Auto merge of #102091 - RalfJung:const_err, r=oli-obk

make const_err a hard error

This lint has been deny-by-default with future incompat wording since [Rust 1.51](#80394) and the stable release of this week starts showing it in cargo's future compat reports. I can't wait to finally get rid of at least some of the mess in our const-err-reporting-code. ;)

r? `@oli-obk`
Fixes #71800
Fixes #100114

compiler/rustc_const_eval/src/const_eval/error.rs

@@ -2,7 +2,6 @@ use std::error::Error;
 use std::fmt;
 
 use rustc_errors::Diagnostic;
-use rustc_hir as hir;
 use rustc_middle::mir::AssertKind;
 use rustc_middle::ty::{layout::LayoutError, query::TyCtxtAt, ConstInt};
 use rustc_span::{Span, Symbol};
@@ -23,11 +22,7 @@ pub enum ConstEvalErrKind {
     Abort(String),
 }
 
-impl MachineStopType for ConstEvalErrKind {
-    fn is_hard_err(&self) -> bool {
-        matches!(self, Self::Panic { .. })
-    }
-}
+impl MachineStopType for ConstEvalErrKind {}
 
 // The errors become `MachineStop` with plain strings when being raised.
 // `ConstEvalErr` (in `librustc_middle/mir/interpret/error.rs`) knows to
@@ -87,48 +82,10 @@ impl<'tcx> ConstEvalErr<'tcx> {
         ConstEvalErr { error: error.into_kind(), stacktrace, span }
     }
 
-    pub fn struct_error(
-        &self,
-        tcx: TyCtxtAt<'tcx>,
-        message: &str,
-        decorate: impl FnOnce(&mut Diagnostic),
-    ) -> ErrorHandled {
-        self.struct_generic(tcx, message, decorate, None)
-    }
-
     pub fn report_as_error(&self, tcx: TyCtxtAt<'tcx>, message: &str) -> ErrorHandled {
         self.struct_error(tcx, message, |_| {})
     }
 
-    pub fn report_as_lint(
-        &self,
-        tcx: TyCtxtAt<'tcx>,
-        message: &str,
-        lint_root: hir::HirId,
-        span: Option<Span>,
-    ) -> ErrorHandled {
-        self.struct_generic(
-            tcx,
-            message,
-            |lint: &mut Diagnostic| {
-                // Apply the span.
-                if let Some(span) = span {
-                    let primary_spans = lint.span.primary_spans().to_vec();
-                    // point at the actual error as the primary span
-                    lint.replace_span_with(span);
-                    // point to the `const` statement as a secondary span
-                    // they don't have any label
-                    for sp in primary_spans {
-                        if sp != span {
-                            lint.span_label(sp, "");
-                        }
-                    }
-                }
-            },
-            Some(lint_root),
-        )
-    }
-
     /// Create a diagnostic for this const eval error.
     ///
     /// Sets the message passed in via `message` and adds span labels with detailed error
@@ -137,13 +94,12 @@ impl<'tcx> ConstEvalErr<'tcx> {
     ///
     /// If `lint_root.is_some()` report it as a lint, else report it as a hard error.
     /// (Except that for some errors, we ignore all that -- see `must_error` below.)
-    #[instrument(skip(self, tcx, decorate, lint_root), level = "debug")]
-    fn struct_generic(
+    #[instrument(skip(self, tcx, decorate), level = "debug")]
+    pub fn struct_error(
         &self,
         tcx: TyCtxtAt<'tcx>,
         message: &str,
         decorate: impl FnOnce(&mut Diagnostic),
-        lint_root: Option<hir::HirId>,
     ) -> ErrorHandled {
         let finish = |err: &mut Diagnostic, span_msg: Option<String>| {
             trace!("reporting const eval failure at {:?}", self.span);
@@ -224,27 +180,9 @@ impl<'tcx> ConstEvalErr<'tcx> {
 
         let err_msg = self.error.to_string();
 
