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Rollup merge of #97739 - a2aaron:let_underscore, r=estebank

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Uplift the `let_underscore` lints from clippy into rustc.

This PR resolves #97241.

This PR adds three lints from clippy--`let_underscore_drop`, `let_underscore_lock`, and `let_underscore_must_use`, which are meant to capture likely-incorrect uses of `let _ = ...` bindings (in particular, doing this on a type with a non-trivial `Drop` causes the `Drop` to occur immediately, instead of at the end of the scope. For a type like `MutexGuard`, this effectively releases the lock immediately, which is almost certainly the wrong behavior)

In porting the lints from clippy I had to copy over a bunch of utility functions from `clippy_util` that these lints also relied upon. Is that the right approach?

Note that I've set the `must_use` and `drop` lints to Allow by default and set `lock` to Deny by default (this matches the same settings that clippy has). In talking with `@estebank` he informed me to do a Crater run (I am not sure what type of Crater run to request here--I think it's just "check only"?)

On the linked issue, there's some discussion about using `must_use` and `Drop` together as a heuristic for when to warn--I did not implement this yet.

r? `@estebank`
This commit is contained in:
Guillaume Gomez 2022-09-02 11:34:45 +02:00 • committed by GitHub
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175
compiler/rustc_lint/src/let_underscore.rs Normal file
View file

@ -0,0 +1,175 @@
use crate::{LateContext, LateLintPass, LintContext};
use rustc_errors::{Applicability, LintDiagnosticBuilder, MultiSpan};
use rustc_hir as hir;
use rustc_middle::ty;
use rustc_span::Symbol;
declare_lint! {
/// The `let_underscore_drop` lint checks for statements which don't bind
/// an expression which has a non-trivial Drop implementation to anything,
/// causing the expression to be dropped immediately instead of at end of
/// scope.
///
/// ### Example
/// ```
/// struct SomeStruct;
/// impl Drop for SomeStruct {
/// fn drop(&mut self) {
/// println!("Dropping SomeStruct");
/// }
/// }
///
/// fn main() {
/// #[warn(let_underscore_drop)]
/// // SomeStuct is dropped immediately instead of at end of scope,
/// // so "Dropping SomeStruct" is printed before "end of main".
/// // The order of prints would be reversed if SomeStruct was bound to
/// // a name (such as "_foo").
/// let _ = SomeStruct;
/// println!("end of main");
/// }
/// ```
///
/// {{produces}}
///
/// ### Explanation
///
/// Statements which assign an expression to an underscore causes the
/// expression to immediately drop instead of extending the expression's
/// lifetime to the end of the scope. This is usually unintended,
/// especially for types like `MutexGuard`, which are typically used to
/// lock a mutex for the duration of an entire scope.
///
/// If you want to extend the expression's lifetime to the end of the scope,
/// assign an underscore-prefixed name (such as `_foo`) to the expression.
/// If you do actually want to drop the expression immediately, then
/// calling `std::mem::drop` on the expression is clearer and helps convey
/// intent.
pub LET_UNDERSCORE_DROP,
Allow,
"non-binding let on a type that implements `Drop`"
}
declare_lint! {
/// The `let_underscore_lock` lint checks for statements which don't bind
/// a mutex to anything, causing the lock to be released immediately instead
/// of at end of scope, which is typically incorrect.
///
/// ### Example
/// ```compile_fail
/// use std::sync::{Arc, Mutex};
/// use std::thread;
/// let data = Arc::new(Mutex::new(0));
///
/// thread::spawn(move || {
/// // The lock is immediately released instead of at the end of the
/// // scope, which is probably not intended.
/// let _ = data.lock().unwrap();
/// println!("doing some work");
/// let mut lock = data.lock().unwrap();
/// *lock += 1;
/// });
/// ```
///
/// {{produces}}
///
/// ### Explanation
///
/// Statements which assign an expression to an underscore causes the
/// expression to immediately drop instead of extending the expression's
/// lifetime to the end of the scope. This is usually unintended,
/// especially for types like `MutexGuard`, which are typically used to
/// lock a mutex for the duration of an entire scope.
///
/// If you want to extend the expression's lifetime to the end of the scope,
/// assign an underscore-prefixed name (such as `_foo`) to the expression.
/// If you do actually want to drop the expression immediately, then
/// calling `std::mem::drop` on the expression is clearer and helps convey
/// intent.
pub LET_UNDERSCORE_LOCK,
Deny,
"non-binding let on a synchronization lock"
}
declare_lint_pass!(LetUnderscore => [LET_UNDERSCORE_DROP, LET_UNDERSCORE_LOCK]);
const SYNC_GUARD_SYMBOLS: [Symbol; 3] = [
rustc_span::sym::MutexGuard,
rustc_span::sym::RwLockReadGuard,
rustc_span::sym::RwLockWriteGuard,
];
impl<'tcx> LateLintPass<'tcx> for LetUnderscore {
fn check_local(&mut self, cx: &LateContext<'_>, local: &hir::Local<'_>) {
if !matches!(local.pat.kind, hir::PatKind::Wild) {
return;
}
if let Some(init) = local.init {
let init_ty = cx.typeck_results().expr_ty(init);
// If the type has a trivial Drop implementation, then it doesn't
// matter that we drop the value immediately.
if !init_ty.needs_drop(cx.tcx, cx.param_env) {
return;
}
let is_sync_lock = match init_ty.kind() {
ty::Adt(adt, _) => SYNC_GUARD_SYMBOLS
.iter()
.any(|guard_symbol| cx.tcx.is_diagnostic_item(*guard_symbol, adt.did())),
_ => false,
};
if is_sync_lock {
let mut span = MultiSpan::from_spans(vec![local.pat.span, init.span]);
span.push_span_label(
local.pat.span,
"this lock is not assigned to a binding and is immediately dropped".to_string(),
);
span.push_span_label(
init.span,
"this binding will immediately drop the value assigned to it".to_string(),
);
cx.struct_span_lint(LET_UNDERSCORE_LOCK, span, |lint| {
build_and_emit_lint(
lint,
local,
init.span,
"non-binding let on a synchronization lock",
)
})
} else {
cx.struct_span_lint(LET_UNDERSCORE_DROP, local.span, |lint| {
build_and_emit_lint(
lint,
local,
init.span,
"non-binding let on a type that implements `Drop`",
);
})
}
}
}
}
fn build_and_emit_lint(
lint: LintDiagnosticBuilder<'_, ()>,
local: &hir::Local<'_>,
init_span: rustc_span::Span,
msg: &str,
) {
lint.build(msg)
.span_suggestion_verbose(
local.pat.span,
"consider binding to an unused variable to avoid immediately dropping the value",
"_unused",
Applicability::MachineApplicable,
)
.multipart_suggestion(
"consider immediately dropping the value",
vec![
(local.span.until(init_span), "drop(".to_string()),
(init_span.shrink_to_hi(), ")".to_string()),
],
Applicability::MachineApplicable,
)
.emit();
}

