HIR const-checking

Author: ecstatic-morse

src/librustc_passes/check_const.rs

@@ -23,15 +23,13 @@ use rustc_span::{sym, Span, Symbol};
 enum NonConstExpr {
  Loop(hir::LoopSource),
  Match(hir::MatchSource),
- OrPattern,
 }
 
 impl NonConstExpr {
  fn name(self) -> String {
  match self {
  Self::Loop(src) => format!("`{}`", src.name()),
  Self::Match(src) => format!("`{}`", src.name()),
- Self::OrPattern => "or-pattern".to_string(),
  }
  }
 
@@ -40,22 +38,19 @@ impl NonConstExpr {
  use hir::MatchSource::*;
 
  let gates: &[_] = match self {
- Self::Match(Normal)
- | Self::Match(IfDesugar { .. })
- | Self::Match(IfLetDesugar { .. })
- | Self::OrPattern => &[sym::const_if_match],
-
- Self::Loop(Loop) => &[sym::const_loop],
+ // A `for` loop's desugaring contains a call to `IntoIterator::into_iter`,
+ // so they are not yet allowed with `#![feature(const_loop)]`.
+ // Likewise, `?` desugars to a call to `Try::into_result`.
+ Self::Loop(ForLoop) | Self::Match(ForLoopDesugar | TryDesugar | AwaitDesugar) => {
+ return None;
+ }
 
- Self::Loop(While)
- | Self::Loop(WhileLet)
- | Self::Match(WhileDesugar | WhileLetDesugar) => {
- &[sym::const_loop, sym::const_if_match]
+ Self::Loop(Loop | While | WhileLet) | Self::Match(WhileDesugar | WhileLetDesugar) => {
+ &[sym::const_loop]
  }
 
- // A `for` loop's desugaring contains a call to `IntoIterator::into_iter`,
- // so they are not yet allowed with `#![feature(const_loop)]`.
- _ => return None,
+ // All other matches are allowed.
+ Self::Match(Normal | IfDesugar { .. } | IfLetDesugar { .. }) => &[],
  };
 
  Some(gates)
@@ -114,17 +109,6 @@ impl<'tcx> CheckConstVisitor<'tcx> {
  match missing_gates.as_slice() {
  &[] => struct_span_err!(self.tcx.sess, span, E0744, "{}", msg).emit(),
 
- // If the user enabled `#![feature(const_loop)]` but not `#![feature(const_if_match)]`,
- // explain why their `while` loop is being rejected.
- &[gate @ sym::const_if_match] if required_gates.contains(&sym::const_loop) => {
- feature_err(&self.tcx.sess.parse_sess, gate, span, &msg)
- .note(
- "`#![feature(const_loop)]` alone is not sufficient, \
- since this loop expression contains an implicit conditional",
- )
- .emit();
- }
-
  &[missing_primary, ref missing_secondary @ ..] => {
  let mut err = feature_err(&self.tcx.sess.parse_sess, missing_primary, span, &msg);
 
@@ -175,15 +159,6 @@ impl<'tcx> Visitor<'tcx> for CheckConstVisitor<'tcx> {
  self.recurse_into(kind, |this| intravisit::walk_body(this, body));
  }
 
- fn visit_pat(&mut self, p: &'tcx hir::Pat<'tcx>) {
- if self.const_kind.is_some() {
- if let hir::PatKind::Or { .. } = p.kind {
- self.const_check_violated(NonConstExpr::OrPattern, p.span);
- }
- }
- intravisit::walk_pat(self, p)
- }
-
  fn visit_expr(&mut self, e: &'tcx hir::Expr<'tcx>) {
  match &e.kind {
  // Skip the following checks if we are not currently in a const context.