Auto merge of #49469 - Nokel81:allow-irrefutable-let-patterns, r=niko…

…matsakis

Implementation of RFC 2086 - Allow Irrefutable Let patterns

This is the set of changes for RFC2086. Tracking issue #44495. Rendered [here](rust-lang/rfcs#2086)

src/doc/unstable-book/src/language-features/irrefutable-let-patterns.md

@@ -0,0 +1,28 @@
+# `irrefutable_let_patterns`
+
+The tracking issue for this feature is: [#44495]
+
+[#44495]: https://github.com/rust-lang/rust/issues/44495
+
+------------------------
+
+This feature changes the way that "irrefutable patterns" are handled
+in the `if let` and `while let` forms. An *irrefutable pattern* is one
+that cannot fail to match -- for example, the `_` pattern matches any
+value, and hence it is "irrefutable". Without this feature, using an
+irrefutable pattern in an `if let` gives a hard error (since often
+this indicates programmer error). But when the feature is enabled, the
+error becomes a lint (since in some cases irrefutable patterns are
+expected). This means you can use `#[allow]` to silence the lint:
+
+```rust
+#![feature(irrefutable_let_patterns)]
+
+#[allow(irrefutable_let_patterns)]
+fn main() {
+    // These two examples used to be errors, but now they
+    // trigger a lint (that is allowed):
+    if let _ = 5 {}
+    while let _ = 5 { break; }
+}
+```

src/librustc/lint/builtin.rs

@@ -280,6 +280,12 @@ declare_lint! {
     "detects name collision with an existing but unstable method"
 }
 
+declare_lint! {
+    pub IRREFUTABLE_LET_PATTERNS,
+    Deny,
+    "detects irrefutable patterns in if-let and while-let statements"
+}
+
 declare_lint! {
     pub UNUSED_LABELS,
     Allow,
@@ -361,6 +367,7 @@ impl LintPass for HardwiredLints {
             BARE_TRAIT_OBJECTS,
             ABSOLUTE_PATHS_NOT_STARTING_WITH_CRATE,
             UNSTABLE_NAME_COLLISIONS,
+            IRREFUTABLE_LET_PATTERNS,
             DUPLICATE_ASSOCIATED_TYPE_BINDINGS,
             DUPLICATE_MACRO_EXPORTS,
             INTRA_DOC_LINK_RESOLUTION_FAILURE,

src/librustc_mir/hair/pattern/check_match.rs

@@ -371,43 +371,56 @@ fn check_arms<'a, 'tcx>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
                 NotUseful => {
                     match source {
                         hir::MatchSource::IfLetDesugar { .. } => {
-                            if printed_if_let_err {
-                                // we already printed an irrefutable if-let pattern error.
-                                // We don't want two, that's just confusing.
+                            if cx.tcx.features().irrefutable_let_patterns {
+                                cx.tcx.lint_node(
+                                    lint::builtin::IRREFUTABLE_LET_PATTERNS,
+                                    hir_pat.id, pat.span,
+                                    "irrefutable if-let pattern");
                             } else {
-                                // find the first arm pattern so we can use its span
-                                let &(ref first_arm_pats, _) = &arms[0];
-                                let first_pat = &first_arm_pats[0];
-                                let span = first_pat.0.span;
-                                struct_span_err!(cx.tcx.sess, span, E0162,
-                                                "irrefutable if-let pattern")
-                                    .span_label(span, "irrefutable pattern")
-                                    .emit();
-                                printed_if_let_err = true;
+                                if printed_if_let_err {
+                                    // we already printed an irrefutable if-let pattern error.
+                                    // We don't want two, that's just confusing.
+                                } else {
+                                    // find the first arm pattern so we can use its span
+                                    let &(ref first_arm_pats, _) = &arms[0];
+                                    let first_pat = &first_arm_pats[0];
+                                    let span = first_pat.0.span;
+                                    struct_span_err!(cx.tcx.sess, span, E0162,
+                                                    "irrefutable if-let pattern")
+                                        .span_label(span, "irrefutable pattern")
+                                        .emit();
+                                    printed_if_let_err = true;
+                                }
                             }
                         },
 
