Add a machine-applicable suggestion to "unreachable pattern"

compiler/rustc_mir_build/messages.ftl

@@ -335,6 +335,7 @@ mir_build_unreachable_pattern = unreachable pattern
     .unreachable_covered_by_catchall = matches any value
     .unreachable_covered_by_one = matches all the values already
     .unreachable_covered_by_many = these patterns collectively make the last one unreachable
+    .suggestion = remove the match arm
 
 mir_build_unsafe_fn_safe_body = an unsafe function restricts its caller, but its body is safe by default
 mir_build_unsafe_not_inherited = items do not inherit unsafety from separate enclosing items

compiler/rustc_mir_build/src/errors.rs

@@ -594,6 +594,8 @@ pub(crate) struct UnreachablePattern<'tcx> {
     pub(crate) covered_by_one: Option<Span>,
     #[note(mir_build_unreachable_covered_by_many)]
     pub(crate) covered_by_many: Option<MultiSpan>,
+    #[suggestion(code = "", applicability = "machine-applicable")]
+    pub(crate) suggest_remove: Option<Span>,
 }
 
 #[derive(Subdiagnostic)]

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -413,7 +413,7 @@ impl<'p, 'tcx> MatchVisitor<'p, 'tcx> {
                 // Emit lints in the order in which they occur in the file.
                 redundant_subpats.sort_unstable_by_key(|(pat, _)| pat.data().span);
                 for (pat, explanation) in redundant_subpats {
-                    report_unreachable_pattern(cx, arm.arm_data, pat, &explanation)
+                    report_unreachable_pattern(cx, arm.arm_data, pat, &explanation, None)
                 }
             }
         }
@@ -474,7 +474,11 @@ impl<'p, 'tcx> MatchVisitor<'p, 'tcx> {
             hir::MatchSource::ForLoopDesugar
             | hir::MatchSource::Postfix
             | hir::MatchSource::Normal
-            | hir::MatchSource::FormatArgs => report_arm_reachability(&cx, &report),
+            | hir::MatchSource::FormatArgs => {
+                let suggest_removing_if_unreachable =
+                    matches!(source, hir::MatchSource::Postfix | hir::MatchSource::Normal);
+                report_arm_reachability(&cx, &report, suggest_removing_if_unreachable);
+            }
             // Unreachable patterns in try and await expressions occur when one of
             // the arms are an uninhabited type. Which is OK.
             hir::MatchSource::AwaitDesugar | hir::MatchSource::TryDesugar(_) => {}
@@ -612,7 +616,8 @@ impl<'p, 'tcx> MatchVisitor<'p, 'tcx> {
         refutability: RefutableFlag,
         scrut: Option<&Expr<'tcx>>,
     ) -> Result<(PatCtxt<'p, 'tcx>, UsefulnessReport<'p, 'tcx>), ErrorGuaranteed> {
-        let cx = self.new_cx(refutability, None, scrut, pat.span);
+        let pat_span = pat.span;
+        let cx = self.new_cx(refutability, None, scrut, pat_span);
         let pat = self.lower_pattern(&cx, pat)?;
         let arms = [MatchArm { pat, arm_data: self.lint_level, has_guard: false }];
         let report = self.analyze_patterns(&cx, &arms, pat.ty().inner())?;
@@ -626,7 +631,7 @@ impl<'p, 'tcx> MatchVisitor<'p, 'tcx> {
     ) -> Result<RefutableFlag, ErrorGuaranteed> {
         let (cx, report) = self.analyze_binding(pat, Refutable, scrut)?;
         // Report if the pattern is unreachable, which can only occur when the type is uninhabited.
-        report_arm_reachability(&cx, &report);
+        report_arm_reachability(&cx, &report, false);
         // If the list of witnesses is empty, the match is exhaustive, i.e. the `if let` pattern is
         // irrefutable.
         Ok(if report.non_exhaustiveness_witnesses.is_empty() { Irrefutable } else { Refutable })
@@ -916,6 +921,7 @@ fn report_unreachable_pattern<'p, 'tcx>(
     hir_id: HirId,
     pat: &DeconstructedPat<'p, 'tcx>,
     explanation: &RedundancyExplanation<'p, 'tcx>,
+    suggest_remove: Option<Span>,
 ) {
     let pat_span = pat.data().span;
     let mut lint = UnreachablePattern {
@@ -924,6 +930,7 @@ fn report_unreachable_pattern<'p, 'tcx>(
         covered_by_catchall: None,
         covered_by_one: None,
         covered_by_many: None,
+        suggest_remove,
     };
     match explanation.covered_by.as_slice() {
         [] => {
@@ -964,10 +971,28 @@ fn report_unreachable_pattern<'p, 'tcx>(
 }
 
