rust/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::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::{ConstKind, Item};

/// An operation that is not *always* allowed in a const context.
pub trait NonConstOp: std::fmt::Debug {
    /// Whether this operation can be evaluated by miri.
    const IS_SUPPORTED_IN_MIRI: bool = true;

    /// 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, item: &Item<'_, '_>) -> bool {
        Self::feature_gate().map_or(false, |gate| item.tcx.features().enabled(gate))
    }

    fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
        let mut err = struct_span_err!(
            item.tcx.sess,
            span,
            E0019,
            "{} contains unimplemented expression type",
            item.const_kind()
        );
        if item.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 `Downcast` projection.
#[derive(Debug)]
pub struct Downcast;
impl NonConstOp for Downcast {
    fn feature_gate() -> Option<Symbol> {
        Some(sym::const_if_match)
    }
}

/// A function call where the callee is a pointer.
#[derive(Debug)]
pub struct FnCallIndirect;
impl NonConstOp for FnCallIndirect {
    fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
        let mut err =
            item.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, item: &Item<'_, '_>, span: Span) {
        let mut err = struct_span_err!(
            item.tcx.sess,
            span,
            E0015,
            "calls in {}s are limited to constant functions, \
             tuple structs and tuple variants",
            item.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, item: &Item<'_, '_>, span: Span) {
        let FnCallUnstable(def_id, feature) = *self;

        let mut err = item.tcx.sess.struct_span_err(
            span,
            &format!("`{}` is not yet stable as a const fn", item.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, item: &Item<'_, '_>, span: Span) {
        let mut err = struct_span_err!(
            item.tcx.sess,
            span,
            E0010,
            "allocations are not allowed in {}s",
            item.const_kind()
        );
        err.span_label(span, format!("allocation not allowed in {}s", item.const_kind()));
        if item.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 IfOrMatch;
impl NonConstOp for IfOrMatch {
    fn feature_gate() -> Option<Symbol> {
        Some(sym::const_if_match)
    }

    fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
        // This should be caught by the HIR const-checker.
        item.tcx.sess.delay_span_bug(span, "complex control flow is forbidden in a const context");
    }
}

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

#[derive(Debug)]
pub struct LiveDrop;
impl NonConstOp for LiveDrop {
    fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
        struct_span_err!(
            item.tcx.sess,
            span,
            E0493,
            "destructors cannot be evaluated at compile-time"
        )
        .span_label(span, format!("{}s cannot evaluate destructors", item.const_kind()))
        .emit();
    }
}

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

    fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
        // This should be caught by the HIR const-checker.
        item.tcx.sess.delay_span_bug(span, "complex control flow is forbidden in a const context");
    }
}

#[derive(Debug)]
pub struct CellBorrow;
impl NonConstOp for CellBorrow {
    fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
        struct_span_err!(
            item.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 feature_gate() -> Option<Symbol> {
        Some(sym::const_mut_refs)
    }

    fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
        let mut err = feature_err(
            &item.tcx.sess.parse_sess,
            sym::const_mut_refs,
            span,
            &format!(
                "references in {}s may only refer \
                      to immutable values",
                item.const_kind()
            ),
        );
        err.span_label(span, format!("{}s require immutable values", item.const_kind()));
        if item.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, item: &Item<'_, '_>, span: Span) {
        feature_err(
            &item.tcx.sess.parse_sess,
            sym::const_mut_refs,
            span,
            &format!("`&raw mut` is not allowed in {}s", item.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, item: &Item<'_, '_>, span: Span) {
        feature_err(
            &item.tcx.sess.parse_sess,
            sym::const_panic,
            span,
            &format!("panicking in {}s is unstable", item.const_kind()),
        )
        .emit();
    }
}

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

    fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
        feature_err(
            &item.tcx.sess.parse_sess,
            sym::const_compare_raw_pointers,
            span,
            &format!("comparing raw pointers inside {}", item.const_kind()),
        )
        .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, item: &Item<'_, '_>, span: Span) {
        feature_err(
            &item.tcx.sess.parse_sess,
            sym::const_raw_ptr_deref,
            span,
            &format!("dereferencing raw pointers in {}s is unstable", item.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, item: &Item<'_, '_>, span: Span) {
        feature_err(
            &item.tcx.sess.parse_sess,
            sym::const_raw_ptr_to_usize_cast,
            span,
            &format!("casting pointers to integers in {}s is unstable", item.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, item: &Item<'_, '_>) -> bool {
        item.const_kind().is_static()
    }

    fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
        let mut err = struct_span_err!(
            item.tcx.sess,
            span,
            E0013,
            "{}s cannot refer to statics",
            item.const_kind()
        );
        err.help(
            "consider extracting the value of the `static` to a `const`, and referring to that",
        );
        if item.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 {
    const IS_SUPPORTED_IN_MIRI: bool = false;

    fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
        struct_span_err!(
            item.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, item: &Item<'_, '_>) -> bool {
        // Union accesses are stable in all contexts except `const fn`.
        item.const_kind() != ConstKind::ConstFn
            || item.tcx.features().enabled(Self::feature_gate().unwrap())
    }

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

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