When finding a match expr with a single arm that requires more, suggest it

Given

```rust
match Some(42) {
    Some(0) => {}
}
```

suggest

```rust
match Some(42) {
    Some(0) => {}
    None | Some(_) => todo!(),
}
```

Author: estebank

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

@@ -591,6 +591,21 @@ fn non_exhaustive_match<'p, 'tcx>(
                 ),
             ));
         }
+        [only] => {
+            let pre_indentation = if let (Some(snippet), true) = (
+                sm.indentation_before(only.span),
+                sm.is_multiline(sp.shrink_to_hi().with_hi(only.span.lo())),
+            ) {
+                format!("\n{}", snippet)
+            } else {
+                " ".to_string()
+            };
+            let comma = if matches!(only.body.kind, hir::ExprKind::Block(..)) { "" } else { "," };
+            suggestion = Some((
+                only.span.shrink_to_hi(),
+                format!("{}{}{} => todo!()", comma, pre_indentation, pattern),
+            ));
+        }
         _ => {}
     }
 

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

@@ -8,7 +8,11 @@ LL |     move |i| match msg_type {
    |                    ^^^^^^^^ patterns `Opcode(0_u8)` and `Opcode(2_u8..=u8::MAX)` not covered
    |
    = 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
+help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
+   |
+LL ~         Opcode::OP1 => unimplemented!(),
+LL ~         Opcode(0_u8) | Opcode(2_u8..=u8::MAX) => todo!(),
+   |
 
 error[E0004]: non-exhaustive patterns: `Opcode2(Opcode(0_u8))` and `Opcode2(Opcode(2_u8..=u8::MAX))` not covered
   --> $DIR/issue-88331.rs:27:20
@@ -20,7 +24,11 @@ LL |     move |i| match msg_type {
    |                    ^^^^^^^^ patterns `Opcode2(Opcode(0_u8))` and `Opcode2(Opcode(2_u8..=u8::MAX))` not covered
    |
    = 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
+help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
+   |
+LL ~         Opcode2::OP2=> unimplemented!(),
+LL ~         Opcode2(Opcode(0_u8)) | Opcode2(Opcode(2_u8..=u8::MAX)) => todo!(),
+   |
 
 error: aborting due to 2 previous errors
 

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

@@ -11,7 +11,10 @@ LL |     let _b = || { match l1 { L1::A => () } };
    |                         ^^ pattern `B` not covered
    |
    = 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
+help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
+   |
+LL |     let _b = || { match l1 { L1::A => (), B => todo!() } };
+   |                                         ++++++++++++++
 
 error[E0004]: non-exhaustive patterns: type `E1` is non-empty
   --> $DIR/non-exhaustive-match.rs:37:25

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

@@ -12,7 +12,11 @@ LL |       match x {
    |             ^ pattern `HastaLaVistaBaby` not covered
    |
    = 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
+help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
+   |
+LL ~         Terminator::TalkToMyHand => {}
+LL +         HastaLaVistaBaby => todo!()
+   |
 
 error: aborting due to previous error
 

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

@@ -7,7 +7,11 @@ LL |     match 0usize {
    = 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
+help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
+   |
+LL ~         0..=usize::MAX => {}
+LL +         _ => todo!()
+   |
 
 error[E0004]: non-exhaustive patterns: `_` not covered
   --> $DIR/feature-gate-precise_pointer_size_matching.rs:10:11
@@ -18,7 +22,11 @@ LL |     match 0isize {
    = 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
+help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
+   |
+LL ~         isize::MIN..=isize::MAX => {}
+LL +         _ => todo!()
+   |
 
 error: aborting due to 2 previous errors