src/librustc_mir/transform/check_consts/ops.rs

//! Concrete error types for all operations which may be invalid in a certain const context.

use rustc_errors::struct_span_err;
use rustc_hir as hir;
use rustc_hir::def_id::DefId;
use rustc_session::config::nightly_options;
use rustc_session::parse::feature_err;
use rustc_span::symbol::sym;
use rustc_span::{Span, Symbol};

use super::ConstCx;

/// Emits an error if `op` is not allowed in the given const context.
pub fn non_const<O: NonConstOp>(ccx: &ConstCx<'_, '_>, op: O, span: Span) {
    debug!("illegal_op: op={:?}", op);

    if op.is_allowed_in_item(ccx) {
        return;
    }

    if ccx.tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you {
        ccx.tcx.sess.miri_unleashed_feature(span, O::feature_gate());
        return;
    }

    op.emit_error(ccx, span);
}

/// An operation that is not *always* allowed in a const context.
pub trait NonConstOp: std::fmt::Debug {
    /// Returns the `Symbol` corresponding to the feature gate that would enable this operation,
    /// or `None` if such a feature gate does not exist.
    fn feature_gate() -> Option<Symbol> {
        None
    }

    /// Returns `true` if this operation is allowed in the given item.
    ///
    /// This check should assume that we are not in a non-const `fn`, where all operations are
    /// legal.
    ///
    /// By default, it returns `true` if and only if this operation has a corresponding feature
    /// gate and that gate is enabled.
    fn is_allowed_in_item(&self, ccx: &ConstCx<'_, '_>) -> bool {
        Self::feature_gate().map_or(false, |gate| ccx.tcx.features().enabled(gate))
    }

    fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
        let mut err = struct_span_err!(
            ccx.tcx.sess,
            span,
            E0019,
            "{} contains unimplemented expression type",
            ccx.const_kind()
        );
        if let Some(feat) = Self::feature_gate() {
            err.help(&format!("add `#![feature({})]` to the crate attributes to enable", feat));
        }
        if ccx.tcx.sess.teach(&err.get_code().unwrap()) {
            err.note(
                "A function call isn't allowed in the const's initialization expression \
                      because the expression's value must be known at compile-time.",
            );
            err.note(
                "Remember: you can't use a function call inside a const's initialization \
                      expression! However, you can use it anywhere else.",
            );
        }
        err.emit();
    }
}

/// A function call where the callee is a pointer.
#[derive(Debug)]
pub struct FnCallIndirect;
impl NonConstOp for FnCallIndirect {
    fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
        let mut err =
            ccx.tcx.sess.struct_span_err(span, "function pointers are not allowed in const fn");
        err.emit();
    }
}

/// A function call where the callee is not marked as `const`.
#[derive(Debug)]
pub struct FnCallNonConst(pub DefId);
impl NonConstOp for FnCallNonConst {
    fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
        let mut err = struct_span_err!(
            ccx.tcx.sess,
            span,
            E0015,
            "calls in {}s are limited to constant functions, \
             tuple structs and tuple variants",
            ccx.const_kind(),
        );
        err.emit();
    }
}

/// A call to a `#[unstable]` const fn or `#[rustc_const_unstable]` function.
///
/// Contains the name of the feature that would allow the use of this function.
#[derive(Debug)]
pub struct FnCallUnstable(pub DefId, pub Symbol);
impl NonConstOp for FnCallUnstable {
    fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
        let FnCallUnstable(def_id, feature) = *self;

        let mut err = ccx.tcx.sess.struct_span_err(
            span,
            &format!("`{}` is not yet stable as a const fn", ccx.tcx.def_path_str(def_id)),
        );
        if nightly_options::is_nightly_build() {
            err.help(&format!("add `#![feature({})]` to the crate attributes to enable", feature));
        }
        err.emit();
    }
}

#[derive(Debug)]
pub struct HeapAllocation;
impl NonConstOp for HeapAllocation {
    fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
        let mut err = struct_span_err!(
            ccx.tcx.sess,
            span,
            E0010,
            "allocations are not allowed in {}s",
            ccx.const_kind()
        );
        err.span_label(span, format!("allocation not allowed in {}s", ccx.const_kind()));
        if ccx.tcx.sess.teach(&err.get_code().unwrap()) {
            err.note(
                "The value of statics and constants must be known at compile time, \
                 and they live for the entire lifetime of a program. Creating a boxed \
                 value allocates memory on the heap at runtime, and therefore cannot \
                 be done at compile time.",
            );
        }
        err.emit();
    }
}

#[derive(Debug)]
pub struct InlineAsm;
impl NonConstOp for InlineAsm {}

#[derive(Debug)]
pub struct LiveDrop(pub Option<Span>);
impl NonConstOp for LiveDrop {
    fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
        let mut diagnostic = struct_span_err!(
            ccx.tcx.sess,
            span,
            E0493,
            "destructors cannot be evaluated at compile-time"
        );
        diagnostic.span_label(span, format!("{}s cannot evaluate destructors", ccx.const_kind()));
        if let Some(span) = self.0 {
            diagnostic.span_label(span, "value is dropped here");
        }
        diagnostic.emit();
    }
}

#[derive(Debug)]
pub struct CellBorrow;
impl NonConstOp for CellBorrow {
    fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
        struct_span_err!(
            ccx.tcx.sess,
            span,
            E0492,
            "cannot borrow a constant which may contain \
            interior mutability, create a static instead"
        )
        .emit();
    }
}

