rust/compiler/rustc_lint/src/noop_method_call.rs
Yuri Astrakhan 5160f8f843 Spellchecking compiler comments
This PR cleans up the rest of the spelling mistakes in the compiler comments. This PR does not change any literal or code spelling issues.
2022-03-30 15:14:15 -04:00

109 lines
4.3 KiB
Rust

use crate::context::LintContext;
use crate::rustc_middle::ty::TypeFoldable;
use crate::LateContext;
use crate::LateLintPass;
use rustc_hir::def::DefKind;
use rustc_hir::{Expr, ExprKind};
use rustc_middle::ty;
use rustc_span::symbol::sym;
declare_lint! {
/// The `noop_method_call` lint detects specific calls to noop methods
/// such as a calling `<&T as Clone>::clone` where `T: !Clone`.
///
/// ### Example
///
/// ```rust
/// # #![allow(unused)]
/// #![warn(noop_method_call)]
/// struct Foo;
/// let foo = &Foo;
/// let clone: &Foo = foo.clone();
/// ```
///
/// {{produces}}
///
/// ### Explanation
///
/// Some method calls are noops meaning that they do nothing. Usually such methods
/// are the result of blanket implementations that happen to create some method invocations
/// that end up not doing anything. For instance, `Clone` is implemented on all `&T`, but
/// calling `clone` on a `&T` where `T` does not implement clone, actually doesn't do anything
/// as references are copy. This lint detects these calls and warns the user about them.
pub NOOP_METHOD_CALL,
Allow,
"detects the use of well-known noop methods"
}
declare_lint_pass!(NoopMethodCall => [NOOP_METHOD_CALL]);
impl<'tcx> LateLintPass<'tcx> for NoopMethodCall {
fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
// We only care about method calls.
let ExprKind::MethodCall(call, elements, _) = &expr.kind else {
return
};
// We only care about method calls corresponding to the `Clone`, `Deref` and `Borrow`
// traits and ignore any other method call.
let (trait_id, did) = match cx.typeck_results().type_dependent_def(expr.hir_id) {
// Verify we are dealing with a method/associated function.
Some((DefKind::AssocFn, did)) => match cx.tcx.trait_of_item(did) {
// Check that we're dealing with a trait method for one of the traits we care about.
Some(trait_id)
if matches!(
cx.tcx.get_diagnostic_name(trait_id),
Some(sym::Borrow | sym::Clone | sym::Deref)
) =>
{
(trait_id, did)
}
_ => return,
},
_ => return,
};
let substs = cx.typeck_results().node_substs(expr.hir_id);
if substs.needs_subst() {
// We can't resolve on types that require monomorphization, so we don't handle them if
// we need to perform substitution.
return;
}
let param_env = cx.tcx.param_env(trait_id);
// Resolve the trait method instance.
let Ok(Some(i)) = ty::Instance::resolve(cx.tcx, param_env, did, substs) else {
return
};
// (Re)check that it implements the noop diagnostic.
let Some(name) = cx.tcx.get_diagnostic_name(i.def_id()) else { return };
if !matches!(
name,
sym::noop_method_borrow | sym::noop_method_clone | sym::noop_method_deref
) {
return;
}
let method = &call.ident.name;
let receiver = &elements[0];
let receiver_ty = cx.typeck_results().expr_ty(receiver);
let expr_ty = cx.typeck_results().expr_ty_adjusted(expr);
if receiver_ty != expr_ty {
// This lint will only trigger if the receiver type and resulting expression \
// type are the same, implying that the method call is unnecessary.
return;
}
let expr_span = expr.span;
let note = format!(
"the type `{:?}` which `{}` is being called on is the same as \
the type returned from `{}`, so the method call does not do \
anything and can be removed",
receiver_ty, method, method,
);
let span = expr_span.with_lo(receiver.span.hi());
cx.struct_span_lint(NOOP_METHOD_CALL, span, |lint| {
let method = &call.ident.name;
let message =
format!("call to `.{}()` on a reference in this situation does nothing", &method,);
lint.build(&message).span_label(span, "unnecessary method call").note(&note).emit();
});
}
}