Suggest _ and .. if a pattern has too few fields

For example, this code:

    struct S(i32, f32);

    let S(x) = S(0, 1.0);

will make the compiler suggest either:

    let S(x, _) = S(0, 1.0);

or:

    let S(x, ..) = S(0, 1.0);

compiler/rustc_typeck/src/check/pat.rs

@@ -15,10 +15,12 @@ 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::{BytePos, DUMMY_SP};
 use rustc_trait_selection::traits::{ObligationCause, Pattern};
 
 use std::cmp;
 use std::collections::hash_map::Entry::{Occupied, Vacant};
+use std::iter;
 
 use super::report_unexpected_variant_res;
 
@@ -1040,6 +1042,33 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 vec![(left, "(".to_string()), (right.shrink_to_hi(), ")".to_string())],
                 Applicability::MachineApplicable,
             );
+        } else if fields.len() > subpats.len() {
+            let after_fields_span = if pat_span == DUMMY_SP {
+                pat_span
+            } else {
+                pat_span.with_hi(pat_span.hi() - BytePos(1)).shrink_to_hi()
+            };
+
+            let mut wildcard_sugg =
+                iter::repeat("_").take(fields.len() - subpats.len()).collect::<Vec<_>>().join(", ");
+            if !subpats.is_empty() {
+                wildcard_sugg = String::from(", ") + &wildcard_sugg;
+            }
+
+            let rest_sugg = if subpats.is_empty() { "..".to_owned() } else { ", ..".to_owned() };
+
+            err.span_suggestion(
+                after_fields_span,
+                "use `_` to explicitly ignore each field",
+                wildcard_sugg,
+                Applicability::MaybeIncorrect,
+            );
+            err.span_suggestion(
+                after_fields_span,
+                "use `..` to ignore all unmentioned fields",
+                rest_sugg,
+                Applicability::MaybeIncorrect,
+            );
         }
 
         err.emit();

src/test/ui/destructuring-assignment/tuple_struct_destructure_fail.stderr

@@ -31,6 +31,15 @@ LL | struct TupleStruct<S, T>(S, T);
 ...
 LL |     TupleStruct(_) = TupleStruct(1, 2);
    |     ^^^^^^^^^^^^^^ expected 2 fields, found 1
+   |
+help: use `_` to explicitly ignore each field
+   |
+LL |     TupleStruct(_, _) = TupleStruct(1, 2);
+   |                  ^^^
+help: use `..` to ignore all unmentioned fields
+   |
+LL |     TupleStruct(_, ..) = TupleStruct(1, 2);
+   |                  ^^^^
 
 error[E0023]: this pattern has 3 fields, but the corresponding tuple variant has 2 fields
   --> $DIR/tuple_struct_destructure_fail.rs:34:5
@@ -49,6 +58,15 @@ LL |     SingleVariant(S, T)
 ...
 LL |     Enum::SingleVariant(_) = Enum::SingleVariant(1, 2);
    |     ^^^^^^^^^^^^^^^^^^^^^^ expected 2 fields, found 1
+   |
+help: use `_` to explicitly ignore each field
+   |
+LL |     Enum::SingleVariant(_, _) = Enum::SingleVariant(1, 2);
+   |                          ^^^
+help: use `..` to ignore all unmentioned fields
+   |
+LL |     Enum::SingleVariant(_, ..) = Enum::SingleVariant(1, 2);
+   |                          ^^^^
 
 error[E0070]: invalid left-hand side of assignment
   --> $DIR/tuple_struct_destructure_fail.rs:40:12

src/test/ui/error-codes/E0023.stderr

@@ -6,6 +6,15 @@ LL |     Apple(String, String),
 ...
 LL |         Fruit::Apple(a) => {},
    |         ^^^^^^^^^^^^^^^ expected 2 fields, found 1
+   |
+help: use `_` to explicitly ignore each field
+   |
+LL |         Fruit::Apple(a, _) => {},
+   |                       ^^^
+help: use `..` to ignore all unmentioned fields
+   |
+LL |         Fruit::Apple(a, ..) => {},
+   |                       ^^^^
 
 error[E0023]: this pattern has 3 fields, but the corresponding tuple variant has 2 fields
   --> $DIR/E0023.rs:12:9

