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|
use crate::{
lints::{NonBindingLet, NonBindingLetSub},
LateContext, LateLintPass, LintContext,
};
use rustc_errors::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
///
/// ```rust
/// 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
/// ```rust,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,
};
let sub = NonBindingLetSub {
suggestion: local.pat.span,
multi_suggestion_start: local.span.until(init.span),
multi_suggestion_end: init.span.shrink_to_hi(),
};
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.emit_spanned_lint(LET_UNDERSCORE_LOCK, span, NonBindingLet::SyncLock { sub });
} else {
cx.emit_spanned_lint(
LET_UNDERSCORE_DROP,
local.span,
NonBindingLet::DropType { sub },
);
}
}
}
}
|
