Auto merge of rust-lang#106934 - DrMeepster:offset_of, r=WaffleLapkin

Add offset_of! macro (RFC 3308)

Implements rust-lang/rfcs#3308 (tracking issue rust-lang#106655) by adding the built in macro `core::mem::offset_of`. Two of the future possibilities are also implemented:

* Nested field accesses (without array indexing)
* DST support (for `Sized` fields)

I wrote this a few months ago, before the RFC merged. Now that it's merged, I decided to rebase and finish it.

cc `@thomcc` (RFC author)

clippy_lints/src/loops/never_loop.rs

@@ -226,7 +226,8 @@ fn never_loop_expr(expr: &Expr<'_>, ignore_ids: &mut Vec<HirId>, main_loop_id: H
                 | InlineAsmOperand::SymStatic { .. } => NeverLoopResult::Otherwise,
             })
             .fold(NeverLoopResult::Otherwise, combine_seq),
-        ExprKind::Yield(_, _)
+        ExprKind::OffsetOf(_, _)
+        | ExprKind::Yield(_, _)
         | ExprKind::Closure { .. }
         | ExprKind::Path(_)
         | ExprKind::ConstBlock(_)

clippy_lints/src/matches/significant_drop_in_scrutinee.rs

@@ -342,6 +342,7 @@ impl<'a, 'tcx> Visitor<'tcx> for SigDropHelper<'a, 'tcx> {
             ExprKind::DropTemps(_) |
             ExprKind::Err(_) |
             ExprKind::InlineAsm(_) |
+            ExprKind::OffsetOf(_, _) |
             ExprKind::Let(_) |
             ExprKind::Lit(_) |
             ExprKind::Loop(_, _, _, _) |

clippy_lints/src/utils/author.rs

@@ -558,6 +558,10 @@ impl<'a, 'tcx> PrintVisitor<'a, 'tcx> {
                 kind!("InlineAsm(_)");
                 out!("// unimplemented: `ExprKind::InlineAsm` is not further destructured at the moment");
             },
+            ExprKind::OffsetOf(container, ref fields) => {
+                bind!(self, container, fields);
+                kind!("OffsetOf({container}, {fields})");
+            }
             ExprKind::Struct(qpath, fields, base) => {
                 bind!(self, qpath, fields);
                 opt_bind!(self, base);

clippy_utils/src/eager_or_lazy.rs

@@ -218,7 +218,8 @@ fn expr_eagerness<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) -> EagernessS
                 | ExprKind::AddrOf(..)
                 | ExprKind::Struct(..)
                 | ExprKind::Repeat(..)
-                | ExprKind::Block(Block { stmts: [], .. }, _) => (),
+                | ExprKind::Block(Block { stmts: [], .. }, _)
+                | ExprKind::OffsetOf(..) => (),
 
                 // Assignment might be to a local defined earlier, so don't eagerly evaluate.
                 // Blocks with multiple statements might be expensive, so don't eagerly evaluate.

clippy_utils/src/hir_utils.rs

@@ -301,6 +301,9 @@ impl HirEqInterExpr<'_, '_, '_> {
             (&ExprKind::Unary(l_op, le), &ExprKind::Unary(r_op, re)) => l_op == r_op && self.eq_expr(le, re),
             (&ExprKind::Array(l), &ExprKind::Array(r)) => self.eq_exprs(l, r),
             (&ExprKind::DropTemps(le), &ExprKind::DropTemps(re)) => self.eq_expr(le, re),
+            (&ExprKind::OffsetOf(l_container, ref l_fields), &ExprKind::OffsetOf(r_container, ref r_fields)) => {
+                self.eq_ty(l_container, r_container) && over(l_fields, r_fields, |l, r| l.name == r.name)
+            },
             _ => false,
         };
         (is_eq && (!self.should_ignore(left) || !self.should_ignore(right)))
@@ -701,6 +704,12 @@ impl<'a, 'tcx> SpanlessHash<'a, 'tcx> {
                     }
                 }
             },
+            ExprKind::OffsetOf(container, fields) => {
+                self.hash_ty(container);
+                for field in fields {
+                    self.hash_name(field.name);
+                }
+            },
             ExprKind::Let(Let { pat, init, ty, .. }) => {
                 self.hash_expr(init);
                 if let Some(ty) = ty {

clippy_utils/src/qualify_min_const_fn.rs

@@ -194,7 +194,7 @@ fn check_rvalue<'tcx>(
                 ))
             }
         },
-        Rvalue::NullaryOp(NullOp::SizeOf | NullOp::AlignOf, _) | Rvalue::ShallowInitBox(_, _) => Ok(()),
+        Rvalue::NullaryOp(NullOp::SizeOf | NullOp::AlignOf | NullOp::OffsetOf(_), _) | Rvalue::ShallowInitBox(_, _) => Ok(()),
         Rvalue::UnaryOp(_, operand) => {
             let ty = operand.ty(body, tcx);
             if ty.is_integral() || ty.is_bool() {

clippy_utils/src/sugg.rs

@@ -139,6 +139,7 @@ impl<'a> Sugg<'a> {
             | hir::ExprKind::Field(..)
             | hir::ExprKind::Index(..)
             | hir::ExprKind::InlineAsm(..)
+            | hir::ExprKind::OffsetOf(..)
             | hir::ExprKind::ConstBlock(..)
             | hir::ExprKind::Lit(..)
             | hir::ExprKind::Loop(..)
@@ -197,6 +198,7 @@ impl<'a> Sugg<'a> {
             | ast::ExprKind::ForLoop(..)
             | ast::ExprKind::Index(..)
             | ast::ExprKind::InlineAsm(..)
+            | ast::ExprKind::OffsetOf(..)
             | ast::ExprKind::ConstBlock(..)
             | ast::ExprKind::Lit(..)
             | ast::ExprKind::IncludedBytes(..)

clippy_utils/src/visitors.rs

@@ -662,6 +662,7 @@ pub fn for_each_unconsumed_temporary<'tcx, B>(
             | ExprKind::Path(_)
             | ExprKind::Continue(_)
             | ExprKind::InlineAsm(_)
+            | ExprKind::OffsetOf(..)
             | ExprKind::Err(_) => (),
         }
         ControlFlow::Continue(())