src/test/ui/issues/issue-67037-pat-tup-scrut-ty-diff-less-fields.stderr

@@ -17,6 +17,15 @@ LL | struct P<T>(T); // 1 type parameter wanted
 ...
 LL |     let P() = U {};
    |         ^^^ expected 1 field, found 0
+   |
+help: use `_` to explicitly ignore each field
+   |
+LL |     let P(_) = U {};
+   |           ^
+help: use `..` to ignore all unmentioned fields
+   |
+LL |     let P(..) = U {};
+   |           ^^
 
 error: aborting due to 2 previous errors
 

src/test/ui/issues/issue-72574-2.stderr

@@ -26,6 +26,15 @@ LL | struct Binder(i32, i32, i32);
 ...
 LL |         Binder(_a, _x @ ..) => {}
    |         ^^^^^^^^^^^^^^^^^^^ expected 3 fields, found 2
+   |
+help: use `_` to explicitly ignore each field
+   |
+LL |         Binder(_a, _x @ .., _) => {}
+   |                           ^^^
+help: use `..` to ignore all unmentioned fields
+   |
+LL |         Binder(_a, _x @ .., ..) => {}
+   |                           ^^^^
 
 error: aborting due to 3 previous errors
 

src/test/ui/match/match-pattern-field-mismatch.stderr

@@ -6,6 +6,15 @@ LL |         Rgb(usize, usize, usize),
 ...
 LL |           Color::Rgb(_, _) => { }
    |           ^^^^^^^^^^^^^^^^ expected 3 fields, found 2
+   |
+help: use `_` to explicitly ignore each field
+   |
+LL |           Color::Rgb(_, _, _) => { }
+   |                          ^^^
+help: use `..` to ignore all unmentioned fields
+   |
+LL |           Color::Rgb(_, _, ..) => { }
+   |                          ^^^^
 
 error: aborting due to previous error
 

src/test/ui/pattern/issue-74539.stderr

@@ -26,6 +26,15 @@ LL |     A(u8, u8),
 ...
 LL |         E::A(x @ ..) => {
    |         ^^^^^^^^^^^^ expected 2 fields, found 1
+   |
+help: use `_` to explicitly ignore each field
+   |
+LL |         E::A(x @ .., _) => {
+   |                    ^^^
+help: use `..` to ignore all unmentioned fields
+   |
+LL |         E::A(x @ .., ..) => {
+   |                    ^^^^
 
 error: aborting due to 3 previous errors
 

src/test/ui/pattern/pat-tuple-underfield.rs

@@ -0,0 +1,49 @@
+struct S(i32, f32);
+enum E {
+    S(i32, f32),
+}
+
+fn main() {
+    match S(0, 1.0) {
+        S(x) => {}
+        //~^ ERROR this pattern has 1 field, but the corresponding tuple struct has 2 fields
+        //~| HELP use `_` to explicitly ignore each field
+        //~| HELP use `..` to ignore all unmentioned fields
+    }
+    match S(0, 1.0) {
+        S(_) => {}
+        //~^ ERROR this pattern has 1 field, but the corresponding tuple struct has 2 fields
+        //~| HELP use `_` to explicitly ignore each field
+        //~| HELP use `..` to ignore all unmentioned fields
+    }
+    match S(0, 1.0) {
+        S() => {}
+        //~^ ERROR this pattern has 0 fields, but the corresponding tuple struct has 2 fields
+        //~| HELP use `_` to explicitly ignore each field
+        //~| HELP use `..` to ignore all unmentioned fields
+    }
+
+    match E::S(0, 1.0) {
+        E::S(x) => {}
+        //~^ ERROR this pattern has 1 field, but the corresponding tuple variant has 2 fields
+        //~| HELP use `_` to explicitly ignore each field
+        //~| HELP use `..` to ignore all unmentioned fields
+    }
+    match E::S(0, 1.0) {
+        E::S(_) => {}
+        //~^ ERROR this pattern has 1 field, but the corresponding tuple variant has 2 fields
+        //~| HELP use `_` to explicitly ignore each field
+        //~| HELP use `..` to ignore all unmentioned fields
+    }
+    match E::S(0, 1.0) {
+        E::S() => {}
+        //~^ ERROR this pattern has 0 fields, but the corresponding tuple variant has 2 fields
+        //~| HELP use `_` to explicitly ignore each field
+        //~| HELP use `..` to ignore all unmentioned fields
+    }
+    match E::S(0, 1.0) {
+        E::S => {}
+        //~^ ERROR expected unit struct, unit variant or constant, found tuple variant `E::S`
+        //~| HELP use the tuple variant pattern syntax instead
+    }
+}

