Stabilise irrefutable if-let and while-let patterns

This stabilises RFC 2086 (#44495).

Co-Authored-By: Sebastian Malton <[email protected]>

Author: varkor

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

@@ -286,7 +286,7 @@ declare_lint! {
 
 declare_lint! {
     pub IRREFUTABLE_LET_PATTERNS,
-    Deny,
+    Warn,
     "detects irrefutable patterns in if-let and while-let statements"
 }
 

src/librustc/lint/builtin.rs

@@ -325,11 +325,13 @@ match Some(42) {
 "##,
 
 E0162: r##"
+#### Note: this error code is no longer emitted by the compiler.
+
 An if-let pattern attempts to match the pattern, and enters the body if the
 match was successful. If the match is irrefutable (when it cannot fail to
 match), use a regular `let`-binding instead. For instance:
 
-```compile_fail,E0162
+```compile_pass
 struct Irrefutable(i32);
 let irr = Irrefutable(0);
 
@@ -352,11 +354,13 @@ println!("{}", x);
 "##,
 
 E0165: r##"
+#### Note: this error code is no longer emitted by the compiler.
+
 A while-let pattern attempts to match the pattern, and enters the body if the
 match was successful. If the match is irrefutable (when it cannot fail to
 match), use a regular `let`-binding inside a `loop` instead. For instance:
 
-```compile_fail,E0165
+```compile_pass,no_run
 struct Irrefutable(i32);
 let irr = Irrefutable(0);
 

src/librustc_mir/diagnostics.rs

@@ -350,7 +350,6 @@ fn check_arms<'a, 'tcx>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
 {
     let mut seen = Matrix::empty();
     let mut catchall = None;
-    let mut printed_if_let_err = false;
     for (arm_index, &(ref pats, guard)) in arms.iter().enumerate() {
         for &(pat, hir_pat) in pats {
             let v = smallvec![pat];
@@ -359,27 +358,12 @@ fn check_arms<'a, 'tcx>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
                 NotUseful => {
                     match source {
                         hir::MatchSource::IfLetDesugar { .. } => {
-                            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 {
-                                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;
-                                }
-                            }
+                            cx.tcx.lint_node(
+                                lint::builtin::IRREFUTABLE_LET_PATTERNS,
+                                hir_pat.id,
+                                pat.span,
+                                "irrefutable if-let pattern",
+                            );
                         }
 
                         hir::MatchSource::WhileLetDesugar => {
@@ -394,21 +378,12 @@ fn check_arms<'a, 'tcx>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
                                 },
                                 // The arm with the wildcard pattern.
                                 1 => {
-                                    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();
-                                    }
+                                    cx.tcx.lint_node(
+                                        lint::builtin::IRREFUTABLE_LET_PATTERNS,
+                                        hir_pat.id,
+                                        pat.span,
+                                        "irrefutable while-let pattern",
+                                    );
                                 },
                                 _ => bug!(),
                             }

src/librustc_mir/hair/pattern/check_match.rs

@@ -414,9 +414,6 @@ declare_features! (
     // `#[doc(alias = "...")]`
     (active, doc_alias, "1.27.0", Some(50146), None),
 
-    // Allows irrefutable patterns in `if let` and `while let` statements (RFC 2086).
-    (active, irrefutable_let_patterns, "1.27.0", Some(44495), None),
-
     // inconsistent bounds in where clauses
     (active, trivial_bounds, "1.28.0", Some(48214), None),
 
@@ -684,6 +681,8 @@ declare_features! (
     (accepted, underscore_imports, "1.33.0", Some(48216), None),
     // Allows `#[repr(packed(N))]` attribute on structs.
     (accepted, repr_packed, "1.33.0", Some(33158), None),
+    // Allows irrefutable patterns in `if let` and `while let` statements (RFC 2086).
+    (accepted, irrefutable_let_patterns, "1.33.0", Some(44495), None),
     // Allows calling `const unsafe fn` inside `unsafe` blocks in `const fn` functions.
     (accepted, min_const_unsafe_fn, "1.33.0", Some(55607), None),
     // `#[cfg_attr(predicate, multiple, attributes, here)]`