                         hir::MatchSource::WhileLetDesugar => {
-                            // find the first arm pattern so we can use its span
-                            let &(ref first_arm_pats, _) = &arms[0];
-                            let first_pat = &first_arm_pats[0];
-                            let span = first_pat.0.span;
-
                             // check which arm we're on.
                             match arm_index {
                                 // The arm with the user-specified pattern.
                                 0 => {
                                     cx.tcx.lint_node(
-                                            lint::builtin::UNREACHABLE_PATTERNS,
+                                        lint::builtin::UNREACHABLE_PATTERNS,
                                         hir_pat.id, pat.span,
                                         "unreachable pattern");
                                 },
                                 // The arm with the wildcard pattern.
                                 1 => {
-                                    struct_span_err!(cx.tcx.sess, span, E0165,
-                                                     "irrefutable while-let pattern")
-                                        .span_label(span, "irrefutable pattern")
-                                        .emit();
+                                    if cx.tcx.features().irrefutable_let_patterns {
+                                        cx.tcx.lint_node(
+                                            lint::builtin::IRREFUTABLE_LET_PATTERNS,
+                                            hir_pat.id, pat.span,
+                                            "irrefutable while-let pattern");
+                                    } else {
+                                        // find the first arm pattern so we can use its span
+                                        let &(ref first_arm_pats, _) = &arms[0];
+                                        let first_pat = &first_arm_pats[0];
+                                        let span = first_pat.0.span;
+                                        struct_span_err!(cx.tcx.sess, span, E0165,
+                                                         "irrefutable while-let pattern")
+                                            .span_label(span, "irrefutable pattern")
+                                            .emit();
+                                    }
                                 },
                                 _ => bug!(),
                             }

src/libsyntax/feature_gate.rs

@@ -458,6 +458,9 @@ declare_features! (
     // Scoped attributes
     (active, tool_attributes, "1.25.0", Some(44690), None),
 
+    // allow irrefutable patterns in if-let and while-let statements (RFC 2086)
+    (active, irrefutable_let_patterns, "1.27.0", Some(44495), None),
+
     // Allows use of the :literal macro fragment specifier (RFC 1576)
     (active, macro_literal_matcher, "1.27.0", Some(35625), None),
 

src/test/compile-fail/feature-gate-without_gate_irrefutable_pattern.rs

@@ -0,0 +1,17 @@
+// gate-test-irrefutable_let_patterns
+
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#[allow(irrefutable_let_patterns)]
+fn main() {
+    if let _ = 5 {}
+    //~^ ERROR 15:12: 15:13: irrefutable if-let pattern [E0162]
+}

src/test/compile-fail/should-fail-no_gate_irrefutable_if_let_pattern.rs

@@ -0,0 +1,15 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// should-fail-irrefutable_let_patterns
+fn main() {
+    if let _ = 5 {}
+    //~^ ERROR irrefutable if-let pattern [E0162]
+}

src/test/compile-fail/should-fail-with_gate_irrefutable_pattern_deny.rs

@@ -0,0 +1,17 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(irrefutable_let_patterns)]
+
+// should-fail-irrefutable_let_patterns_with_gate
+fn main() {
+    if let _ = 5 {}
+    //~^ ERROR irrefutable if-let pattern [irrefutable_let_patterns]
+}

src/test/run-pass/allow_irrefutable_let_patterns.rs

@@ -0,0 +1,21 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(irrefutable_let_patterns)]
+
+// must-compile-successfully-irrefutable_let_patterns_with_gate
+#[allow(irrefutable_let_patterns)]
+fn main() {
+    if let _ = 5 {}
+
+    while let _ = 5 {
+        break;
+    }
+}