead using `std::mem::needs_drop` to detect if a type has a destructor + --> $DIR/drop-bounds.rs:9:15 + | +LL | struct Foo { + | ^^^^ + +error: bounds on `U: Drop` are useless, consider instead using `std::mem::needs_drop` to detect if a type has a destructor + --> $DIR/drop-bounds.rs:12:24 + | +LL | struct Bar where U: Drop { + | ^^^^ + +error: bounds on `Self: Drop` are useless, consider instead using `std::mem::needs_drop` to detect if a type has a destructor + --> $DIR/drop-bounds.rs:15:12 + | +LL | trait Baz: Drop { + | ^^^^ + +error: bounds on `T: Drop` are useless, consider instead using `std::mem::needs_drop` to detect if a type has a destructor + --> $DIR/drop-bounds.rs:17:9 + | +LL | impl Baz for T { + | ^^^^ + +error: aborting due to 7 previous errors +