Check bindings around never patterns

rust-lang · Jan 5, 2024 · 997103a · 997103a
1 parent fb6d10d
commit 997103a
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diff --git a/compiler/rustc_resolve/messages.ftl b/compiler/rustc_resolve/messages.ftl
@@ -36,6 +36,10 @@ resolve_attempt_to_use_non_constant_value_in_constant_with_suggestion =
 resolve_attempt_to_use_non_constant_value_in_constant_without_suggestion =
     this would need to be a `{$suggestion}`
 
+resolve_binding_in_never_pattern =
+    never patterns cannot contain variable bindings
+    .suggestion = use a wildcard `_` instead
+
 resolve_binding_shadows_something_unacceptable =
     {$shadowing_binding}s cannot shadow {$shadowed_binding}s
     .label = cannot be named the same as {$article} {$shadowed_binding}

diff --git a/compiler/rustc_resolve/src/diagnostics.rs b/compiler/rustc_resolve/src/diagnostics.rs
@@ -958,6 +958,9 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                 .create_err(errs::TraitImplDuplicate { span, name, trait_item_span, old_span }),
             ResolutionError::InvalidAsmSym => self.dcx().create_err(errs::InvalidAsmSym { span }),
             ResolutionError::LowercaseSelf => self.dcx().create_err(errs::LowercaseSelf { span }),
+            ResolutionError::BindingInNeverPattern => {
+                self.dcx().create_err(errs::BindingInNeverPattern { span })
+            }
         }
     }
 

diff --git a/compiler/rustc_resolve/src/errors.rs b/compiler/rustc_resolve/src/errors.rs
@@ -486,6 +486,15 @@ pub(crate) struct LowercaseSelf {
     pub(crate) span: Span,
 }
 
+#[derive(Debug)]
+#[derive(Diagnostic)]
+#[diag(resolve_binding_in_never_pattern)]
+pub(crate) struct BindingInNeverPattern {
+    #[primary_span]
+    #[suggestion(code = "_", applicability = "machine-applicable", style = "short")]
+    pub(crate) span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(resolve_trait_impl_duplicate, code = "E0201")]
 pub(crate) struct TraitImplDuplicate {

diff --git a/compiler/rustc_resolve/src/late.rs b/compiler/rustc_resolve/src/late.rs
@@ -65,6 +65,8 @@ enum IsRepeatExpr {
     Yes,
 }
 
+struct IsNeverPattern;
+
 /// Describes whether an `AnonConst` is a type level const arg or
 /// some other form of anon const (i.e. inline consts or enum discriminants)
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
@@ -3182,11 +3184,15 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
     }
 
     /// Build a map from pattern identifiers to binding-info's, and check the bindings are
-    /// consistent when encountering or-patterns.
+    /// consistent when encountering or-patterns and never patterns.
     /// This is done hygienically: this could arise for a macro that expands into an or-pattern
     /// where one 'x' was from the user and one 'x' came from the macro.
-    fn compute_and_check_binding_map(&mut self, pat: &Pat) -> FxIndexMap<Ident, BindingInfo> {
+    fn compute_and_check_binding_map(
+        &mut self,
+        pat: &Pat,
+    ) -> Result<FxIndexMap<Ident, BindingInfo>, IsNeverPattern> {
         let mut binding_map = FxIndexMap::default();
+        let mut is_never_pat = false;
 
         pat.walk(&mut |pat| {
             match pat.kind {
@@ -3198,17 +3204,26 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
                 PatKind::Or(ref ps) => {
                     // Check the consistency of this or-pattern and
                     // then add all bindings to the larger map.
-                    let bm = self.compute_and_check_or_pat_binding_map(ps);
+                    let (bm, np) = self.compute_and_check_or_pat_binding_map(ps);
                     binding_map.extend(bm);
+                    is_never_pat |= np;
                     return false;
                 }
+                PatKind::Never => is_never_pat = true,
                 _ => {}
             }
 
             true
         });
 
-        binding_map
+        if is_never_pat {
+            for (_, binding) in binding_map {
+                self.report_error(binding.span, ResolutionError::BindingInNeverPattern);
+            }
+            Err(IsNeverPattern)
+        } else {
+            Ok(binding_map)
+        }
     }
 
     fn is_base_res_local(&self, nid: NodeId) -> bool {
@@ -3220,24 +3235,29 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
 
