Fix an invalid error for a suggestion to add a slice in pattern-matching

compiler/rustc_typeck/src/check/pat.rs

@@ -2044,63 +2044,60 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             self.tcx.sess,
             span,
             E0529,
-            "expected an array or slice, found `{}`",
-            expected_ty
+            "expected an array or slice, found `{expected_ty}`"
         );
-        if let ty::Ref(_, ty, _) = expected_ty.kind() {
-            if let ty::Array(..) | ty::Slice(..) = ty.kind() {
-                err.help("the semantics of slice patterns changed recently; see issue #62254");
-            }
+        if let ty::Ref(_, ty, _) = expected_ty.kind()
+            && let ty::Array(..) | ty::Slice(..) = ty.kind()
+        {
+            err.help("the semantics of slice patterns changed recently; see issue #62254");
         } else if Autoderef::new(&self.infcx, self.param_env, self.body_id, span, expected_ty, span)
             .any(|(ty, _)| matches!(ty.kind(), ty::Slice(..) | ty::Array(..)))
+            && let (Some(span), true) = (ti.span, ti.origin_expr)
+            && let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span)
         {
-            if let (Some(span), true) = (ti.span, ti.origin_expr) {
-                if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
-                    let applicability = Autoderef::new(
-                        &self.infcx,
-                        self.param_env,
-                        self.body_id,
-                        span,
-                        self.resolve_vars_if_possible(ti.expected),
+            let ty = self.resolve_vars_if_possible(ti.expected);
+            let is_slice_or_array_or_vector = self.is_slice_or_array_or_vector(&mut err, snippet.clone(), ty);
+            match is_slice_or_array_or_vector.1.kind() {
+                ty::Adt(adt_def, _)
+                    if self.tcx.is_diagnostic_item(sym::Option, adt_def.did())
+                        || self.tcx.is_diagnostic_item(sym::Result, adt_def.did()) =>
+                {
+                    // Slicing won't work here, but `.as_deref()` might (issue #91328).
+                    err.span_suggestion(
                         span,
-                    )
-                    .find_map(|(ty, _)| {
-                        match ty.kind() {
-                            ty::Adt(adt_def, _)
-                                if self.tcx.is_diagnostic_item(sym::Option, adt_def.did())
-                                    || self.tcx.is_diagnostic_item(sym::Result, adt_def.did()) =>
-                            {
-                                // Slicing won't work here, but `.as_deref()` might (issue #91328).
-                                err.span_suggestion(
-                                    span,
-                                    "consider using `as_deref` here",
-                                    format!("{}.as_deref()", snippet),
-                                    Applicability::MaybeIncorrect,
-                                );
-                                Some(None)
-                            }
-
-                            ty::Slice(..) | ty::Array(..) => {
-                                Some(Some(Applicability::MachineApplicable))
-                            }
-
-                            _ => None,
-                        }
-                    })
-                    .unwrap_or(Some(Applicability::MaybeIncorrect));
-
-                    if let Some(applicability) = applicability {
-                        err.span_suggestion(
-                            span,
-                            "consider slicing here",
-                            format!("{}[..]", snippet),
-                            applicability,
-                        );
-                    }
+                        "consider using `as_deref` here",
+                        format!("{snippet}.as_deref()"),
+                        Applicability::MaybeIncorrect,
+                    );
                 }
+                _ => ()
+            }
+            if is_slice_or_array_or_vector.0 {
+                err.span_suggestion(
+                    span,
+                    "consider slicing here",
+                    format!("{snippet}[..]"),
+                    Applicability::MachineApplicable,
+                );
             }
         }
-        err.span_label(span, format!("pattern cannot match with input type `{}`", expected_ty));
+        err.span_label(span, format!("pattern cannot match with input type `{expected_ty}`"));
         err.emit();
     }
+
+    fn is_slice_or_array_or_vector(
+        &self,
+        err: &mut Diagnostic,
+        snippet: String,
+        ty: Ty<'tcx>,
+    ) -> (bool, Ty<'tcx>) {
+        match ty.kind() {
+            ty::Adt(adt_def, _) if self.tcx.is_diagnostic_item(sym::Vec, adt_def.did()) => {
+                (true, ty)
+            }
+            ty::Ref(_, ty, _) => self.is_slice_or_array_or_vector(err, snippet, *ty),
+            ty::Slice(..) | ty::Array(..) => (true, ty),
+            _ => (false, ty),
+        }
+    }
 }

src/test/ui/suggestions/pattern-struct-with-slice-vec-field.rs

@@ -0,0 +1,35 @@
+use std::ops::Deref;
+
+struct Foo {
+    v: Vec<u32>,
+}
+
+struct Bar {
+    v: Vec<u32>,
+}
+
+impl Deref for Bar {
+    type Target = Vec<u32>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.v
+    }
+}
+
+fn f(foo: &Foo) {
+    match foo {
+        Foo { v: [1, 2] } => {}
+        //~^ ERROR expected an array or slice, found `Vec<u32>
+        _ => {}
+    }
+}
+
+fn bar(bar: &Bar) {
+    match bar {
+        Bar { v: [1, 2] } => {}
+        //~^ ERROR expected an array or slice, found `Vec<u32>
+        _ => {}
+    }
+}
+
+fn main() {}

src/test/ui/suggestions/pattern-struct-with-slice-vec-field.stderr

@@ -0,0 +1,15 @@
+error[E0529]: expected an array or slice, found `Vec<u32>`
+  --> $DIR/pattern-struct-with-slice-vec-field.rs:21:18
+   |
+LL |         Foo { v: [1, 2] } => {}
+   |                  ^^^^^^ pattern cannot match with input type `Vec<u32>`
+
+error[E0529]: expected an array or slice, found `Vec<u32>`
+  --> $DIR/pattern-struct-with-slice-vec-field.rs:29:18
+   |
+LL |         Bar { v: [1, 2] } => {}
+   |                  ^^^^^^ pattern cannot match with input type `Vec<u32>`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0529`.