-        // Regular case - emit a lint.
-        if let Some(lint_root) = lint_root {
-            // Report as lint.
-            let hir_id =
-                self.stacktrace.iter().rev().find_map(|frame| frame.lint_root).unwrap_or(lint_root);
-            tcx.struct_span_lint_hir(
-                rustc_session::lint::builtin::CONST_ERR,
-                hir_id,
-                tcx.span,
-                message,
-                |lint| {
-                    finish(lint, Some(err_msg));
-                    lint
-                },
-            );
-            ErrorHandled::Linted
-        } else {
-            // Report as hard error.
-            let mut err = struct_error(tcx, message);
-            finish(&mut err, Some(err_msg));
-            ErrorHandled::Reported(err.emit())
-        }
+        // Report as hard error.
+        let mut err = struct_error(tcx, message);
+        finish(&mut err, Some(err_msg));
+        ErrorHandled::Reported(err.emit())
     }
 }

compiler/rustc_const_eval/src/const_eval/eval_queries.rs

@@ -317,45 +317,23 @@ pub fn eval_to_allocation_raw_provider<'tcx>(
     match res.and_then(|body| eval_body_using_ecx(&mut ecx, cid, &body)) {
         Err(error) => {
             let err = ConstEvalErr::new(&ecx, error, None);
-            // Some CTFE errors raise just a lint, not a hard error; see
-            // <https://github.com/rust-lang/rust/issues/71800>.
-            let is_hard_err = if let Some(def) = def.as_local() {
-                // (Associated) consts only emit a lint, since they might be unused.
-                !matches!(tcx.def_kind(def.did.to_def_id()), DefKind::Const | DefKind::AssocConst)
-                    // check if the inner InterpError is hard
-                    || err.error.is_hard_err()
+            let msg = if is_static {
+                Cow::from("could not evaluate static initializer")
             } else {
-                // use of broken constant from other crate: always an error
-                true
-            };
-
-            if is_hard_err {
-                let msg = if is_static {
-                    Cow::from("could not evaluate static initializer")
+                // If the current item has generics, we'd like to enrich the message with the
+                // instance and its substs: to show the actual compile-time values, in addition to
+                // the expression, leading to the const eval error.
+                let instance = &key.value.instance;
+                if !instance.substs.is_empty() {
+                    let instance = with_no_trimmed_paths!(instance.to_string());
+                    let msg = format!("evaluation of `{}` failed", instance);
+                    Cow::from(msg)
                 } else {
-                    // If the current item has generics, we'd like to enrich the message with the
-                    // instance and its substs: to show the actual compile-time values, in addition to
-                    // the expression, leading to the const eval error.
-                    let instance = &key.value.instance;
-                    if !instance.substs.is_empty() {
-                        let instance = with_no_trimmed_paths!(instance.to_string());
-                        let msg = format!("evaluation of `{}` failed", instance);
-                        Cow::from(msg)
-                    } else {
-                        Cow::from("evaluation of constant value failed")
-                    }
-                };
+                    Cow::from("evaluation of constant value failed")
+                }
+            };
 
-                Err(err.report_as_error(ecx.tcx.at(err.span), &msg))
-            } else {
-                let hir_id = tcx.hir().local_def_id_to_hir_id(def.as_local().unwrap().did);
-                Err(err.report_as_lint(
-                    tcx.at(tcx.def_span(def.did)),
-                    "any use of this value will cause an error",
-                    hir_id,
-                    Some(err.span),
-                ))
-            }
+            Err(err.report_as_error(ecx.tcx.at(err.span), &msg))
         }
         Ok(mplace) => {
             // Since evaluation had no errors, validate the resulting constant.

compiler/rustc_const_eval/src/interpret/eval_context.rs

@@ -258,6 +258,9 @@ impl<'tcx> fmt::Display for FrameInfo<'tcx> {
             {
                 write!(f, "inside closure")?;
             } else {
+                // Note: this triggers a `good_path_bug` state, which means that if we ever get here
+                // we must emit a diagnostic. We should never display a `FrameInfo` unless we
+                // actually want to emit a warning or error to the user.
                 write!(f, "inside `{}`", self.instance)?;
             }
             if !self.span.is_dummy() {

