cleanup spaghetti code

clippy_utils/src/lib.rs

@@ -20,8 +20,8 @@
 extern crate rustc_ast;
 extern crate rustc_ast_pretty;
 extern crate rustc_attr;
-extern crate rustc_data_structures;
 extern crate rustc_const_eval;
+extern crate rustc_data_structures;
 // The `rustc_driver` crate seems to be required in order to use the `rust_ast` crate.
 #[allow(unused_extern_crates)]
 extern crate rustc_driver;

clippy_utils/src/qualify_min_const_fn.rs

@@ -4,7 +4,7 @@
 // differ from the time of `rustc` even if the name stays the same.
 
 use crate::msrvs::Msrv;
-use hir::{Constness, LangItem};
+use hir::LangItem;
 use rustc_const_eval::transform::check_consts::ConstCx;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
@@ -14,14 +14,14 @@ use rustc_middle::mir::{
     Body, CastKind, NonDivergingIntrinsic, NullOp, Operand, Place, ProjectionElem, Rvalue, Statement, StatementKind,
     Terminator, TerminatorKind,
 };
-use rustc_middle::traits::ObligationCause;
+use rustc_middle::traits::{ImplSource, ObligationCause};
 use rustc_middle::ty::subst::GenericArgKind;
 use rustc_middle::ty::{self, adjustment::PointerCast, Ty, TyCtxt};
 use rustc_middle::ty::{BoundConstness, TraitRef};
 use rustc_semver::RustcVersion;
 use rustc_span::symbol::sym;
 use rustc_span::Span;
-use rustc_trait_selection::traits::SelectionContext;
+use rustc_trait_selection::traits::{ObligationCtxt, SelectionContext};
 use std::borrow::Cow;
 
 type McfResult = Result<(), (Span, Cow<'static, str>)>;
@@ -135,9 +135,9 @@ fn check_rvalue<'tcx>(
     match rvalue {
         Rvalue::ThreadLocalRef(_) => Err((span, "cannot access thread local storage in const fn".into())),
         Rvalue::Len(place) | Rvalue::Discriminant(place) | Rvalue::Ref(_, _, place) | Rvalue::AddressOf(_, place) => {
-            check_place(tcx, *place, span, body)
+            check_place(tcx, *place, span, body, false)
         },
-        Rvalue::CopyForDeref(place) => check_place(tcx, *place, span, body),
+        Rvalue::CopyForDeref(place) => check_place(tcx, *place, span, body, false),
         Rvalue::Repeat(operand, _)
         | Rvalue::Use(operand)
         | Rvalue::Cast(
@@ -230,14 +230,14 @@ fn check_statement<'tcx>(
     let span = statement.source_info.span;
     match &statement.kind {
         StatementKind::Assign(box (place, rval)) => {
-            check_place(tcx, *place, span, body)?;
+            check_place(tcx, *place, span, body, false)?;
             check_rvalue(tcx, body, def_id, rval, span)
         },
 
-        StatementKind::FakeRead(box (_, place)) => check_place(tcx, *place, span, body),
+        StatementKind::FakeRead(box (_, place)) => check_place(tcx, *place, span, body, false),
         // just an assignment
         StatementKind::SetDiscriminant { place, .. } | StatementKind::Deinit(place) => {
-            check_place(tcx, **place, span, body)
+            check_place(tcx, **place, span, body, false)
         },
 
         StatementKind::Intrinsic(box NonDivergingIntrinsic::Assume(op)) => check_operand(tcx, op, span, body),
@@ -263,7 +263,8 @@ fn check_statement<'tcx>(
 
 fn check_operand<'tcx>(tcx: TyCtxt<'tcx>, operand: &Operand<'tcx>, span: Span, body: &Body<'tcx>) -> McfResult {
     match operand {
-        Operand::Move(place) | Operand::Copy(place) => check_place(tcx, *place, span, body),
+        Operand::Move(place) => check_place(tcx, *place, span, body, true),
+        Operand::Copy(place) => check_place(tcx, *place, span, body, false),
         Operand::Constant(c) => match c.check_static_ptr(tcx) {
             Some(_) => Err((span, "cannot access `static` items in const fn".into())),
             None => Ok(()),
@@ -271,13 +272,20 @@ fn check_operand<'tcx>(tcx: TyCtxt<'tcx>, operand: &Operand<'tcx>, span: Span, b
     }
 }
 