#[derive(Debug)]
pub struct MutBorrow;
impl NonConstOp for MutBorrow {
    fn is_allowed_in_item(&self, ccx: &ConstCx<'_, '_>) -> bool {
        // Forbid everywhere except in const fn
        ccx.const_kind() == hir::ConstContext::ConstFn
            && ccx.tcx.features().enabled(Self::feature_gate().unwrap())
    }

    fn feature_gate() -> Option<Symbol> {
        Some(sym::const_mut_refs)
    }

    fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
        let mut err = if ccx.const_kind() == hir::ConstContext::ConstFn {
            feature_err(
                &ccx.tcx.sess.parse_sess,
                sym::const_mut_refs,
                span,
                &format!("mutable references are not allowed in {}s", ccx.const_kind()),
            )
        } else {
            struct_span_err!(
                ccx.tcx.sess,
                span,
                E0764,
                "mutable references are not allowed in {}s",
                ccx.const_kind(),
            )
        };
        err.span_label(span, "`&mut` is only allowed in `const fn`".to_string());
        if ccx.tcx.sess.teach(&err.get_code().unwrap()) {
            err.note(
                "References in statics and constants may only refer \
                      to immutable values.\n\n\
                      Statics are shared everywhere, and if they refer to \
                      mutable data one might violate memory safety since \
                      holding multiple mutable references to shared data \
                      is not allowed.\n\n\
                      If you really want global mutable state, try using \
                      static mut or a global UnsafeCell.",
            );
        }
        err.emit();
    }
}

#[derive(Debug)]
pub struct MutAddressOf;
impl NonConstOp for MutAddressOf {
    fn feature_gate() -> Option<Symbol> {
        Some(sym::const_mut_refs)
    }

    fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
        feature_err(
            &ccx.tcx.sess.parse_sess,
            sym::const_mut_refs,
            span,
            &format!("`&raw mut` is not allowed in {}s", ccx.const_kind()),
        )
        .emit();
    }
}

#[derive(Debug)]
pub struct MutDeref;
impl NonConstOp for MutDeref {
    fn feature_gate() -> Option<Symbol> {
        Some(sym::const_mut_refs)
    }
}

#[derive(Debug)]
pub struct Panic;
impl NonConstOp for Panic {
    fn feature_gate() -> Option<Symbol> {
        Some(sym::const_panic)
    }

    fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
        feature_err(
            &ccx.tcx.sess.parse_sess,
            sym::const_panic,
            span,
            &format!("panicking in {}s is unstable", ccx.const_kind()),
        )
        .emit();
    }
}

#[derive(Debug)]
pub struct RawPtrComparison;
impl NonConstOp for RawPtrComparison {
    fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
        let mut err = ccx
            .tcx
            .sess
            .struct_span_err(span, "pointers cannot be reliably compared during const eval.");
        err.note(
            "see issue #53020 <https://github.com/rust-lang/rust/issues/53020> \
            for more information",
        );
        err.emit();
    }
}

#[derive(Debug)]
pub struct RawPtrDeref;
impl NonConstOp for RawPtrDeref {
    fn feature_gate() -> Option<Symbol> {
        Some(sym::const_raw_ptr_deref)
    }

    fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
        feature_err(
            &ccx.tcx.sess.parse_sess,
            sym::const_raw_ptr_deref,
            span,
            &format!("dereferencing raw pointers in {}s is unstable", ccx.const_kind(),),
        )
        .emit();
    }
}

#[derive(Debug)]
pub struct RawPtrToIntCast;
impl NonConstOp for RawPtrToIntCast {
    fn feature_gate() -> Option<Symbol> {
        Some(sym::const_raw_ptr_to_usize_cast)
    }

    fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
        feature_err(
            &ccx.tcx.sess.parse_sess,
            sym::const_raw_ptr_to_usize_cast,
            span,
            &format!("casting pointers to integers in {}s is unstable", ccx.const_kind(),),
        )
        .emit();
    }
}

/// An access to a (non-thread-local) `static`.
#[derive(Debug)]
pub struct StaticAccess;
impl NonConstOp for StaticAccess {
    fn is_allowed_in_item(&self, ccx: &ConstCx<'_, '_>) -> bool {
        matches!(ccx.const_kind(), hir::ConstContext::Static(_))
    }

    fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
        let mut err = struct_span_err!(
            ccx.tcx.sess,
            span,
            E0013,
            "{}s cannot refer to statics",
            ccx.const_kind()
        );
        err.help(
            "consider extracting the value of the `static` to a `const`, and referring to that",
        );
        if ccx.tcx.sess.teach(&err.get_code().unwrap()) {
            err.note(
                "`static` and `const` variables can refer to other `const` variables. \
                    A `const` variable, however, cannot refer to a `static` variable.",
            );
            err.help("To fix this, the value can be extracted to a `const` and then used.");
        }
        err.emit();
    }
}

/// An access to a thread-local `static`.
#[derive(Debug)]
pub struct ThreadLocalAccess;
impl NonConstOp for ThreadLocalAccess {
    fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
        struct_span_err!(
            ccx.tcx.sess,
            span,
            E0625,
            "thread-local statics cannot be \
            accessed at compile-time"
        )
        .emit();
    }
}

#[derive(Debug)]
pub struct UnionAccess;
impl NonConstOp for UnionAccess {
    fn is_allowed_in_item(&self, ccx: &ConstCx<'_, '_>) -> bool {
        // Union accesses are stable in all contexts except `const fn`.
        ccx.const_kind() != hir::ConstContext::ConstFn
            || ccx.tcx.features().enabled(Self::feature_gate().unwrap())
    }

    fn feature_gate() -> Option<Symbol> {
        Some(sym::const_fn_union)
    }

    fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
        feature_err(
            &ccx.tcx.sess.parse_sess,
            sym::const_fn_union,
            span,
            "unions in const fn are unstable",
        )
        .emit();
    }
}