Don't suggest an arm when suggesting a never pattern

Author: Nadrieril

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

@@ -1020,6 +1020,14 @@ fn report_non_exhaustive_match<'p, 'tcx>(
 
     let mut suggestion = None;
     let sm = cx.tcx.sess.source_map();
+    let suggested_arm = if witnesses.len() < 4
+        && witnesses.iter().all(|p| p.is_never_pattern())
+        && cx.tcx.features().never_patterns
+    {
+        pattern
+    } else {
+        format!("{pattern} => todo!()")
+    };
     match arms {
         [] if sp.eq_ctxt(expr_span) => {
             // Get the span for the empty match body `{}`.
@@ -1030,7 +1038,7 @@ fn report_non_exhaustive_match<'p, 'tcx>(
             };
             suggestion = Some((
                 sp.shrink_to_hi().with_hi(expr_span.hi()),
-                format!(" {{{indentation}{more}{pattern} => todo!(),{indentation}}}",),
+                format!(" {{{indentation}{more}{suggested_arm},{indentation}}}",),
             ));
         }
         [only] => {
@@ -1056,7 +1064,7 @@ fn report_non_exhaustive_match<'p, 'tcx>(
             };
             suggestion = Some((
                 only.span.shrink_to_hi(),
-                format!("{comma}{pre_indentation}{pattern} => todo!()"),
+                format!("{comma}{pre_indentation}{suggested_arm}"),
             ));
         }
         [.., prev, last] => {
@@ -1079,7 +1087,7 @@ fn report_non_exhaustive_match<'p, 'tcx>(
                 if let Some(spacing) = spacing {
                     suggestion = Some((
                         last.span.shrink_to_hi(),
-                        format!("{comma}{spacing}{pattern} => todo!()"),
+                        format!("{comma}{spacing}{suggested_arm}"),
                     ));
                 }
             }

compiler/rustc_pattern_analysis/src/pat.rs

@@ -321,6 +321,14 @@ impl<Cx: TypeCx> WitnessPat<Cx> {
         &self.ty
     }
 
+    pub fn is_never_pattern(&self) -> bool {
+        match self.ctor() {
+            Never => true,
+            Or => self.fields.iter().all(|p| p.is_never_pattern()),
+            _ => self.fields.iter().any(|p| p.is_never_pattern()),
+        }
+    }
+
     pub fn iter_fields(&self) -> impl Iterator<Item = &WitnessPat<Cx>> {
         self.fields.iter()
     }

tests/ui/pattern/usefulness/empty-types.never_pats.stderr

@@ -111,7 +111,7 @@ note: `Result<u32, !>` defined here
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~         Ok(_) => {},
-LL +         Err(!) => todo!()
+LL +         Err(!)
    |
 
 error[E0004]: non-exhaustive patterns: `Ok(1_u32..=u32::MAX)` not covered
@@ -192,7 +192,7 @@ note: `Result<!, !>` defined here
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern, a match arm with multiple or-patterns as shown, or multiple match arms
    |
 LL ~     match result_never {
-LL +         Ok(!) | Err(!) => todo!(),
+LL +         Ok(!) | Err(!),
 LL +     }
    |
 
@@ -210,8 +210,8 @@ note: `Result<!, !>` defined here
    = note: the matched value is of type `Result<!, !>`
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
-LL |         Ok(_) => {}, Err(!) => todo!()
-   |                    +++++++++++++++++++
+LL |         Ok(_) => {}, Err(!)
+   |                    ++++++++
 
 error: unreachable pattern
   --> $DIR/empty-types.rs:140:13
@@ -240,7 +240,7 @@ note: `Option<Void>` defined here
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~             None => {},
-LL +             Some(!) => todo!()
+LL +             Some(!)
    |
 
 error[E0004]: non-exhaustive patterns: `Some(!)` not covered
@@ -258,7 +258,7 @@ note: `Option<Void>` defined here
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~             None => {},
-LL +             Some(!) => todo!()
+LL +             Some(!)
    |
 
 error: unreachable pattern
@@ -343,7 +343,7 @@ note: `Result<!, !>` defined here
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern, a match arm with multiple or-patterns as shown, or multiple match arms
    |
 LL ~     match *x {
-LL +         Ok(!) | Err(!) => todo!(),
+LL +         Ok(!) | Err(!),
 LL ~     }
    |
 
@@ -385,7 +385,7 @@ LL |     match slice_never {
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~         [] => {},
-LL +         &[!, ..] => todo!()
+LL +         &[!, ..]
    |
 
 error[E0004]: non-exhaustive patterns: `&[]`, `&[!]` and `&[!, !]` not covered
@@ -492,7 +492,7 @@ note: `Option<!>` defined here
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~         &None => {},
-LL +         &Some(!) => todo!()
+LL +         &Some(!)
    |
 
 error[E0004]: non-exhaustive patterns: `Some(!)` not covered
@@ -510,7 +510,7 @@ note: `Option<!>` defined here
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~         None => {},
-LL +         Some(!) => todo!()
+LL +         Some(!)
    |
 
 error[E0004]: non-exhaustive patterns: `Err(!)` not covered
@@ -528,7 +528,7 @@ note: `Result<!, !>` defined here
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~         Ok(_) => {},
-LL +         Err(!) => todo!()
+LL +         Err(!)
    |
 
 error[E0004]: non-exhaustive patterns: `Err(!)` not covered
@@ -546,7 +546,7 @@ note: `Result<!, !>` defined here
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~         Ok(_a) => {},
-LL +         Err(!) => todo!()
+LL +         Err(!)
    |
 
 error[E0004]: non-exhaustive patterns: type `(u32, !)` is non-empty
@@ -599,7 +599,7 @@ LL |     match ref_never {
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~         &_a if false => {},
-LL +         &! => todo!()
+LL +         &!
    |
 
 error[E0004]: non-exhaustive patterns: `Ok(!)` not covered
@@ -617,7 +617,7 @@ note: `Result<!, !>` defined here
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~         Err(_) => {},
-LL +         Ok(!) => todo!()
+LL +         Ok(!)
    |
 
 error[E0004]: non-exhaustive patterns: `Some(!)` not covered
@@ -635,7 +635,7 @@ note: `Option<Result<!, !>>` defined here
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~         None => {},
-LL +         Some(!) => todo!()
+LL +         Some(!)
    |
 
 error: aborting due to 49 previous errors; 1 warning emitted

tests/ui/rfcs/rfc-0000-never_patterns/check.stderr

@@ -46,7 +46,7 @@ note: `Option<Void>` defined here
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~         None => {},
-LL +         Some(!) => todo!()
+LL +         Some(!)
    |
 
 error[E0004]: non-exhaustive patterns: `Some(!)` not covered
@@ -64,7 +64,7 @@ note: `Option<Void>` defined here
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~         None => {},
-LL +         Some(!) => todo!()
+LL +         Some(!)
    |
 
 error: aborting due to 6 previous errors