-fn check_place<'tcx>(tcx: TyCtxt<'tcx>, place: Place<'tcx>, span: Span, body: &Body<'tcx>) -> McfResult {
+fn check_place<'tcx>(tcx: TyCtxt<'tcx>, place: Place<'tcx>, span: Span, body: &Body<'tcx>, in_move: bool) -> McfResult {
     let mut cursor = place.projection.as_ref();
 
     while let [ref proj_base @ .., elem] = *cursor {
         cursor = proj_base;
         match elem {
-            ProjectionElem::Field(..) => {
+            ProjectionElem::Field(_, ty) => {
+                if !is_ty_const_destruct(tcx, ty, body) && in_move {
+                    return Err((
+                        span,
+                        "cannot drop locals with a non constant destructor in const fn".into(),
+                    ));
+                }
+
                 let base_ty = Place::ty_from(place.local, proj_base, body, tcx).ty;
                 if let Some(def) = base_ty.ty_adt_def() {
                     // No union field accesses in `const fn`
@@ -320,7 +328,7 @@ fn check_terminator<'tcx>(
                     "cannot drop locals with a non constant destructor in const fn".into(),
                 ));
             }
-            check_place(tcx, *place, span, body)
+            check_place(tcx, *place, span, body, false)
         },
         TerminatorKind::SwitchInt { discr, targets: _ } => check_operand(tcx, discr, span, body),
         TerminatorKind::GeneratorDrop | TerminatorKind::Yield { .. } => {
@@ -408,6 +416,7 @@ fn is_const_fn(tcx: TyCtxt<'_>, def_id: DefId, msrv: &Msrv) -> bool {
         })
 }
 
+#[expect(clippy::similar_names)] // bit too pedantic
 fn is_ty_const_destruct<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, body: &Body<'tcx>) -> bool {
     // Avoid selecting for simple cases, such as builtin types.
     if ty::util::is_trivially_const_drop(ty) {
@@ -417,23 +426,24 @@ fn is_ty_const_destruct<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, body: &Body<'tcx>
     let obligation = Obligation::new(
         tcx,
         ObligationCause::dummy_with_span(body.span),
-        ConstCx::new(tcx, body).param_env.with_constness(Constness::Const),
+        ConstCx::new(tcx, body).param_env.with_const(),
         TraitRef::from_lang_item(tcx, LangItem::Destruct, body.span, [ty]).with_constness(BoundConstness::ConstIfConst),
     );
 
-    let fields_all_const_destruct = if let ty::Adt(def, subst) = ty.kind() && !ty.is_union() {
-        // This is such a mess even rustfmt doesn't wanna touch it
-        def.all_fields()
-            .map(|field| is_ty_const_destruct(tcx, field.ty(tcx, subst), body))
-            .all(|f| f)
-            && def.variants().iter()
-                .map(|variant| variant.fields.iter().map(|field| is_ty_const_destruct(tcx, field.ty(tcx, subst), body)))
-                .all(|mut fs| fs.all(|f| f))
-    } else {
-        true
-    };
-
     let infcx = tcx.infer_ctxt().build();
     let mut selcx = SelectionContext::new(&infcx);
-    selcx.select(&obligation).is_ok() && fields_all_const_destruct
+    let Some(impl_src) = selcx.select(&obligation).ok().flatten() else {
+        return false;
+    };
+
+    if !matches!(
+        impl_src,
+        ImplSource::ConstDestruct(_) | ImplSource::Param(_, ty::BoundConstness::ConstIfConst)
+    ) {
+        return false;
+    }
+
+    let ocx = ObligationCtxt::new(&infcx);
+    ocx.register_obligations(impl_src.nested_obligations());
+    ocx.select_all_or_error().is_empty()
 }

tests/ui/missing_const_for_fn/cant_be_const.rs

@@ -127,6 +127,7 @@ with_span! {
     fn dont_check_in_proc_macro() {}
 }
 
+// Do not lint `String` has `Vec<u8>`, which cannot be dropped in const contexts
 fn a(this: String) {}
 
 enum A {
@@ -134,6 +135,25 @@ enum A {
     N,
 }
 
+// Same here.
 fn b(this: A) {}
 
+// Minimized version of `a`.
 fn c(this: Vec<u16>) {}
+
+struct F(A);
+
+// Do not lint
+fn f(this: F) {}
+
+// Do not lint
+fn g<T>(this: T) {}
+
+struct Issue10617(String);
+
+impl Issue10617 {
+    // Do not lint
+    pub fn name(self) -> String {
+        self.0
+    }
+}

tests/ui/missing_const_for_fn/could_be_const.rs

@@ -1,5 +1,7 @@
 #![warn(clippy::missing_const_for_fn)]
 #![allow(incomplete_features, clippy::let_and_return)]
+#![feature(const_mut_refs)]
+#![feature(const_trait_impl)]
 
 use std::mem::transmute;
 