     /// Compute the binding map for an or-pattern. Checks that all of the arms in the or-pattern
     /// have exactly the same set of bindings, with the same binding modes for each.
-    /// Returns the computed binding map.
+    /// Returns the computed binding map and a boolean indicating whether the pattern is a never
+    /// pattern.
     fn compute_and_check_or_pat_binding_map(
         &mut self,
         pats: &[P<Pat>],
-    ) -> FxIndexMap<Ident, BindingInfo> {
+    ) -> (FxIndexMap<Ident, BindingInfo>, bool) {
         let mut missing_vars = FxIndexMap::default();
         let mut inconsistent_vars = FxIndexMap::default();
 
-        // 1) Compute the binding maps of all arms.
-        let maps =
-            pats.iter().map(|pat| self.compute_and_check_binding_map(pat)).collect::<Vec<_>>();
+        // 1) Compute the binding maps of all arms; never patterns don't participate in this.
+        let not_never_pats = pats
+            .iter()
+            .filter_map(|pat| {
+                let binding_map = self.compute_and_check_binding_map(pat).ok()?;
+                Some((binding_map, pat))
+            })
+            .collect::<Vec<_>>();
 
         // 2) Record any missing bindings or binding mode inconsistencies.
-        for (map_outer, pat_outer) in maps.iter().zip(pats.iter()) {
+        for (map_outer, pat_outer) in not_never_pats.iter() {
             // Check against all arms except for the same pattern which is always self-consistent.
-            let inners = maps
+            let inners = not_never_pats
                 .iter()
-                .zip(pats.iter())
                 .filter(|(_, pat)| pat.id != pat_outer.id)
                 .flat_map(|(map, _)| map);
 
@@ -3285,22 +3305,17 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
         }
 
         // 5) Bubble up the final binding map.
+        let is_never_pat = not_never_pats.is_empty();
         let mut binding_map = FxIndexMap::default();
-        for bm in maps {
+        for (bm, _) in not_never_pats {
             binding_map.extend(bm);
         }
-        binding_map
+        (binding_map, is_never_pat)
     }
 
-    /// Check the consistency of bindings wrt or-patterns.
+    /// Check the consistency of bindings wrt or-patterns and never patterns.
     fn check_consistent_bindings(&mut self, pat: &'ast Pat) {
-        pat.walk(&mut |pat| match pat.kind {
-            PatKind::Or(ref ps) => {
-                let _ = self.compute_and_check_or_pat_binding_map(ps);
-                false
-            }
-            _ => true,
-        })
+        let _ = self.compute_and_check_binding_map(pat);
     }
 
     fn resolve_arm(&mut self, arm: &'ast Arm) {

diff --git a/compiler/rustc_resolve/src/lib.rs b/compiler/rustc_resolve/src/lib.rs
@@ -265,6 +265,8 @@ enum ResolutionError<'a> {
     InvalidAsmSym,
     /// `self` used instead of `Self` in a generic parameter
     LowercaseSelf,
+    /// A never pattern has a binding.
+    BindingInNeverPattern,
 }
 
 enum VisResolutionError<'a> {

diff --git a/tests/ui/feature-gates/feature-gate-never_patterns.rs b/tests/ui/feature-gates/feature-gate-never_patterns.rs
@@ -7,7 +7,6 @@ fn main() {
     let res: Result<u32, Void> = Ok(0);
     let (Ok(_x) | Err(&!)) = res.as_ref();
     //~^ ERROR `!` patterns are experimental
-    //~| ERROR: is not bound in all patterns
 
     unsafe {
         let ptr: *const Void = NonNull::dangling().as_ptr();

diff --git a/tests/ui/feature-gates/feature-gate-never_patterns.stderr b/tests/ui/feature-gates/feature-gate-never_patterns.stderr
@@ -1,5 +1,5 @@
 error: unexpected `,` in pattern
-  --> $DIR/feature-gate-never_patterns.rs:34:16
+  --> $DIR/feature-gate-never_patterns.rs:33:16
    |
 LL |         Some(_),
    |                ^
@@ -13,14 +13,6 @@ help: ...or a vertical bar to match on multiple alternatives
 LL |         Some(_) |
    |
 
