Provide suggestion for missing fields in patterns

Author: estebank

compiler/rustc_typeck/src/check/pat.rs

@@ -1121,7 +1121,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             if no_accessible_unmentioned_fields {
                 unmentioned_err = Some(self.error_no_accessible_fields(pat, &fields));
             } else {
-                unmentioned_err = Some(self.error_unmentioned_fields(pat, &unmentioned_fields));
+                unmentioned_err =
+                    Some(self.error_unmentioned_fields(pat, &unmentioned_fields, &fields));
             }
         }
         match (inexistent_fields_err, unmentioned_err) {
@@ -1376,6 +1377,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         &self,
         pat: &Pat<'_>,
         unmentioned_fields: &[(&ty::FieldDef, Ident)],
+        fields: &'tcx [hir::FieldPat<'tcx>],
     ) -> DiagnosticBuilder<'tcx> {
         let field_names = if unmentioned_fields.len() == 1 {
             format!("field `{}`", unmentioned_fields[0].1)
@@ -1395,14 +1397,52 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             field_names
         );
         err.span_label(pat.span, format!("missing {}", field_names));
-        if self.tcx.sess.teach(&err.get_code().unwrap()) {
-            err.note(
-                "This error indicates that a pattern for a struct fails to specify a \
-                 sub-pattern for every one of the struct's fields. Ensure that each field \
-                 from the struct's definition is mentioned in the pattern, or use `..` to \
-                 ignore unwanted fields.",
-            );
-        }
+        let len = unmentioned_fields.len();
+        let (prefix, postfix, sp) = match fields {
+            [] => match &pat.kind {
+                PatKind::Struct(path, [], false) => {
+                    (" { ", " }", path.span().shrink_to_hi().until(pat.span.shrink_to_hi()))
+                }
+                _ => return err,
+            },
+            [.., field] => (
+                match pat.kind {
+                    PatKind::Struct(_, [_, ..], _) => ", ",
+                    _ => "",
+                },
+                "",
+                field.span.shrink_to_hi(),
+            ),
+        };
+        err.span_suggestion(
+            sp,
+            &format!(
+                "include the missing field{} in the pattern",
+                if len == 1 { "" } else { "s" },
+            ),
+            format!(
+                "{}{}{}",
+                prefix,
+                unmentioned_fields
+                    .iter()
+                    .map(|(_, name)| name.to_string())
+                    .collect::<Vec<_>>()
+                    .join(", "),
+                postfix,
+            ),
+            Applicability::MachineApplicable,
+        );
+        err.span_suggestion(
+            sp,
+            &format!(
+                "if you don't care about {} missing field{}, you can explicitely ignore {}",
+                if len == 1 { "this" } else { "these" },
+                if len == 1 { "" } else { "s" },
+                if len == 1 { "it" } else { "them" },
+            ),
+            format!("{}..{}", prefix, postfix),
+            Applicability::MachineApplicable,
+        );
         err
     }
 

src/test/ui/error-codes/E0027-teach.rs

@@ -7,7 +7,9 @@ fn main() {
     let d = Dog { name: "Rusty".to_string(), age: 8 };
 
     match d {
-        Dog { age: x } => {}
-        //~^ ERROR pattern does not mention field `name`
+        Dog { age: x } => {} //~ ERROR pattern does not mention field `name`
+    }
+    match d {
+        Dog {} => {} //~ ERROR pattern does not mention fields `name`, `age`
     }
 }

src/test/ui/error-codes/E0027-teach.stderr

@@ -3,7 +3,31 @@ error[E0027]: pattern does not mention field `name`
    |
 LL |         Dog { age: x } => {}
    |         ^^^^^^^^^^^^^^ missing field `name`
+   |
+help: include the missing field in the pattern
+   |
+LL |         Dog { age: x, name } => {}
+   |                     ^^^^^^
+help: if you don't care about this missing field, you can explicitely ignore it
+   |
+LL |         Dog { age: x, .. } => {}
+   |                     ^^^^
+
+error[E0027]: pattern does not mention fields `name`, `age`
+  --> $DIR/E0027.rs:13:9
+   |
+LL |         Dog {} => {}
+   |         ^^^^^^ missing fields `name`, `age`
+   |
+help: include the missing fields in the pattern
+   |
+LL |         Dog { name, age } => {}
+   |             ^^^^^^^^^^^^^
+help: if you don't care about these missing fields, you can explicitely ignore them
+   |
+LL |         Dog { .. } => {}
+   |             ^^^^^^
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0027`.

src/test/ui/error-codes/E0027.rs

@@ -17,6 +17,15 @@ error[E0027]: pattern does not mention field `present`
    |
 LL |     let Foo { #[cfg(any())] present: () } = foo;
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing field `present`
+   |
+help: include the missing field in the pattern
+   |
+LL |     let Foo { present } = foo;
+   |             ^^^^^^^^^^^
+help: if you don't care about this missing field, you can explicitely ignore it
+   |
+LL |     let Foo { .. } = foo;
+   |             ^^^^^^
 
 error[E0026]: struct `Foo` does not have a field named `absent`
   --> $DIR/struct-field-cfg.rs:16:42

src/test/ui/error-codes/E0027.stderr

@@ -15,6 +15,15 @@ error[E0027]: pattern does not mention fields `b`, `c`
    |
 LL |         let A { x, y } = self.d;
    |             ^^^^^^^^^^ missing fields `b`, `c`
+   |
+help: include the missing fields in the pattern
+   |
+LL |         let A { x, y, b, c } = self.d;
+   |                     ^^^^^^
+help: if you don't care about these missing fields, you can explicitely ignore them
+   |
+LL |         let A { x, y, .. } = self.d;
+   |                     ^^^^
 
 error: aborting due to 3 previous errors