5
compiler/rustc_lint/src/lib.rs
View file

@ -55,6 +55,7 @@ mod expect;
pub mod hidden_unicode_codepoints;
mod internal;
mod late;
mod let_underscore;
mod levels;
mod methods;
mod non_ascii_idents;
@ -86,6 +87,7 @@ use builtin::*;
use enum_intrinsics_non_enums::EnumIntrinsicsNonEnums;
use hidden_unicode_codepoints::*;
use internal::*;
use let_underscore::*;
use methods::*;
use non_ascii_idents::*;
use non_fmt_panic::NonPanicFmt;
@ -189,6 +191,7 @@ macro_rules! late_lint_mod_passes {
VariantSizeDifferences: VariantSizeDifferences,
BoxPointers: BoxPointers,
PathStatements: PathStatements,
LetUnderscore: LetUnderscore,
// Depends on referenced function signatures in expressions
UnusedResults: UnusedResults,
NonUpperCaseGlobals: NonUpperCaseGlobals,
@ -315,6 +318,8 @@ fn register_builtins(store: &mut LintStore, no_interleave_lints: bool) {
REDUNDANT_SEMICOLONS
);
add_lint_group!("let_underscore", LET_UNDERSCORE_DROP, LET_UNDERSCORE_LOCK);
add_lint_group!(
"rust_2018_idioms",
BARE_TRAIT_OBJECTS,