-error[E0408]: variable `_x` is not bound in all patterns
-  --> $DIR/feature-gate-never_patterns.rs:8:19
-   |
-LL |     let (Ok(_x) | Err(&!)) = res.as_ref();
-   |             --    ^^^^^^^ pattern doesn't bind `_x`
-   |             |
-   |             variable not in all patterns
-
 error[E0658]: `!` patterns are experimental
   --> $DIR/feature-gate-never_patterns.rs:8:24
    |
@@ -31,7 +23,7 @@ LL |     let (Ok(_x) | Err(&!)) = res.as_ref();
    = help: add `#![feature(never_patterns)]` to the crate attributes to enable
 
 error[E0658]: `!` patterns are experimental
-  --> $DIR/feature-gate-never_patterns.rs:15:13
+  --> $DIR/feature-gate-never_patterns.rs:14:13
    |
 LL |             !
    |             ^
@@ -40,7 +32,7 @@ LL |             !
    = help: add `#![feature(never_patterns)]` to the crate attributes to enable
 
 error[E0658]: `!` patterns are experimental
-  --> $DIR/feature-gate-never_patterns.rs:21:13
+  --> $DIR/feature-gate-never_patterns.rs:20:13
    |
 LL |             !
    |             ^
@@ -49,7 +41,7 @@ LL |             !
    = help: add `#![feature(never_patterns)]` to the crate attributes to enable
 
 error[E0658]: `!` patterns are experimental
-  --> $DIR/feature-gate-never_patterns.rs:26:13
+  --> $DIR/feature-gate-never_patterns.rs:25:13
    |
 LL |             ! => {}
    |             ^
@@ -58,25 +50,25 @@ LL |             ! => {}
    = help: add `#![feature(never_patterns)]` to the crate attributes to enable
 
 error: `match` arm with no body
-  --> $DIR/feature-gate-never_patterns.rs:39:9
+  --> $DIR/feature-gate-never_patterns.rs:38:9
    |
 LL |         Some(_)
    |         ^^^^^^^- help: add a body after the pattern: `=> todo!(),`
 
 error: `match` arm with no body
-  --> $DIR/feature-gate-never_patterns.rs:44:9
+  --> $DIR/feature-gate-never_patterns.rs:43:9
    |
 LL |         Some(_) if false,
    |         ^^^^^^^- help: add a body after the pattern: `=> todo!(),`
 
 error: `match` arm with no body
-  --> $DIR/feature-gate-never_patterns.rs:46:9
+  --> $DIR/feature-gate-never_patterns.rs:45:9
    |
 LL |         Some(_) if false
    |         ^^^^^^^- help: add a body after the pattern: `=> todo!(),`
 
 error[E0658]: `!` patterns are experimental
-  --> $DIR/feature-gate-never_patterns.rs:51:13
+  --> $DIR/feature-gate-never_patterns.rs:50:13
    |
 LL |         Err(!),
    |             ^
@@ -85,7 +77,7 @@ LL |         Err(!),
    = help: add `#![feature(never_patterns)]` to the crate attributes to enable
 
 error[E0658]: `!` patterns are experimental
-  --> $DIR/feature-gate-never_patterns.rs:55:13
+  --> $DIR/feature-gate-never_patterns.rs:54:13
    |
 LL |         Err(!) if false,
    |             ^
@@ -94,24 +86,23 @@ LL |         Err(!) if false,
    = help: add `#![feature(never_patterns)]` to the crate attributes to enable
 
 error: `match` arm with no body
-  --> $DIR/feature-gate-never_patterns.rs:65:9
+  --> $DIR/feature-gate-never_patterns.rs:64:9
    |
 LL |         Some(_)
    |         ^^^^^^^- help: add a body after the pattern: `=> todo!(),`
 
 error: `match` arm with no body
-  --> $DIR/feature-gate-never_patterns.rs:71:9
+  --> $DIR/feature-gate-never_patterns.rs:70:9
    |
 LL |         Some(_) if false
    |         ^^^^^^^- help: add a body after the pattern: `=> todo!(),`
 
 error: a guard on a never pattern will never be run
-  --> $DIR/feature-gate-never_patterns.rs:55:19
+  --> $DIR/feature-gate-never_patterns.rs:54:19
    |
 LL |         Err(!) if false,
    |                   ^^^^^ help: remove this guard
 
