rust/compiler/rustc_passes/src/check_const.rs

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//! 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_attr as attr;
use rustc_errors::struct_span_err;
use rustc_hir as hir;
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use rustc_hir::def_id::LocalDefId;
use rustc_hir::intravisit::{self, NestedVisitorMap, Visitor};
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use rustc_middle::hir::map::Map;
use rustc_middle::ty::query::Providers;
use rustc_middle::ty::TyCtxt;
use rustc_session::parse::feature_err;
use rustc_span::{sym, Span, Symbol};
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/// An expression that is not *always* legal in a const context.
#[derive(Clone, Copy)]
enum NonConstExpr {
Loop(hir::LoopSource),
Match(hir::MatchSource),
}
impl NonConstExpr {
fn name(self) -> String {
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match self {
Self::Loop(src) => format!("`{}`", src.name()),
Self::Match(src) => format!("`{}`", src.name()),
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}
}
fn required_feature_gates(self) -> Option<&'static [Symbol]> {
use hir::LoopSource::*;
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use hir::MatchSource::*;
let gates: &[_] = match self {
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// A `for` loop's desugaring contains a call to `IntoIterator::into_iter`,
// so they are not yet allowed.
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// Likewise, `?` desugars to a call to `Try::into_result`.
Self::Loop(ForLoop) | Self::Match(ForLoopDesugar | TryDesugar | AwaitDesugar) => {
return None;
}
Self::Match(IfLetGuardDesugar) => bug!("`if let` guard outside a `match` expression"),
// All other expressions are allowed.
Self::Loop(Loop | While | WhileLet)
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| Self::Match(WhileDesugar | WhileLetDesugar | Normal | IfLetDesugar { .. }) => &[],
};
Some(gates)
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}
}
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fn check_mod_const_bodies(tcx: TyCtxt<'_>, module_def_id: LocalDefId) {
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) {
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*providers = Providers { check_mod_const_bodies, ..*providers };
}
#[derive(Copy, Clone)]
struct CheckConstVisitor<'tcx> {
tcx: TyCtxt<'tcx>,
const_kind: Option<hir::ConstContext>,
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def_id: Option<LocalDefId>,
}
impl<'tcx> CheckConstVisitor<'tcx> {
fn new(tcx: TyCtxt<'tcx>) -> Self {
CheckConstVisitor { tcx, const_kind: None, def_id: None }
}
/// Emits an error when an unsupported expression is found in a const context.
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fn const_check_violated(&self, expr: NonConstExpr, span: Span) {
let Self { tcx, def_id, const_kind } = *self;
let features = tcx.features();
let required_gates = expr.required_feature_gates();
let is_feature_allowed = |feature_gate| {
// All features require that the corresponding gate be enabled,
// even if the function has `#[rustc_allow_const_fn_unstable(the_gate)]`.
if !tcx.features().enabled(feature_gate) {
return false;
}
// If `def_id` is `None`, we don't need to consider stability attributes.
let def_id = match def_id {
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Some(x) => x.to_def_id(),
None => return true,
};
// If this crate is not using stability attributes, or this function is not claiming to be a
// stable `const fn`, that is all that is required.
if !tcx.features().staged_api || tcx.has_attr(def_id, sym::rustc_const_unstable) {
return true;
}
// However, we cannot allow stable `const fn`s to use unstable features without an explicit
// opt-in via `rustc_allow_const_fn_unstable`.
attr::rustc_allow_const_fn_unstable(&tcx.sess, &tcx.get_attrs(def_id))
.any(|name| name == feature_gate)
};
match required_gates {
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// Don't emit an error if the user has enabled the requisite feature gates.
Some(gates) if gates.iter().copied().all(is_feature_allowed) => return,
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// `-Zunleash-the-miri-inside-of-you` only works for expressions that don't have a
// corresponding feature gate. This encourages nightly users to use feature gates when
// possible.
None if tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you => {
tcx.sess.span_warn(span, "skipping const checks");
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return;
}
_ => {}
}
let const_kind =
const_kind.expect("`const_check_violated` may only be called inside a const context");
let msg = format!("{} is not allowed in a `{}`", expr.name(), const_kind.keyword_name());
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let required_gates = required_gates.unwrap_or(&[]);
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let missing_gates: Vec<_> =
required_gates.iter().copied().filter(|&g| !features.enabled(g)).collect();
match missing_gates.as_slice() {
&[] => struct_span_err!(tcx.sess, span, E0744, "{}", msg).emit(),
&[missing_primary, ref missing_secondary @ ..] => {
let mut err = feature_err(&tcx.sess.parse_sess, missing_primary, span, &msg);
// If multiple feature gates would be required to enable this expression, include
// them as help messages. Don't emit a separate error for each missing feature gate.
//
// FIXME(ecstaticmorse): Maybe this could be incorporated into `feature_err`? This
// is a pretty narrow case, however.
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if tcx.sess.is_nightly_build() {
for gate in missing_secondary {
let note = format!(
"add `#![feature({})]` to the crate attributes to enable",
gate,
);
err.help(&note);
}
}
err.emit();
}
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}
}
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/// Saves the parent `const_kind` before calling `f` and restores it afterwards.
fn recurse_into(
&mut self,
kind: Option<hir::ConstContext>,
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def_id: Option<LocalDefId>,
f: impl FnOnce(&mut Self),
) {
let parent_def_id = self.def_id;
let parent_kind = self.const_kind;
self.def_id = def_id;
self.const_kind = kind;
f(self);
self.def_id = parent_def_id;
self.const_kind = parent_kind;
}
}
impl<'tcx> Visitor<'tcx> for CheckConstVisitor<'tcx> {
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type Map = Map<'tcx>;
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fn nested_visit_map(&mut self) -> intravisit::NestedVisitorMap<Self::Map> {
NestedVisitorMap::OnlyBodies(self.tcx.hir())
}
fn visit_anon_const(&mut self, anon: &'tcx hir::AnonConst) {
let kind = Some(hir::ConstContext::Const);
self.recurse_into(kind, None, |this| intravisit::walk_anon_const(this, anon));
}
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fn visit_body(&mut self, body: &'tcx hir::Body<'tcx>) {
let owner = self.tcx.hir().body_owner_def_id(body.id());
let kind = self.tcx.hir().body_const_context(owner);
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self.recurse_into(kind, Some(owner), |this| intravisit::walk_body(this, body));
}
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fn visit_expr(&mut self, e: &'tcx hir::Expr<'tcx>) {
match &e.kind {
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// Skip the following checks if we are not currently in a const context.
_ if self.const_kind.is_none() => {}
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hir::ExprKind::Loop(_, _, source, _) => {
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self.const_check_violated(NonConstExpr::Loop(*source), e.span);
}
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hir::ExprKind::Match(_, _, source) => {
let non_const_expr = match source {
// These are handled by `ExprKind::Loop` above.
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hir::MatchSource::WhileDesugar
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| hir::MatchSource::WhileLetDesugar
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| hir::MatchSource::ForLoopDesugar => None,
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_ => Some(NonConstExpr::Match(*source)),
};
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if let Some(expr) = non_const_expr {
self.const_check_violated(expr, e.span);
}
}
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_ => {}
}
intravisit::walk_expr(self, e);
}
}