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Auto merge of #66507 - ecstatic-morse:const-if-match, r=oli-obk
Enable `if` and `match` in constants behind a feature flag This PR is an initial implementation of #49146. It introduces a `const_if_match` feature flag and does the following if it is enabled: - Allows `Downcast` projections, `SwitchInt` terminators and `FakeRead`s for matched places through the MIR const-checker. - Allows `if` and `match` expressions through the HIR const-checker. - Stops converting `&&` to `&` and `||` to `|` in `const` and `static` items. As a result, the following operations are now allowed in a const context behind the feature flag: - `if` and `match` - short circuiting logic operators (`&&` and `||`) - the `assert` and `debug_assert` macros (if the `const_panic` feature flag is also enabled) However, the following operations remain forbidden: - `while`, `loop` and `for` (see #52000) - the `?` operator (calls `From::from` on its error variant) - the `assert_eq` and `assert_ne` macros, along with their `debug` variants (calls `fmt::Debug`) This PR is possible now that we use dataflow for const qualification (see #64470 and #66385). r? @oli-obk cc @rust-lang/wg-const-eval @eddyb
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‎src/doc/unstable-book/src/language-features/const-if-match.md

+14
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,14 @@
1+
# `const_if_match`
2+
3+
The tracking issue for this feature is: [#49146]
4+
5+
[#49146]: https://github.com/rust-lang/rust/issues/49146
6+
7+
------------------------
8+
9+
Allows for the use of conditionals (`if` and `match`) in a const context.
10+
Const contexts include `static`, `static mut`, `const`, `const fn`, const
11+
generics, and array initializers. Enabling this feature flag will also make
12+
`&&` and `||` function normally in a const-context by removing the hack that
13+
replaces them with their non-short-circuiting equivalents, `&` and `|`, in a
14+
`const` or `static`.

‎src/librustc/hir/mod.rs

+15-2
Original file line numberDiff line numberDiff line change
@@ -1749,6 +1749,20 @@ pub enum MatchSource {
17491749
AwaitDesugar,
17501750
}
17511751

1752+
impl MatchSource {
1753+
pub fn name(self) -> &'static str {
1754+
use MatchSource::*;
1755+
match self {
1756+
Normal => "match",
1757+
IfDesugar { .. } | IfLetDesugar { .. } => "if",
1758+
WhileDesugar | WhileLetDesugar => "while",
1759+
ForLoopDesugar => "for",
1760+
TryDesugar => "?",
1761+
AwaitDesugar => ".await",
1762+
}
1763+
}
1764+
}
1765+
17521766
/// The loop type that yielded an `ExprKind::Loop`.
17531767
#[derive(Copy, Clone, PartialEq, RustcEncodable, RustcDecodable, Debug, HashStable)]
17541768
pub enum LoopSource {
@@ -1766,8 +1780,7 @@ impl LoopSource {
17661780
pub fn name(self) -> &'static str {
17671781
match self {
17681782
LoopSource::Loop => "loop",
1769-
LoopSource::While => "while",
1770-
LoopSource::WhileLet => "while let",
1783+
LoopSource::While | LoopSource::WhileLet => "while",
17711784
LoopSource::ForLoop => "for",
17721785
}
17731786
}

‎src/librustc_mir/hair/cx/expr.rs

+9-6
Original file line numberDiff line numberDiff line change
@@ -341,9 +341,10 @@ fn make_mirror_unadjusted<'a, 'tcx>(
341341
} else {
342342
// FIXME overflow
343343
match (op.node, cx.constness) {
344-
// FIXME(eddyb) use logical ops in constants when
345-
// they can handle that kind of control-flow.
346-
(hir::BinOpKind::And, hir::Constness::Const) => {
344+
// Destroy control flow if `#![feature(const_if_match)]` is not enabled.
345+
(hir::BinOpKind::And, hir::Constness::Const)
346+
if !cx.tcx.features().const_if_match =>
347+
{
347348
cx.control_flow_destroyed.push((
348349
op.span,
349350
"`&&` operator".into(),
@@ -354,7 +355,9 @@ fn make_mirror_unadjusted<'a, 'tcx>(
354355
rhs: rhs.to_ref(),
355356
}
356357
}
357-
(hir::BinOpKind::Or, hir::Constness::Const) => {
358+
(hir::BinOpKind::Or, hir::Constness::Const)
359+
if !cx.tcx.features().const_if_match =>
360+
{
358361
cx.control_flow_destroyed.push((
359362
op.span,
360363
"`||` operator".into(),
@@ -366,14 +369,14 @@ fn make_mirror_unadjusted<'a, 'tcx>(
366369
}
367370
}
368371

