Fix suggestion to slice if scrutinee is a Result or Option

compiler/rustc_typeck/src/check/pat.rs

@@ -15,7 +15,7 @@ use rustc_session::lint::builtin::NON_EXHAUSTIVE_OMITTED_PATTERNS;
 use rustc_span::hygiene::DesugaringKind;
 use rustc_span::lev_distance::find_best_match_for_name;
 use rustc_span::source_map::{Span, Spanned};
-use rustc_span::symbol::Ident;
+use rustc_span::symbol::{sym, Ident};
 use rustc_span::{BytePos, MultiSpan, DUMMY_SP};
 use rustc_trait_selection::autoderef::Autoderef;
 use rustc_trait_selection::traits::{ObligationCause, Pattern};
@@ -2033,12 +2033,39 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         {
             if let (Some(span), true) = (ti.span, ti.origin_expr) {
                 if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
-                    err.span_suggestion(
-                        span,
-                        "consider slicing here",
-                        format!("{}[..]", snippet),
-                        Applicability::MachineApplicable,
-                    );
+                    let applicability = match self.resolve_vars_if_possible(ti.expected).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,
+                            );
+                            None
+                        }
+                        // FIXME: instead of checking for Vec only, we could check whether the
+                        // type implements `Deref<Target=X>`; see
+                        // https://github.com/rust-lang/rust/pull/91343#discussion_r761466979
+                        ty::Adt(adt_def, _)
+                            if self.tcx.is_diagnostic_item(sym::Vec, adt_def.did) =>
+                        {
+                            Some(Applicability::MachineApplicable)
+                        }
+                        _ => Some(Applicability::MaybeIncorrect),
+                    };
+
+                    if let Some(applicability) = applicability {
+                        err.span_suggestion(
+                            span,
+                            "consider slicing here",
+                            format!("{}[..]", snippet),
+                            applicability,
+                        );
+                    }
                 }
             }
         }

src/test/ui/typeck/issue-91328.fixed

@@ -0,0 +1,37 @@
+// Regression test for issue #91328.
+
+// run-rustfix
+
+#![allow(dead_code)]
+
+fn foo(r: Result<Vec<i32>, i32>) -> i32 {
+    match r.as_deref() {
+    //~^ HELP: consider using `as_deref` here
+        Ok([a, b]) => a + b,
+        //~^ ERROR: expected an array or slice
+        //~| NOTE: pattern cannot match with input type
+        _ => 42,
+    }
+}
+
+fn bar(o: Option<Vec<i32>>) -> i32 {
+    match o.as_deref() {
+    //~^ HELP: consider using `as_deref` here
+        Some([a, b]) => a + b,
+        //~^ ERROR: expected an array or slice
+        //~| NOTE: pattern cannot match with input type
+        _ => 42,
+    }
+}
+
+fn baz(v: Vec<i32>) -> i32 {
+    match v[..] {
+    //~^ HELP: consider slicing here
+        [a, b] => a + b,
+        //~^ ERROR: expected an array or slice
+        //~| NOTE: pattern cannot match with input type
+        _ => 42,
+    }
+}
+
+fn main() {}

src/test/ui/typeck/issue-91328.rs

@@ -0,0 +1,37 @@
+// Regression test for issue #91328.
+
+// run-rustfix
+
+#![allow(dead_code)]
+
+fn foo(r: Result<Vec<i32>, i32>) -> i32 {
+    match r {
+    //~^ HELP: consider using `as_deref` here
+        Ok([a, b]) => a + b,
+        //~^ ERROR: expected an array or slice
+        //~| NOTE: pattern cannot match with input type
+        _ => 42,
+    }
+}
+
+fn bar(o: Option<Vec<i32>>) -> i32 {
+    match o {
+    //~^ HELP: consider using `as_deref` here
+        Some([a, b]) => a + b,
+        //~^ ERROR: expected an array or slice
+        //~| NOTE: pattern cannot match with input type
+        _ => 42,
+    }
+}
+
+fn baz(v: Vec<i32>) -> i32 {
+    match v {
+    //~^ HELP: consider slicing here
+        [a, b] => a + b,
+        //~^ ERROR: expected an array or slice
+        //~| NOTE: pattern cannot match with input type
+        _ => 42,
+    }
+}
+
+fn main() {}

src/test/ui/typeck/issue-91328.stderr

@@ -0,0 +1,30 @@
+error[E0529]: expected an array or slice, found `Vec<i32>`
+  --> $DIR/issue-91328.rs:10:12
+   |
+LL |     match r {
+   |           - help: consider using `as_deref` here: `r.as_deref()`
+LL |
+LL |         Ok([a, b]) => a + b,
+   |            ^^^^^^ pattern cannot match with input type `Vec<i32>`
+
+error[E0529]: expected an array or slice, found `Vec<i32>`
+  --> $DIR/issue-91328.rs:20:14
+   |
+LL |     match o {
+   |           - help: consider using `as_deref` here: `o.as_deref()`
+LL |
+LL |         Some([a, b]) => a + b,
+   |              ^^^^^^ pattern cannot match with input type `Vec<i32>`
+
+error[E0529]: expected an array or slice, found `Vec<i32>`
+  --> $DIR/issue-91328.rs:30:9
+   |
+LL |     match v {
+   |           - help: consider slicing here: `v[..]`
+LL |
+LL |         [a, b] => a + b,
+   |         ^^^^^^ pattern cannot match with input type `Vec<i32>`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0529`.