 /// Report unreachable arms, if any.
-fn report_arm_reachability<'p, 'tcx>(cx: &PatCtxt<'p, 'tcx>, report: &UsefulnessReport<'p, 'tcx>) {
+fn report_arm_reachability<'p, 'tcx>(
+    cx: &PatCtxt<'p, 'tcx>,
+    report: &UsefulnessReport<'p, 'tcx>,
+    is_match_arm: bool,
+) {
+    let sm = cx.tcx.sess.source_map();
     for (arm, is_useful) in report.arm_usefulness.iter() {
         if let Usefulness::Redundant(explanation) = is_useful {
-            report_unreachable_pattern(cx, arm.arm_data, arm.pat, explanation)
+            let hir_id = arm.arm_data;
+            let arm_span = cx.tcx.hir().span(hir_id);
+            let suggest_remove = if is_match_arm {
+                // If the arm is followed by a comma, extend the span to include it.
+                let with_whitespace = sm.span_extend_while_whitespace(arm_span);
+                if let Some(comma) = sm.span_look_ahead(with_whitespace, ",", Some(1)) {
+                    Some(arm_span.to(comma))
+                } else {
+                    Some(arm_span)
+                }
+            } else {
+                None
+            };
+            report_unreachable_pattern(cx, hir_id, arm.pat, explanation, suggest_remove)
         }
     }
 }

compiler/rustc_pattern_analysis/src/rustc.rs

@@ -921,6 +921,7 @@ impl<'p, 'tcx: 'p> PatCx for RustcPatCtxt<'p, 'tcx> {
     type Error = ErrorGuaranteed;
     type VariantIdx = VariantIdx;
     type StrLit = Const<'tcx>;
+    // The `HirId` is used for lints, the `Span` is the span of the arm.
     type ArmData = HirId;
     type PatData = &'p Pat<'tcx>;
 

tests/ui/consts/packed_pattern.stderr

@@ -4,7 +4,10 @@ warning: unreachable pattern
 LL |         Foo { field: (5, 6, 7, 8) } => {},
    |         --------------------------- matches all the values already
 LL |         FOO => unreachable!(),
-   |         ^^^ unreachable pattern
+   |         ^^^-------------------
+   |         |
+   |         unreachable pattern
+   |         help: remove the match arm
    |
    = note: `#[warn(unreachable_patterns)]` on by default
 

tests/ui/consts/packed_pattern2.stderr

@@ -4,7 +4,10 @@ warning: unreachable pattern
 LL |         Bar { a: Foo { field: (5, 6) } } => {},
    |         -------------------------------- matches all the values already
 LL |         FOO => unreachable!(),
-   |         ^^^ unreachable pattern
+   |         ^^^-------------------
+   |         |
+   |         unreachable pattern
+   |         help: remove the match arm
    |
    = note: `#[warn(unreachable_patterns)]` on by default
 

tests/ui/error-codes/E0001.stderr

@@ -2,7 +2,10 @@ error: unreachable pattern
   --> $DIR/E0001.rs:8:9
    |
 LL |         _ => {/* ... */}
-   |         ^ unreachable pattern
+   |         ^---------------
+   |         |
+   |         unreachable pattern
+   |         help: remove the match arm
    |
 note: these patterns collectively make the last one unreachable
   --> $DIR/E0001.rs:8:9

tests/ui/lint/issue-30302.stderr

@@ -13,7 +13,10 @@ LL |         Nil => true,
    |         --- matches any value
 LL |
 LL |         _ => false
-   |         ^ unreachable pattern
+   |         ^---------
+   |         |
+   |         unreachable pattern
+   |         help: remove the match arm
    |
 note: the lint level is defined here
   --> $DIR/issue-30302.rs:4:9

tests/ui/or-patterns/exhaustiveness-unreachable-pattern.stderr

@@ -4,7 +4,10 @@ error: unreachable pattern
 LL |         (1 | 2,) => {}
    |         -------- matches all the values already
 LL |         (1,) => {}
-   |         ^^^^ unreachable pattern
+   |         ^^^^------
+   |         |
+   |         unreachable pattern
+   |         help: remove the match arm
    |
 note: the lint level is defined here
   --> $DIR/exhaustiveness-unreachable-pattern.rs:1:9
@@ -18,13 +21,19 @@ error: unreachable pattern
 LL |         (1 | 2,) => {}
    |         -------- matches all the values already
 LL |         (2,) => {}
-   |         ^^^^ unreachable pattern
+   |         ^^^^------
+   |         |
+   |         unreachable pattern
+   |         help: remove the match arm
 