369-
(hir::BinOpKind::And, hir::Constness::NotConst) => {
372+
(hir::BinOpKind::And, _) => {
370373
ExprKind::LogicalOp {
371374
op: LogicalOp::And,
372375
lhs: lhs.to_ref(),
373376
rhs: rhs.to_ref(),
374377
}
375378
}
376-
(hir::BinOpKind::Or, hir::Constness::NotConst) => {
379+
(hir::BinOpKind::Or, _) => {
377380
ExprKind::LogicalOp {
378381
op: LogicalOp::Or,
379382
lhs: lhs.to_ref(),

‎src/librustc_mir/transform/check_consts/ops.rs

+9-1
Original file line numberDiff line numberDiff line change
@@ -52,7 +52,11 @@ pub trait NonConstOp: std::fmt::Debug {
5252
/// A `Downcast` projection.
5353
#[derive(Debug)]
5454
pub struct Downcast;
55-
impl NonConstOp for Downcast {}
55+
impl NonConstOp for Downcast {
56+
fn feature_gate(tcx: TyCtxt<'_>) -> Option<bool> {
57+
Some(tcx.features().const_if_match)
58+
}
59+
}
5660

5761
/// A function call where the callee is a pointer.
5862
#[derive(Debug)]
@@ -139,6 +143,10 @@ impl NonConstOp for HeapAllocation {
139143
#[derive(Debug)]
140144
pub struct IfOrMatch;
141145
impl NonConstOp for IfOrMatch {
146+
fn feature_gate(tcx: TyCtxt<'_>) -> Option<bool> {
147+
Some(tcx.features().const_if_match)
148+
}
149+
142150
fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
143151
// This should be caught by the HIR const-checker.
144152
item.tcx.sess.delay_span_bug(

‎src/librustc_mir/transform/qualify_min_const_fn.rs

+17-5
Original file line numberDiff line numberDiff line change
@@ -216,7 +216,9 @@ fn check_statement(
216216
check_rvalue(tcx, body, def_id, rval, span)
217217
}
218218

219-
StatementKind::FakeRead(FakeReadCause::ForMatchedPlace, _) => {
219+
| StatementKind::FakeRead(FakeReadCause::ForMatchedPlace, _)
220+
if !tcx.features().const_if_match
221+
=> {
220222
Err((span, "loops and conditional expressions are not stable in const fn".into()))
221223
}
222224

@@ -267,9 +269,10 @@ fn check_place(
267269
while let &[ref proj_base @ .., elem] = cursor {
268270
cursor = proj_base;
269271
match elem {
270-
ProjectionElem::Downcast(..) => {
271-
return Err((span, "`match` or `if let` in `const fn` is unstable".into()));
272-
}
272+
ProjectionElem::Downcast(..) if !tcx.features().const_if_match
273+
=> return Err((span, "`match` or `if let` in `const fn` is unstable".into())),
274+
ProjectionElem::Downcast(_symbol, _variant_index) => {}
275+
273276
ProjectionElem::Field(..) => {
274277
let base_ty = Place::ty_from(&place.base, &proj_base, body, tcx).ty;
275278
if let Some(def) = base_ty.ty_adt_def() {
@@ -321,10 +324,19 @@ fn check_terminator(
321324
check_operand(tcx, value, span, def_id, body)
322325
},
323326

324-
TerminatorKind::FalseEdges { .. } | TerminatorKind::SwitchInt { .. } => Err((
327+
| TerminatorKind::FalseEdges { .. }
328+
| TerminatorKind::SwitchInt { .. }
329+
if !tcx.features().const_if_match
330+
=> Err((
325331
span,
326332
"loops and conditional expressions are not stable in const fn".into(),
327333
)),
334+
335+
TerminatorKind::FalseEdges { .. } => Ok(()),
336+
TerminatorKind::SwitchInt { discr, switch_ty: _, values: _, targets: _ } => {
337+
check_operand(tcx, discr, span, def_id, body)
338+
}
339+
328340
| TerminatorKind::Abort | TerminatorKind::Unreachable => {
329341
Err((span, "const fn with unreachable code is not stable".into()))
330342
}

‎src/librustc_passes/check_const.rs

+73-25
Original file line numberDiff line numberDiff line change
@@ -11,16 +11,46 @@ use rustc::hir::def_id::DefId;
1111
use rustc::hir::intravisit::{Visitor, NestedVisitorMap};
1212
use rustc::hir::map::Map;
1313
use rustc::hir;
14-
use rustc::session::Session;
1514
use rustc::ty::TyCtxt;
1615
use rustc::ty::query::Providers;
1716
use syntax::ast::Mutability;
17+
use syntax::feature_gate::{emit_feature_err, Features, GateIssue};
1818
use syntax::span_err;
19-
use syntax_pos::Span;
19+
use syntax_pos::{sym, Span};
2020
use rustc_error_codes::*;
2121

