rust/src/librustc_passes/check_const.rs

//! This pass checks HIR bodies that may be evaluated at compile-time (e.g., `const`, `static`,
//! `const fn`) for structured control flow (e.g. `if`, `while`), which is forbidden in a const
//! context.
//!
//! By the time the MIR const-checker runs, these high-level constructs have been lowered to
//! control-flow primitives (e.g., `Goto`, `SwitchInt`), making it tough to properly attribute
//! errors. We still look for those primitives in the MIR const-checker to ensure nothing slips
//! through, but errors for structured control flow in a `const` should be emitted here.

use rustc::hir::def_id::DefId;
use rustc::hir::intravisit::{Visitor, NestedVisitorMap};
use rustc::hir::map::Map;
use rustc::hir;
use rustc::session::Session;
use rustc::ty::TyCtxt;
use rustc::ty::query::Providers;
use syntax::span_err;
use syntax_pos::Span;

use std::fmt;

#[derive(Copy, Clone)]
enum ConstKind {
    Static,
    StaticMut,
    ConstFn,
    Const,
    AnonConst,
}

impl ConstKind {
    fn for_body(body: &hir::Body, hir_map: &Map<'_>) -> Option<Self> {
        let is_const_fn = |id| hir_map.fn_sig_by_hir_id(id).unwrap().header.is_const();

        let owner = hir_map.body_owner(body.id());
        let const_kind = match hir_map.body_owner_kind(owner) {
            hir::BodyOwnerKind::Const => Self::Const,
            hir::BodyOwnerKind::Static(hir::Mutability::MutMutable) => Self::StaticMut,
            hir::BodyOwnerKind::Static(hir::Mutability::MutImmutable) => Self::Static,

            hir::BodyOwnerKind::Fn if is_const_fn(owner) => Self::ConstFn,
            hir::BodyOwnerKind::Fn | hir::BodyOwnerKind::Closure => return None,
        };

        Some(const_kind)
    }
}

impl fmt::Display for ConstKind {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let s = match self {
            Self::Static => "static",
            Self::StaticMut => "static mut",
            Self::Const | Self::AnonConst => "const",
            Self::ConstFn => "const fn",
        };

        write!(f, "{}", s)
    }
}

fn check_mod_const_bodies(tcx: TyCtxt<'_>, module_def_id: DefId) {
    if tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you {
        return;
    }

    let mut vis = CheckConstVisitor::new(tcx);
    tcx.hir().visit_item_likes_in_module(module_def_id, &mut vis.as_deep_visitor());
}

pub(crate) fn provide(providers: &mut Providers<'_>) {
    *providers = Providers {
        check_mod_const_bodies,
        ..*providers
    };
}

#[derive(Copy, Clone)]
struct CheckConstVisitor<'tcx> {
    sess: &'tcx Session,
    hir_map: &'tcx Map<'tcx>,
    const_kind: Option<ConstKind>,
}

impl<'tcx> CheckConstVisitor<'tcx> {
    fn new(tcx: TyCtxt<'tcx>) -> Self {
        CheckConstVisitor {
            sess: &tcx.sess,
            hir_map: tcx.hir(),
            const_kind: None,
        }
    }

    /// Emits an error when an unsupported expression is found in a const context.
    fn const_check_violated(&self, bad_op: &str, span: Span) {
        let const_kind = self.const_kind
            .expect("`const_check_violated` may only be called inside a const context");

        span_err!(self.sess, span, E0744, "`{}` is not allowed in a `{}`", bad_op, const_kind);
    }

    /// Saves the parent `const_kind` before calling `f` and restores it afterwards.
    fn recurse_into(&mut self, kind: Option<ConstKind>, f: impl FnOnce(&mut Self)) {
        let parent_kind = self.const_kind;
        self.const_kind = kind;
        f(self);
        self.const_kind = parent_kind;
    }
}

impl<'tcx> Visitor<'tcx> for CheckConstVisitor<'tcx> {
    fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
        NestedVisitorMap::OnlyBodies(&self.hir_map)
    }

    fn visit_anon_const(&mut self, anon: &'tcx hir::AnonConst) {
        let kind = Some(ConstKind::AnonConst);
        self.recurse_into(kind, |this| hir::intravisit::walk_anon_const(this, anon));
    }

    fn visit_body(&mut self, body: &'tcx hir::Body) {
        let kind = ConstKind::for_body(body, self.hir_map);
        self.recurse_into(kind, |this| hir::intravisit::walk_body(this, body));
    }

    fn visit_expr(&mut self, e: &'tcx hir::Expr) {
        match &e.kind {
            // Skip the following checks if we are not currently in a const context.
            _ if self.const_kind.is_none() => {}

            hir::ExprKind::Loop(_, _, source) => {
                self.const_check_violated(source.name(), e.span);
            }

            hir::ExprKind::Match(_, _, source) => {
                use hir::MatchSource::*;

                let op = match source {
                    Normal => Some("match"),
                    IfDesugar { .. } | IfLetDesugar { .. } => Some("if"),
                    TryDesugar => Some("?"),
                    AwaitDesugar => Some(".await"),

                    // These are handled by `ExprKind::Loop` above.
                    WhileDesugar | WhileLetDesugar | ForLoopDesugar => None,
                };

                if let Some(op) = op {
                    self.const_check_violated(op, e.span);
                }
            }

            _ => {},
        }

        hir::intravisit::walk_expr(self, e);
    }
}