compiler/rustc_const_eval/src/interpret/intern.rs

@@ -332,8 +332,6 @@ pub enum InternKind {
 ///
 /// This *cannot raise an interpreter error*.  Doing so is left to validation, which
 /// tracks where in the value we are and thus can show much better error messages.
-/// Any errors here would anyway be turned into `const_err` lints, whereas validation failures
-/// are hard errors.
 #[instrument(level = "debug", skip(ecx))]
 pub fn intern_const_alloc_recursive<
     'mir,

compiler/rustc_lint/src/lib.rs

@@ -522,6 +522,11 @@ fn register_builtins(store: &mut LintStore, no_interleave_lints: bool) {
         "now allowed, see issue #59159 \
          <https://github.com/rust-lang/rust/issues/59159> for more information",
     );
+    store.register_removed(
+        "const_err",
+        "converted into hard error, see issue #71800 \
+         <https://github.com/rust-lang/rust/issues/71800> for more information",
+    );
 }
 
 fn register_internals(store: &mut LintStore) {

compiler/rustc_lint_defs/src/builtin.rs

@@ -263,37 +263,6 @@ declare_lint! {
     "operation will cause a panic at runtime"
 }
 
-declare_lint! {
-    /// The `const_err` lint detects an erroneous expression while doing
-    /// constant evaluation.
-    ///
-    /// ### Example
-    ///
-    /// ```rust,compile_fail
-    /// #![allow(unconditional_panic)]
-    /// const C: i32 = 1/0;
-    /// ```
-    ///
-    /// {{produces}}
-    ///
-    /// ### Explanation
-    ///
-    /// This lint detects constants that fail to evaluate. Allowing the lint will accept the
-    /// constant declaration, but any use of this constant will still lead to a hard error. This is
-    /// a future incompatibility lint; the plan is to eventually entirely forbid even declaring
-    /// constants that cannot be evaluated.  See [issue #71800] for more details.
-    ///
-    /// [issue #71800]: https://github.com/rust-lang/rust/issues/71800
-    pub CONST_ERR,
-    Deny,
-    "constant evaluation encountered erroneous expression",
-    @future_incompatible = FutureIncompatibleInfo {
-        reference: "issue #71800 <https://github.com/rust-lang/rust/issues/71800>",
-        reason: FutureIncompatibilityReason::FutureReleaseErrorReportNow,
-    };
-    report_in_external_macro
-}
-
 declare_lint! {
     /// The `unused_imports` lint detects imports that are never used.
     ///
@@ -3295,7 +3264,6 @@ declare_lint_pass! {
         EXPORTED_PRIVATE_DEPENDENCIES,
         PUB_USE_OF_PRIVATE_EXTERN_CRATE,
         INVALID_TYPE_PARAM_DEFAULT,
-        CONST_ERR,
         RENAMED_AND_REMOVED_LINTS,
         UNALIGNED_REFERENCES,
         CONST_ITEM_MUTATION,

compiler/rustc_middle/src/mir/interpret/error.rs

@@ -479,12 +479,7 @@ impl<T: Any> AsAny for T {
 }
 
 /// A trait for machine-specific errors (or other "machine stop" conditions).
-pub trait MachineStopType: AsAny + fmt::Display + Send {
-    /// If `true`, emit a hard error instead of going through the `CONST_ERR` lint
-    fn is_hard_err(&self) -> bool {
-        false
-    }
-}
+pub trait MachineStopType: AsAny + fmt::Display + Send {}
 
 impl dyn MachineStopType {
     #[inline(always)]
@@ -543,16 +538,4 @@ impl InterpError<'_> {
                 | InterpError::UndefinedBehavior(UndefinedBehaviorInfo::Ub(_))
         )
     }
-
-    /// Should this error be reported as a hard error, preventing compilation, or a soft error,
-    /// causing a deny-by-default lint?
-    pub fn is_hard_err(&self) -> bool {
-        use InterpError::*;
-        match *self {
-            MachineStop(ref err) => err.is_hard_err(),
-            UndefinedBehavior(_) => true,
-            ResourceExhaustion(ResourceExhaustionInfo::MemoryExhausted) => true,
-            _ => false,
-        }
-    }
 }

