When encountering a match expr with no arms, suggest it

Given

```rust
match Some(42) {}
```

suggest

```rust
match Some(42) { None | Some(_) => todo!(), }
```

Author: estebank

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

@@ -64,7 +64,9 @@ impl<'tcx> Visitor<'tcx> for MatchVisitor<'_, '_, 'tcx> {
     fn visit_expr(&mut self, ex: &'tcx hir::Expr<'tcx>) {
         intravisit::walk_expr(self, ex);
         match &ex.kind {
-            hir::ExprKind::Match(scrut, arms, source) => self.check_match(scrut, arms, *source),
+            hir::ExprKind::Match(scrut, arms, source) => {
+                self.check_match(scrut, arms, *source, ex.span)
+            }
             hir::ExprKind::Let(hir::Let { pat, init, span, .. }) => {
                 self.check_let(pat, init, *span)
             }
@@ -163,6 +165,7 @@ impl<'p, 'tcx> MatchVisitor<'_, 'p, 'tcx> {
         scrut: &hir::Expr<'_>,
         hir_arms: &'tcx [hir::Arm<'tcx>],
         source: hir::MatchSource,
+        expr_span: Span,
     ) {
         let mut cx = self.new_cx(scrut.hir_id);
 
@@ -208,15 +211,14 @@ impl<'p, 'tcx> MatchVisitor<'_, 'p, 'tcx> {
         }
 
         // Check if the match is exhaustive.
-        let is_empty_match = arms.is_empty();
         let witnesses = report.non_exhaustiveness_witnesses;
         if !witnesses.is_empty() {
             if source == hir::MatchSource::ForLoopDesugar && hir_arms.len() == 2 {
                 // the for loop pattern is not irrefutable
                 let pat = hir_arms[1].pat.for_loop_some().unwrap();
                 self.check_irrefutable(pat, "`for` loop binding", None);
             } else {
-                non_exhaustive_match(&cx, scrut_ty, scrut.span, witnesses, is_empty_match);
+                non_exhaustive_match(&cx, scrut_ty, scrut.span, witnesses, hir_arms, expr_span);
             }
         }
     }
@@ -494,21 +496,25 @@ fn non_exhaustive_match<'p, 'tcx>(
     scrut_ty: Ty<'tcx>,
     sp: Span,
     witnesses: Vec<DeconstructedPat<'p, 'tcx>>,
-    is_empty_match: bool,
+    arms: &[hir::Arm<'tcx>],
+    expr_span: Span,
 ) {
+    let is_empty_match = arms.is_empty();
     let non_empty_enum = match scrut_ty.kind() {
         ty::Adt(def, _) => def.is_enum() && !def.variants.is_empty(),
         _ => false,
     };
     // In the case of an empty match, replace the '`_` not covered' diagnostic with something more
     // informative.
     let mut err;
+    let pattern;
     if is_empty_match && !non_empty_enum {
         err = create_e0004(
             cx.tcx.sess,
             sp,
             format!("non-exhaustive patterns: type `{}` is non-empty", scrut_ty),
         );
+        pattern = "_".to_string();
     } else {
         let joined_patterns = joined_uncovered_patterns(cx, &witnesses);
         err = create_e0004(
@@ -517,6 +523,15 @@ fn non_exhaustive_match<'p, 'tcx>(
             format!("non-exhaustive patterns: {} not covered", joined_patterns),
         );
         err.span_label(sp, pattern_not_covered_label(&witnesses, &joined_patterns));
+        pattern = if witnesses.len() < 4 {
+            witnesses
+                .iter()
+                .map(|witness| witness.to_pat(cx).to_string())
+                .collect::<Vec<String>>()
+                .join(" | ")
+        } else {
+            "_".to_string()
+        };
     };
 
     let is_variant_list_non_exhaustive = match scrut_ty.kind() {
@@ -525,10 +540,6 @@ fn non_exhaustive_match<'p, 'tcx>(
     };
 
     adt_defined_here(cx, &mut err, scrut_ty, &witnesses);
-    err.help(
-        "ensure that all possible cases are being handled, \
-              possibly by adding wildcards or more match arms",
-    );
     err.note(&format!(
         "the matched value is of type `{}`{}",
         scrut_ty,
@@ -540,14 +551,14 @@ fn non_exhaustive_match<'p, 'tcx>(
         && matches!(witnesses[0].ctor(), Constructor::NonExhaustive)
     {
         err.note(&format!(
-            "`{}` does not have a fixed maximum value, \
-                so a wildcard `_` is necessary to match exhaustively",
+            "`{}` does not have a fixed maximum value, so a wildcard `_` is necessary to match \
+             exhaustively",
             scrut_ty,
         ));
         if cx.tcx.sess.is_nightly_build() {
             err.help(&format!(
-                "add `#![feature(precise_pointer_size_matching)]` \
-                    to the crate attributes to enable precise `{}` matching",
+                "add `#![feature(precise_pointer_size_matching)]` to the crate attributes to \
+                 enable precise `{}` matching",
                 scrut_ty,
             ));
         }
@@ -557,6 +568,37 @@ fn non_exhaustive_match<'p, 'tcx>(
             err.note("references are always considered inhabited");
         }
     }
+
+    let mut suggestion = None;
+    let sm = cx.tcx.sess.source_map();
+    match arms {
+        [] if sp.ctxt() == expr_span.ctxt() => {
+            // Get the span for the empty match body `{}`.
+            let (indentation, more) = if let Some(snippet) = sm.indentation_before(sp) {
+                (format!("\n{}", snippet), "    ")
+            } else {
+                (" ".to_string(), "")
+            };
+            suggestion = Some((
+                sp.shrink_to_hi().with_hi(expr_span.hi()),
+                format!(
+                    " {{{indentation}{more}{pattern} => todo!(),{indentation}}}",
+                    indentation = indentation,
+                    more = more,
+                    pattern = pattern,
+                ),
+            ));
+        }
+        _ => {}
+    }
+
+    let msg = "ensure that all possible cases are being handled, possibly by adding wildcards \
+        or more match arms";
+    if let Some((span, sugg)) = suggestion {
+        err.span_suggestion_verbose(span, msg, sugg, Applicability::HasPlaceholders);
+    } else {
+        err.help(msg);
+    }
     err.emit();
 }
 