src/test/ui/pattern/pat-tuple-underfield.stderr

@@ -0,0 +1,121 @@
+error[E0532]: expected unit struct, unit variant or constant, found tuple variant `E::S`
+  --> $DIR/pat-tuple-underfield.rs:45:9
+   |
+LL |     S(i32, f32),
+   |     ----------- `E::S` defined here
+...
+LL |         E::S => {}
+   |         ^^^^ help: use the tuple variant pattern syntax instead: `E::S(_, _)`
+
+error[E0023]: this pattern has 1 field, but the corresponding tuple struct has 2 fields
+  --> $DIR/pat-tuple-underfield.rs:8:9
+   |
+LL | struct S(i32, f32);
+   | ------------------- tuple struct defined here
+...
+LL |         S(x) => {}
+   |         ^^^^ expected 2 fields, found 1
+   |
+help: use `_` to explicitly ignore each field
+   |
+LL |         S(x, _) => {}
+   |            ^^^
+help: use `..` to ignore all unmentioned fields
+   |
+LL |         S(x, ..) => {}
+   |            ^^^^
+
+error[E0023]: this pattern has 1 field, but the corresponding tuple struct has 2 fields
+  --> $DIR/pat-tuple-underfield.rs:14:9
+   |
+LL | struct S(i32, f32);
+   | ------------------- tuple struct defined here
+...
+LL |         S(_) => {}
+   |         ^^^^ expected 2 fields, found 1
+   |
+help: use `_` to explicitly ignore each field
+   |
+LL |         S(_, _) => {}
+   |            ^^^
+help: use `..` to ignore all unmentioned fields
+   |
+LL |         S(_, ..) => {}
+   |            ^^^^
+
+error[E0023]: this pattern has 0 fields, but the corresponding tuple struct has 2 fields
+  --> $DIR/pat-tuple-underfield.rs:20:9
+   |
+LL | struct S(i32, f32);
+   | ------------------- tuple struct defined here
+...
+LL |         S() => {}
+   |         ^^^ expected 2 fields, found 0
+   |
+help: use `_` to explicitly ignore each field
+   |
+LL |         S(_, _) => {}
+   |           ^^^^
+help: use `..` to ignore all unmentioned fields
+   |
+LL |         S(..) => {}
+   |           ^^
+
+error[E0023]: this pattern has 1 field, but the corresponding tuple variant has 2 fields
+  --> $DIR/pat-tuple-underfield.rs:27:9
+   |
+LL |     S(i32, f32),
+   |     ----------- tuple variant defined here
+...
+LL |         E::S(x) => {}
+   |         ^^^^^^^ expected 2 fields, found 1
+   |
+help: use `_` to explicitly ignore each field
+   |
+LL |         E::S(x, _) => {}
+   |               ^^^
+help: use `..` to ignore all unmentioned fields
+   |
+LL |         E::S(x, ..) => {}
+   |               ^^^^
+
+error[E0023]: this pattern has 1 field, but the corresponding tuple variant has 2 fields
+  --> $DIR/pat-tuple-underfield.rs:33:9
+   |
+LL |     S(i32, f32),
+   |     ----------- tuple variant defined here
+...
+LL |         E::S(_) => {}
+   |         ^^^^^^^ expected 2 fields, found 1
+   |
+help: use `_` to explicitly ignore each field
+   |
+LL |         E::S(_, _) => {}
+   |               ^^^
+help: use `..` to ignore all unmentioned fields
+   |
+LL |         E::S(_, ..) => {}
+   |               ^^^^
+
+error[E0023]: this pattern has 0 fields, but the corresponding tuple variant has 2 fields
+  --> $DIR/pat-tuple-underfield.rs:39:9
+   |
+LL |     S(i32, f32),
+   |     ----------- tuple variant defined here
+...
+LL |         E::S() => {}
+   |         ^^^^^^ expected 2 fields, found 0
+   |
+help: use `_` to explicitly ignore each field
+   |
+LL |         E::S(_, _) => {}
+   |              ^^^^
+help: use `..` to ignore all unmentioned fields
+   |
+LL |         E::S(..) => {}
+   |              ^^
+
+error: aborting due to 7 previous errors
+
+Some errors have detailed explanations: E0023, E0532.
+For more information about an error, try `rustc --explain E0023`.