From 696084c764e5fe3a318eb75a180763fdeab194da Mon Sep 17 00:00:00 2001
From: Dylan MacKenzie <ecstaticmorse@gmail.com>
Date: Thu, 21 May 2020 12:47:09 -0700
Subject: [PATCH] HIR const-checking

---
 src/librustc_passes/check_const.rs | 47 +++++++-----------------------
 1 file changed, 11 insertions(+), 36 deletions(-)

diff --git a/src/librustc_passes/check_const.rs b/src/librustc_passes/check_const.rs
index 90a076eeded..22a65b6730a 100644
--- a/src/librustc_passes/check_const.rs
+++ b/src/librustc_passes/check_const.rs
@@ -23,7 +23,6 @@
 enum NonConstExpr {
     Loop(hir::LoopSource),
     Match(hir::MatchSource),
-    OrPattern,
 }
 
 impl NonConstExpr {
@@ -31,7 +30,6 @@ 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 @@ fn required_feature_gates(self) -> Option<&'static [Symbol]> {
         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],
-
-            Self::Loop(While)
-            | Self::Loop(WhileLet)
-            | Self::Match(WhileDesugar | WhileLetDesugar) => {
-                &[sym::const_loop, sym::const_if_match]
-            }
-
             // A `for` loop's desugaring contains a call to `IntoIterator::into_iter`,
             // so they are not yet allowed with `#![feature(const_loop)]`.
-            _ => return None,
+            // Likewise, `?` desugars to a call to `Try::into_result`.
+            Self::Loop(ForLoop) | Self::Match(ForLoopDesugar | TryDesugar | AwaitDesugar) => {
+                return None;
+            }
+
+            Self::Loop(Loop | While | WhileLet) | Self::Match(WhileDesugar | WhileLetDesugar) => {
+                &[sym::const_loop]
+            }
+
+            // All other matches are allowed.
+            Self::Match(Normal | IfDesugar { .. } | IfLetDesugar { .. }) => &[],
         };
 
         Some(gates)
@@ -114,17 +109,6 @@ fn const_check_violated(&self, expr: NonConstExpr, span: Span) {
         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 @@ fn visit_body(&mut self, body: &'tcx hir::Body<'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.