2222
use std::fmt;
2323

24+
/// An expression that is not *always* legal in a const context.
25+
#[derive(Clone, Copy)]
26+
enum NonConstExpr {
27+
Loop(hir::LoopSource),
28+
Match(hir::MatchSource),
29+
}
30+
31+
impl NonConstExpr {
32+
fn name(self) -> &'static str {
33+
match self {
34+
Self::Loop(src) => src.name(),
35+
Self::Match(src) => src.name(),
36+
}
37+
}
38+
39+
/// Returns `true` if all feature gates required to enable this expression are turned on, or
40+
/// `None` if there is no feature gate corresponding to this expression.
41+
fn is_feature_gate_enabled(self, features: &Features) -> Option<bool> {
42+
use hir::MatchSource::*;
43+
match self {
44+
| Self::Match(Normal)
45+
| Self::Match(IfDesugar { .. })
46+
| Self::Match(IfLetDesugar { .. })
47+
=> Some(features.const_if_match),
48+
49+
_ => None,
50+
}
51+
}
52+
}
53+
2454
#[derive(Copy, Clone)]
2555
enum ConstKind {
2656
Static,
@@ -75,31 +105,51 @@ pub(crate) fn provide(providers: &mut Providers<'_>) {
75105

76106
#[derive(Copy, Clone)]
77107
struct CheckConstVisitor<'tcx> {
78-
sess: &'tcx Session,
79-
hir_map: &'tcx Map<'tcx>,
108+
tcx: TyCtxt<'tcx>,
80109
const_kind: Option<ConstKind>,
81110
}
82111

83112
impl<'tcx> CheckConstVisitor<'tcx> {
84113
fn new(tcx: TyCtxt<'tcx>) -> Self {
85114
CheckConstVisitor {
86-
sess: &tcx.sess,
87-
hir_map: tcx.hir(),
115+
tcx,
88116
const_kind: None,
89117
}
90118
}
91119

92120
/// Emits an error when an unsupported expression is found in a const context.
93-
fn const_check_violated(&self, bad_op: &str, span: Span) {
94-
if self.sess.opts.debugging_opts.unleash_the_miri_inside_of_you {
95-
self.sess.span_warn(span, "skipping const checks");
96-
return;
121+
fn const_check_violated(&self, expr: NonConstExpr, span: Span) {
122+
match expr.is_feature_gate_enabled(self.tcx.features()) {
123+
// Don't emit an error if the user has enabled the requisite feature gates.
124+
Some(true) => return,
125+
126+
// Users of `-Zunleash-the-miri-inside-of-you` must use feature gates when possible.
127+
None if self.tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you => {
128+
self.tcx.sess.span_warn(span, "skipping const checks");
129+
return;
130+
}
131+
132+
_ => {}
97133
}
98134

99135
let const_kind = self.const_kind
100136
.expect("`const_check_violated` may only be called inside a const context");
101137

102-
span_err!(self.sess, span, E0744, "`{}` is not allowed in a `{}`", bad_op, const_kind);
138+
let msg = format!("`{}` is not allowed in a `{}`", expr.name(), const_kind);
139+
match expr {
140+
| NonConstExpr::Match(hir::MatchSource::Normal)
141+
| NonConstExpr::Match(hir::MatchSource::IfDesugar { .. })
142+
| NonConstExpr::Match(hir::MatchSource::IfLetDesugar { .. })
143+
=> emit_feature_err(
144+
&self.tcx.sess.parse_sess,
145+
sym::const_if_match,
146+
span,
147+
GateIssue::Language,
148+
&msg
149+
),
150+
151+
_ => span_err!(self.tcx.sess, span, E0744, "{}", msg),
152+
}
103153
}
104154

105155
/// Saves the parent `const_kind` before calling `f` and restores it afterwards.
@@ -113,7 +163,7 @@ impl<'tcx> CheckConstVisitor<'tcx> {
113163

114164
impl<'tcx> Visitor<'tcx> for CheckConstVisitor<'tcx> {
115165
fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
116-
NestedVisitorMap::OnlyBodies(&self.hir_map)
166+
NestedVisitorMap::OnlyBodies(&self.tcx.hir())
117167
}
118168

119169
fn visit_anon_const(&mut self, anon: &'tcx hir::AnonConst) {
@@ -122,7 +172,7 @@ impl<'tcx> Visitor<'tcx> for CheckConstVisitor<'tcx> {
122172
}
123173

