Auto merge of #115524 - RalfJung:misalign, r=wesleywiser

const-eval: make misalignment a hard error

It's been a future-incompat error (showing up in cargo's reports) since https://github.com/rust-lang/rust/pull/104616, Rust 1.68, released in March.  That should be long enough.

The question for the lang team is simply -- should we move ahead with this, making const-eval alignment failures a hard error? (It turns out some of them accidentally already were hard errors since #104616. But not all so this is still a breaking change. Crater found no regression.)

compiler/rustc_const_eval/src/const_eval/eval_queries.rs

@@ -1,6 +1,3 @@
-use crate::const_eval::CheckAlignment;
-use crate::errors::ConstEvalError;
-
 use either::{Left, Right};
 
 use rustc_hir::def::DefKind;
@@ -15,7 +12,9 @@ use rustc_span::source_map::Span;
 use rustc_target::abi::{self, Abi};
 
 use super::{CanAccessStatics, CompileTimeEvalContext, CompileTimeInterpreter};
+use crate::const_eval::CheckAlignment;
 use crate::errors;
+use crate::errors::ConstEvalError;
 use crate::interpret::eval_nullary_intrinsic;
 use crate::interpret::{
     intern_const_alloc_recursive, CtfeValidationMode, GlobalId, Immediate, InternKind, InterpCx,
@@ -290,14 +289,7 @@ pub fn eval_to_allocation_raw_provider<'tcx>(
         key.param_env,
         // Statics (and promoteds inside statics) may access other statics, because unlike consts
         // they do not have to behave "as if" they were evaluated at runtime.
-        CompileTimeInterpreter::new(
-            CanAccessStatics::from(is_static),
-            if tcx.sess.opts.unstable_opts.extra_const_ub_checks {
-                CheckAlignment::Error
-            } else {
-                CheckAlignment::FutureIncompat
-            },
-        ),
+        CompileTimeInterpreter::new(CanAccessStatics::from(is_static), CheckAlignment::Error),
     );
 
     let res = ecx.load_mir(cid.instance.def, cid.promoted);

compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -1,10 +1,9 @@
 use rustc_hir::def::DefKind;
-use rustc_hir::{LangItem, CRATE_HIR_ID};
+use rustc_hir::LangItem;
 use rustc_middle::mir;
 use rustc_middle::mir::interpret::PointerArithmetic;
 use rustc_middle::ty::layout::{FnAbiOf, TyAndLayout};
 use rustc_middle::ty::{self, TyCtxt};
-use rustc_session::lint::builtin::INVALID_ALIGNMENT;
 use std::borrow::Borrow;
 use std::hash::Hash;
 use std::ops::ControlFlow;
@@ -21,11 +20,11 @@ use rustc_target::abi::{Align, Size};
 use rustc_target::spec::abi::Abi as CallAbi;
 
 use crate::errors::{LongRunning, LongRunningWarn};
+use crate::fluent_generated as fluent;
 use crate::interpret::{
     self, compile_time_machine, AllocId, ConstAllocation, FnArg, FnVal, Frame, ImmTy, InterpCx,
     InterpResult, OpTy, PlaceTy, Pointer, Scalar,
 };
-use crate::{errors, fluent_generated as fluent};
 
 use super::error::*;
 
@@ -65,22 +64,11 @@ pub struct CompileTimeInterpreter<'mir, 'tcx> {
 
 #[derive(Copy, Clone)]
 pub enum CheckAlignment {
-    /// Ignore alignment when following relocations.
+    /// Ignore all alignment requirements.
     /// This is mainly used in interning.
     No,
     /// Hard error when dereferencing a misaligned pointer.
     Error,
-    /// Emit a future incompat lint when dereferencing a misaligned pointer.
-    FutureIncompat,
-}
-
-impl CheckAlignment {
-    pub fn should_check(&self) -> bool {
-        match self {
-            CheckAlignment::No => false,
-            CheckAlignment::Error | CheckAlignment::FutureIncompat => true,
-        }
-    }
 }
 
 #[derive(Copy, Clone, PartialEq)]
@@ -358,8 +346,8 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
     const PANIC_ON_ALLOC_FAIL: bool = false; // will be raised as a proper error
 
     #[inline(always)]
-    fn enforce_alignment(ecx: &InterpCx<'mir, 'tcx, Self>) -> CheckAlignment {
-        ecx.machine.check_alignment
+    fn enforce_alignment(ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
+        matches!(ecx.machine.check_alignment, CheckAlignment::Error)
     }
 
     #[inline(always)]
@@ -367,39 +355,6 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
         ecx.tcx.sess.opts.unstable_opts.extra_const_ub_checks || layout.abi.is_uninhabited()
     }
 
-    fn alignment_check_failed(
-        ecx: &InterpCx<'mir, 'tcx, Self>,
-        has: Align,
-        required: Align,
-        check: CheckAlignment,
-    ) -> InterpResult<'tcx, ()> {
-        let err = err_ub!(AlignmentCheckFailed { has, required }).into();
-        match check {
-            CheckAlignment::Error => Err(err),
-            CheckAlignment::No => span_bug!(
-                ecx.cur_span(),
-                "`alignment_check_failed` called when no alignment check requested"
-            ),
-            CheckAlignment::FutureIncompat => {
-                let (_, backtrace) = err.into_parts();
-                backtrace.print_backtrace();
-                let (span, frames) = super::get_span_and_frames(&ecx);
-
-                ecx.tcx.emit_spanned_lint(
-                    INVALID_ALIGNMENT,
-                    ecx.stack().iter().find_map(|frame| frame.lint_root()).unwrap_or(CRATE_HIR_ID),
-                    span,
-                    errors::AlignmentCheckFailed {
-                        has: has.bytes(),
-                        required: required.bytes(),
-                        frames,
-                    },
-                );
-                Ok(())
-            }
-        }
-    }
-
     fn load_mir(
         ecx: &InterpCx<'mir, 'tcx, Self>,
         instance: ty::InstanceDef<'tcx>,