 error: unreachable pattern
   --> $DIR/exhaustiveness-unreachable-pattern.rs:19:9
    |
 LL |         (1 | 2,) => {}
-   |         ^^^^^^^^ unreachable pattern
+   |         ^^^^^^^^------
+   |         |
+   |         unreachable pattern
+   |         help: remove the match arm
    |
 note: these patterns collectively make the last one unreachable
   --> $DIR/exhaustiveness-unreachable-pattern.rs:19:9
@@ -42,7 +51,10 @@ error: unreachable pattern
 LL |         (1 | 2, 3 | 4) => {}
    |         -------------- matches all the values already
 LL |         (1, 3) => {}
-   |         ^^^^^^ unreachable pattern
+   |         ^^^^^^------
+   |         |
+   |         unreachable pattern
+   |         help: remove the match arm
 
 error: unreachable pattern
   --> $DIR/exhaustiveness-unreachable-pattern.rs:25:9
@@ -51,7 +63,10 @@ LL |         (1 | 2, 3 | 4) => {}
    |         -------------- matches all the values already
 LL |         (1, 3) => {}
 LL |         (1, 4) => {}
-   |         ^^^^^^ unreachable pattern
+   |         ^^^^^^------
+   |         |
+   |         unreachable pattern
+   |         help: remove the match arm
 
 error: unreachable pattern
   --> $DIR/exhaustiveness-unreachable-pattern.rs:26:9
@@ -60,7 +75,10 @@ LL |         (1 | 2, 3 | 4) => {}
    |         -------------- matches all the values already
 ...
 LL |         (2, 4) => {}
-   |         ^^^^^^ unreachable pattern
+   |         ^^^^^^------
+   |         |
+   |         unreachable pattern
+   |         help: remove the match arm
 
 error: unreachable pattern
   --> $DIR/exhaustiveness-unreachable-pattern.rs:27:9
@@ -69,13 +87,19 @@ LL |         (1 | 2, 3 | 4) => {}
    |         -------------- matches all the values already
 ...
 LL |         (2 | 1, 4) => {}
-   |         ^^^^^^^^^^ unreachable pattern
+   |         ^^^^^^^^^^------
+   |         |
+   |         unreachable pattern
+   |         help: remove the match arm
 
 error: unreachable pattern
   --> $DIR/exhaustiveness-unreachable-pattern.rs:29:9
    |
 LL |         (1, 4 | 5) => {}
-   |         ^^^^^^^^^^ unreachable pattern
+   |         ^^^^^^^^^^------
+   |         |
+   |         unreachable pattern
+   |         help: remove the match arm
    |
 note: these patterns collectively make the last one unreachable
   --> $DIR/exhaustiveness-unreachable-pattern.rs:29:9
@@ -102,7 +126,10 @@ error: unreachable pattern
 LL |         (None | Some(1 | 2),) => {}
    |         --------------------- matches all the values already
 LL |         (Some(1),) => {}
-   |         ^^^^^^^^^^ unreachable pattern
+   |         ^^^^^^^^^^------
+   |         |
+   |         unreachable pattern
+   |         help: remove the match arm
 
 error: unreachable pattern
   --> $DIR/exhaustiveness-unreachable-pattern.rs:43:9
@@ -111,15 +138,21 @@ LL |         (None | Some(1 | 2),) => {}
    |         --------------------- matches all the values already
 LL |         (Some(1),) => {}
 LL |         (None,) => {}
-   |         ^^^^^^^ unreachable pattern
+   |         ^^^^^^^------
+   |         |
+   |         unreachable pattern
+   |         help: remove the match arm
 
 error: unreachable pattern
   --> $DIR/exhaustiveness-unreachable-pattern.rs:48:9
    |
 LL |         ((1 | 2,) | (3 | 4,),) => {}
    |         ---------------------- matches all the values already
 LL |         ((1..=4,),) => {}
-   |         ^^^^^^^^^^^ unreachable pattern
+   |         ^^^^^^^^^^^------
+   |         |
+   |         unreachable pattern
+   |         help: remove the match arm
 
 error: unreachable pattern
   --> $DIR/exhaustiveness-unreachable-pattern.rs:53:14

tests/ui/pattern/issue-14221.stderr

@@ -19,7 +19,10 @@ LL |             A => "A",
    |             - matches any value
 LL |
 LL |             B => "B",
-   |             ^ unreachable pattern
+   |             ^--------
+   |             |
+   |             unreachable pattern
+   |             help: remove the match arm
    |
 note: the lint level is defined here
   --> $DIR/issue-14221.rs:1:9