124174
fn visit_body(&mut self, body: &'tcx hir::Body) {
125-
let kind = ConstKind::for_body(body, self.hir_map);
175+
let kind = ConstKind::for_body(body, self.tcx.hir());
126176
self.recurse_into(kind, |this| hir::intravisit::walk_body(this, body));
127177
}
128178

@@ -132,24 +182,22 @@ impl<'tcx> Visitor<'tcx> for CheckConstVisitor<'tcx> {
132182
_ if self.const_kind.is_none() => {}
133183

134184
hir::ExprKind::Loop(_, _, source) => {
135-
self.const_check_violated(source.name(), e.span);
185+
self.const_check_violated(NonConstExpr::Loop(*source), e.span);
136186
}
137187

138188
hir::ExprKind::Match(_, _, source) => {
139-
use hir::MatchSource::*;
140-
141-
let op = match source {
142-
Normal => Some("match"),
143-
IfDesugar { .. } | IfLetDesugar { .. } => Some("if"),
144-
TryDesugar => Some("?"),
145-
AwaitDesugar => Some(".await"),
146-
189+
let non_const_expr = match source {
147190
// These are handled by `ExprKind::Loop` above.
148-
WhileDesugar | WhileLetDesugar | ForLoopDesugar => None,
191+
| hir::MatchSource::WhileDesugar
192+
| hir::MatchSource::WhileLetDesugar
193+
| hir::MatchSource::ForLoopDesugar
194+
=> None,
195+
196+
_ => Some(NonConstExpr::Match(*source)),
149197
};
150198

151-
if let Some(op) = op {
152-
self.const_check_violated(op, e.span);
199+
if let Some(expr) = non_const_expr {
200+
self.const_check_violated(expr, e.span);
153201
}
154202
}
155203

‎src/libsyntax/feature_gate/active.rs

+3
Original file line numberDiff line numberDiff line change
@@ -529,6 +529,9 @@ declare_features! (
529529
/// Allows using the `#[register_attr]` attribute.
530530
(active, register_tool, "1.41.0", Some(66079), None),
531531

532+
/// Allows the use of `if` and `match` in constants.
533+
(active, const_if_match, "1.41.0", Some(49146), None),
534+
532535
// -------------------------------------------------------------------------
533536
// feature-group-end: actual feature gates
534537
// -------------------------------------------------------------------------

‎src/libsyntax_pos/symbol.rs

+1
Original file line numberDiff line numberDiff line change
@@ -203,6 +203,7 @@ symbols! {
203203
const_fn,
204204
const_fn_union,
205205
const_generics,
206+
const_if_match,
206207
const_indexing,
207208
const_in_array_repeat_expressions,
208209
const_let,

‎src/test/ui/borrowck/issue-64453.stderr

+4-2
Original file line numberDiff line numberDiff line change
@@ -1,11 +1,13 @@
1-
error[E0744]: `match` is not allowed in a `static`
1+
error[E0658]: `match` is not allowed in a `static`
22
--> $DIR/issue-64453.rs:4:31
33
|
44
LL | static settings_dir: String = format!("");
55
| ^^^^^^^^^^^
66
|
7+
= note: for more information, see https://github.com/rust-lang/rust/issues/49146
8+
= help: add `#![feature(const_if_match)]` to the crate attributes to enable
79
= note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
810

911
error: aborting due to previous error
1012

11-
For more information about this error, try `rustc --explain E0744`.
13+
For more information about this error, try `rustc --explain E0658`.

‎src/test/ui/consts/const-eval/infinite_loop.stderr

+5-2
Original file line numberDiff line numberDiff line change
@@ -9,11 +9,14 @@ LL | |
99
LL | | }
1010
| |_________^
1111

12-
error[E0744]: `if` is not allowed in a `const`
12+
error[E0658]: `if` is not allowed in a `const`
1313
--> $DIR/infinite_loop.rs:9:17
1414
|
1515
LL | n = if n % 2 == 0 { n/2 } else { 3*n + 1 };
1616
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
17+
|
18+
= note: for more information, see https://github.com/rust-lang/rust/issues/49146
19+
= help: add `#![feature(const_if_match)]` to the crate attributes to enable
1720

1821
warning: Constant evaluating a complex constant, this might take some time
1922
--> $DIR/infinite_loop.rs:4:18
@@ -36,5 +39,5 @@ LL | n = if n % 2 == 0 { n/2 } else { 3*n + 1 };
3639

3740
error: aborting due to 3 previous errors
3841

39-
Some errors have detailed explanations: E0080, E0744.
42+
Some errors have detailed explanations: E0080, E0658, E0744.
4043
For more information about an error, try `rustc --explain E0080`.

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