-error: aborting due to 14 previous errors
+error: aborting due to 13 previous errors
 
-Some errors have detailed explanations: E0408, E0658.
-For more information about an error, try `rustc --explain E0408`.
+For more information about this error, try `rustc --explain E0658`.
diff --git a/tests/ui/pattern/never_patterns.rs b/tests/ui/pattern/never_patterns.rs
@@ -7,12 +7,9 @@ fn main() {}
 
 // The classic use for empty types.
 fn safe_unwrap_result<T>(res: Result<T, Void>) {
-    let Ok(_x) = res;
-    // FIXME(never_patterns): These should be allowed
+    let Ok(_x) = res; //~ ERROR refutable pattern in local binding
     let (Ok(_x) | Err(!)) = &res;
-    //~^ ERROR: is not bound in all patterns
     let (Ok(_x) | Err(&!)) = res.as_ref();
-    //~^ ERROR: is not bound in all patterns
 }
 
 // Check we only accept `!` where we want to.

diff --git a/tests/ui/pattern/never_patterns.stderr b/tests/ui/pattern/never_patterns.stderr
@@ -1,19 +1,17 @@
-error[E0408]: variable `_x` is not bound in all patterns
-  --> $DIR/never_patterns.rs:12:19
+error[E0005]: refutable pattern in local binding
+  --> $DIR/never_patterns.rs:10:9
    |
-LL |     let (Ok(_x) | Err(!)) = &res;
-   |             --    ^^^^^^ pattern doesn't bind `_x`
-   |             |
-   |             variable not in all patterns
-
-error[E0408]: variable `_x` is not bound in all patterns
-  --> $DIR/never_patterns.rs:14:19
+LL |     let Ok(_x) = res;
+   |         ^^^^^^ pattern `Err(_)` not covered
+   |
+   = note: `let` bindings require an "irrefutable pattern", like a `struct` or an `enum` with only one variant
+   = note: for more information, visit https://doc.rust-lang.org/book/ch18-02-refutability.html
+   = note: the matched value is of type `Result<T, Void>`
+help: you might want to use `let else` to handle the variant that isn't matched
    |
-LL |     let (Ok(_x) | Err(&!)) = res.as_ref();
-   |             --    ^^^^^^^ pattern doesn't bind `_x`
-   |             |
-   |             variable not in all patterns
+LL |     let Ok(_x) = res else { todo!() };
+   |                      ++++++++++++++++
 
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0408`.
+For more information about this error, try `rustc --explain E0005`.
diff --git a/tests/ui/rfcs/rfc-0000-never_patterns/bindings.rs b/tests/ui/rfcs/rfc-0000-never_patterns/bindings.rs
@@ -6,23 +6,20 @@ enum Void {}
 fn main() {
     let x: Result<bool, &(u32, u32, Void)> = Ok(false);
 
-    // FIXME(never_patterns): Never patterns in or-patterns don't need to share the same bindings.
     match x {
         Ok(_x) | Err(&!) => {}
-        //~^ ERROR: is not bound in all patterns
     }
     let (Ok(_x) | Err(&!)) = x;
-    //~^ ERROR: is not bound in all patterns
 
-    // FIXME(never_patterns): A never pattern mustn't have bindings.
     match x {
         Ok(_) => {}
         Err(&(_a, _b, !)),
+        //~^ ERROR: never patterns cannot contain variable bindings
+        //~| ERROR: never patterns cannot contain variable bindings
     }
     match x {
         Ok(_ok) | Err(&(_a, _b, !)) => {}
-        //~^ ERROR: is not bound in all patterns
-        //~| ERROR: is not bound in all patterns
-        //~| ERROR: is not bound in all patterns
+        //~^ ERROR: never patterns cannot contain variable bindings
+        //~| ERROR: never patterns cannot contain variable bindings
     }
 }