@@ -87,3 +89,14 @@ fn msrv_1_46() -> i32 {
 
 // Should not be const
 fn main() {}
+
+struct D;
+
+impl const Drop for D {
+    fn drop(&mut self) {
+        todo!();
+    }
+}
+
+// Lint this, since it can be dropped in const contexts
+fn d(this: D) {}

tests/ui/missing_const_for_fn/could_be_const.stderr

@@ -1,5 +1,5 @@
 error: this could be a `const fn`
-  --> $DIR/could_be_const.rs:12:5
+  --> $DIR/could_be_const.rs:14:5
    |
 LL | /     pub fn new() -> Self {
 LL | |         Self { guess: 42 }
@@ -9,7 +9,7 @@ LL | |     }
    = note: `-D clippy::missing-const-for-fn` implied by `-D warnings`
 
 error: this could be a `const fn`
-  --> $DIR/could_be_const.rs:16:5
+  --> $DIR/could_be_const.rs:18:5
    |
 LL | /     fn const_generic_params<'a, T, const N: usize>(&self, b: &'a [T; N]) -> &'a [T; N] {
 LL | |         b
@@ -17,7 +17,7 @@ LL | |     }
    | |_____^
 
 error: this could be a `const fn`
-  --> $DIR/could_be_const.rs:22:1
+  --> $DIR/could_be_const.rs:24:1
    |
 LL | / fn one() -> i32 {
 LL | |     1
@@ -25,7 +25,7 @@ LL | | }
    | |_^
 
 error: this could be a `const fn`
-  --> $DIR/could_be_const.rs:27:1
+  --> $DIR/could_be_const.rs:29:1
    |
 LL | / fn two() -> i32 {
 LL | |     let abc = 2;
@@ -34,7 +34,7 @@ LL | | }
    | |_^
 
 error: this could be a `const fn`
-  --> $DIR/could_be_const.rs:33:1
+  --> $DIR/could_be_const.rs:35:1
    |
 LL | / fn string() -> String {
 LL | |     String::new()
@@ -42,7 +42,7 @@ LL | | }
    | |_^
 
 error: this could be a `const fn`
-  --> $DIR/could_be_const.rs:38:1
+  --> $DIR/could_be_const.rs:40:1
    |
 LL | / unsafe fn four() -> i32 {
 LL | |     4
@@ -50,7 +50,7 @@ LL | | }
    | |_^
 
 error: this could be a `const fn`
-  --> $DIR/could_be_const.rs:43:1
+  --> $DIR/could_be_const.rs:45:1
    |
 LL | / fn generic<T>(t: T) -> T {
 LL | |     t
@@ -58,7 +58,7 @@ LL | | }
    | |_^
 
 error: this could be a `const fn`
-  --> $DIR/could_be_const.rs:51:1
+  --> $DIR/could_be_const.rs:53:1
    |
 LL | / fn generic_arr<T: Copy>(t: [T; 1]) -> T {
 LL | |     t[0]
@@ -66,7 +66,7 @@ LL | | }
    | |_^
 
 error: this could be a `const fn`
-  --> $DIR/could_be_const.rs:64:9
+  --> $DIR/could_be_const.rs:66:9
    |
 LL | /         pub fn b(self, a: &A) -> B {
 LL | |             B
@@ -74,7 +74,7 @@ LL | |         }
    | |_________^
 
 error: this could be a `const fn`
-  --> $DIR/could_be_const.rs:73:5
+  --> $DIR/could_be_const.rs:75:5
    |
 LL | /     fn const_fn_stabilized_before_msrv(byte: u8) {
 LL | |         byte.is_ascii_digit();
@@ -82,12 +82,18 @@ LL | |     }
    | |_____^
 
 error: this could be a `const fn`
-  --> $DIR/could_be_const.rs:84:1
+  --> $DIR/could_be_const.rs:86:1
    |
 LL | / fn msrv_1_46() -> i32 {
 LL | |     46
 LL | | }
    | |_^
 
-error: aborting due to 11 previous errors
+error: this could be a `const fn`
+  --> $DIR/could_be_const.rs:102:1
+   |
+LL | fn d(this: D) {}
+   | ^^^^^^^^^^^^^^^^
+
+error: aborting due to 12 previous errors