src/test/ui/closures/2229_closure_analysis/match/issue-88331.stderr

@@ -7,8 +7,8 @@ LL | pub struct Opcode(pub u8);
 LL |     move |i| match msg_type {
    |                    ^^^^^^^^ patterns `Opcode(0_u8)` and `Opcode(2_u8..=u8::MAX)` not covered
    |
-   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
    = note: the matched value is of type `Opcode`
+   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 
 error[E0004]: non-exhaustive patterns: `Opcode2(Opcode(0_u8))` and `Opcode2(Opcode(2_u8..=u8::MAX))` not covered
   --> $DIR/issue-88331.rs:27:20
@@ -19,8 +19,8 @@ LL | pub struct Opcode2(Opcode);
 LL |     move |i| match msg_type {
    |                    ^^^^^^^^ patterns `Opcode2(Opcode(0_u8))` and `Opcode2(Opcode(2_u8..=u8::MAX))` not covered
    |
-   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
    = note: the matched value is of type `Opcode2`
+   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 
 error: aborting due to 2 previous errors
 

src/test/ui/closures/2229_closure_analysis/match/non-exhaustive-match.stderr

@@ -10,26 +10,31 @@ LL | enum L1 { A, B }
 LL |     let _b = || { match l1 { L1::A => () } };
    |                         ^^ pattern `B` not covered
    |
-   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
    = note: the matched value is of type `L1`
+   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 
 error[E0004]: non-exhaustive patterns: type `E1` is non-empty
   --> $DIR/non-exhaustive-match.rs:37:25
    |
 LL |     let _d = || { match e1 {} };
    |                         ^^
    |
-   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
    = note: the matched value is of type `E1`, which is marked as non-exhaustive
+help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
+   |
+LL ~     let _d = || { match e1 {
+LL +         _ => todo!(),
+LL ~     } };
+   |
 
 error[E0004]: non-exhaustive patterns: `_` not covered
   --> $DIR/non-exhaustive-match.rs:39:25
    |
 LL |     let _e = || { match e2 { E2::A => (), E2::B => () } };
    |                         ^^ pattern `_` not covered
    |
-   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
    = note: the matched value is of type `E2`, which is marked as non-exhaustive
+   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 
 error[E0505]: cannot move out of `e3` because it is borrowed
   --> $DIR/non-exhaustive-match.rs:46:22

src/test/ui/closures/2229_closure_analysis/match/pattern-matching-should-fail.stderr

@@ -4,8 +4,13 @@ error[E0004]: non-exhaustive patterns: type `u8` is non-empty
 LL |     let c1 = || match x { };
    |                       ^
    |
-   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
    = note: the matched value is of type `u8`
+help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
+   |
+LL ~     let c1 = || match x {
+LL +         _ => todo!(),
+LL ~     };
+   |
 
 error[E0381]: use of possibly-uninitialized variable: `x`
   --> $DIR/pattern-matching-should-fail.rs:8:23

src/test/ui/error-codes/E0004-2.stderr

@@ -12,8 +12,13 @@ LL |     None,
 LL |     Some(#[stable(feature = "rust1", since = "1.0.0")] T),
    |     ---- not covered
    |
-   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
    = note: the matched value is of type `Option<i32>`
+help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
+   |
+LL ~     match x {
+LL +         None | Some(_) => todo!(),
+LL ~     }
+   |
 
 error: aborting due to previous error
 

src/test/ui/error-codes/E0004.stderr

@@ -11,8 +11,8 @@ LL | | }
 LL |       match x {
    |             ^ pattern `HastaLaVistaBaby` not covered
    |
-   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
    = note: the matched value is of type `Terminator`
+   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 
 error: aborting due to previous error
 

src/test/ui/feature-gates/feature-gate-precise_pointer_size_matching.stderr

@@ -4,21 +4,21 @@ error[E0004]: non-exhaustive patterns: `_` not covered
 LL |     match 0usize {
    |           ^^^^^^ pattern `_` not covered
    |
-   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
    = note: the matched value is of type `usize`
    = note: `usize` does not have a fixed maximum value, so a wildcard `_` is necessary to match exhaustively
    = help: add `#![feature(precise_pointer_size_matching)]` to the crate attributes to enable precise `usize` matching
+   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 
 error[E0004]: non-exhaustive patterns: `_` not covered
   --> $DIR/feature-gate-precise_pointer_size_matching.rs:10:11
    |
 LL |     match 0isize {
    |           ^^^^^^ pattern `_` not covered
    |
-   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
    = note: the matched value is of type `isize`
    = note: `isize` does not have a fixed maximum value, so a wildcard `_` is necessary to match exhaustively
    = help: add `#![feature(precise_pointer_size_matching)]` to the crate attributes to enable precise `isize` matching
+   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 
 error: aborting due to 2 previous errors