compiler/rustc_mir_transform/src/const_prop_lint.rs

@@ -16,9 +16,9 @@ use rustc_index::bit_set::BitSet;
 use rustc_index::vec::IndexVec;
 use rustc_middle::mir::visit::Visitor;
 use rustc_middle::mir::{
-    self, AssertKind, BinOp, Body, Constant, ConstantKind, Local, LocalDecl, Location, Operand,
-    Place, Rvalue, SourceInfo, SourceScope, SourceScopeData, Statement, StatementKind, Terminator,
-    TerminatorKind, UnOp, RETURN_PLACE,
+    AssertKind, BinOp, Body, Constant, Local, LocalDecl, Location, Operand, Place, Rvalue,
+    SourceInfo, SourceScope, SourceScopeData, Statement, StatementKind, Terminator, TerminatorKind,
+    UnOp, RETURN_PLACE,
 };
 use rustc_middle::ty::layout::{LayoutError, LayoutOf, LayoutOfHelpers, TyAndLayout};
 use rustc_middle::ty::InternalSubsts;
@@ -286,7 +286,11 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
     }
 
     /// Returns the value, if any, of evaluating `c`.
-    fn eval_constant(&mut self, c: &Constant<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
+    fn eval_constant(
+        &mut self,
+        c: &Constant<'tcx>,
+        _source_info: SourceInfo,
+    ) -> Option<OpTy<'tcx>> {
         // FIXME we need to revisit this for #67176
         if c.needs_subst() {
             return None;
@@ -297,28 +301,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
             Err(error) => {
                 let tcx = self.ecx.tcx.at(c.span);
                 let err = ConstEvalErr::new(&self.ecx, error, Some(c.span));
-                if let Some(lint_root) = self.lint_root(source_info) {
-                    let lint_only = match c.literal {
-                        ConstantKind::Ty(ct) => ct.needs_subst(),
-                        ConstantKind::Unevaluated(
-                            mir::UnevaluatedConst { def: _, substs: _, promoted: Some(_) },
-                            _,
-                        ) => {
-                            // Promoteds must lint and not error as the user didn't ask for them
-                            true
-                        }
-                        ConstantKind::Unevaluated(..) | ConstantKind::Val(..) => c.needs_subst(),
-                    };
-                    if lint_only {
-                        // Out of backwards compatibility we cannot report hard errors in unused
-                        // generic functions using associated constants of the generic parameters.
-                        err.report_as_lint(tcx, "erroneous constant used", lint_root, Some(c.span));
-                    } else {
-                        err.report_as_error(tcx, "erroneous constant used");
-                    }
-                } else {
-                    err.report_as_error(tcx, "erroneous constant used");
-                }
+                err.report_as_error(tcx, "erroneous constant used");
                 None
             }
         }

library/core/tests/num/wrapping.rs

@@ -75,8 +75,6 @@ wrapping_test!(test_wrapping_u64, u64, u64::MIN, u64::MAX);
 wrapping_test!(test_wrapping_u128, u128, u128::MIN, u128::MAX);
 wrapping_test!(test_wrapping_usize, usize, usize::MIN, usize::MAX);
 
-// Don't warn about overflowing ops on 32-bit platforms
-#[cfg_attr(target_pointer_width = "32", allow(const_err))]
 #[test]
 fn wrapping_int_api() {
     assert_eq!(i8::MAX.wrapping_add(1), i8::MIN);

library/core/tests/slice.rs

@@ -1284,7 +1284,6 @@ fn test_windows_zip() {
 }
 
 #[test]
-#[allow(const_err)]
 fn test_iter_ref_consistency() {
     use std::fmt::Debug;
 

src/test/mir-opt/remove-never-const.rs

@@ -6,7 +6,6 @@
 // compile-flags: --emit mir,link
 
 #![feature(never_type)]
-#![warn(const_err)]
 
 struct PrintName<T>(T);
 

src/test/ui/array-slice-vec/array_const_index-0.rs

@@ -1,8 +1,7 @@
 const A: &'static [i32] = &[];
 const B: i32 = (&A)[1];
 //~^ index out of bounds: the length is 0 but the index is 1
-//~| ERROR any use of this value will cause an error
-//~| WARN this was previously accepted by the compiler but is being phased out
+//~| ERROR evaluation of constant value failed
 
 fn main() {
     let _ = B;