Suggest if let on let refutable binding

src/librustc_mir/hair/pattern/check_match.rs

@@ -62,12 +62,13 @@ impl<'tcx> Visitor<'tcx> for MatchVisitor<'_, 'tcx> {
     fn visit_local(&mut self, loc: &'tcx hir::Local) {
         intravisit::walk_local(self, loc);
 
-        self.check_irrefutable(&loc.pat, match loc.source {
-            hir::LocalSource::Normal => "local binding",
-            hir::LocalSource::ForLoopDesugar => "`for` loop binding",
-            hir::LocalSource::AsyncFn => "async fn binding",
-            hir::LocalSource::AwaitDesugar => "`await` future binding",
-        });
+        let (msg, sp) = match loc.source {
+            hir::LocalSource::Normal => ("local binding", Some(loc.span)),
+            hir::LocalSource::ForLoopDesugar => ("`for` loop binding", None),
+            hir::LocalSource::AsyncFn => ("async fn binding", None),
+            hir::LocalSource::AwaitDesugar => ("`await` future binding", None),
+        };
+        self.check_irrefutable(&loc.pat, msg, sp);
 
         // Check legality of move bindings and `@` patterns.
         self.check_patterns(false, &loc.pat);
@@ -77,7 +78,7 @@ impl<'tcx> Visitor<'tcx> for MatchVisitor<'_, 'tcx> {
         intravisit::walk_body(self, body);
 
         for param in &body.params {
-            self.check_irrefutable(&param.pat, "function argument");
+            self.check_irrefutable(&param.pat, "function argument", None);
             self.check_patterns(false, &param.pat);
         }
     }
@@ -242,7 +243,7 @@ impl<'tcx> MatchVisitor<'_, 'tcx> {
         })
     }
 
-    fn check_irrefutable(&self, pat: &'tcx Pat, origin: &str) {
+    fn check_irrefutable(&self, pat: &'tcx Pat, origin: &str, sp: Option<Span>) {
         let module = self.tcx.hir().get_module_parent(pat.hir_id);
         MatchCheckCtxt::create_and_enter(self.tcx, self.param_env, module, |ref mut cx| {
             let mut patcx = PatCtxt::new(self.tcx,
@@ -266,18 +267,35 @@ impl<'tcx> MatchVisitor<'_, 'tcx> {
                 "refutable pattern in {}: {} not covered",
                 origin, joined_patterns
             );
-            match &pat.kind {
+            let suggest_if_let = match &pat.kind {
                 hir::PatKind::Path(hir::QPath::Resolved(None, path))
                     if path.segments.len() == 1 && path.segments[0].args.is_none() =>
                 {
                     const_not_var(&mut err, cx.tcx, pat, path);
+                    false
                 }
                 _ => {
                     err.span_label(
                         pat.span,
                         pattern_not_covered_label(&witnesses, &joined_patterns),
                     );
+                    true
+                }
+            };
+
+            if let (Some(span), true) = (sp, suggest_if_let) {
+                err.note("`let` bindings require an \"irrefutable pattern\", like a `struct` or \
+                          an `enum` with only one variant");
+                if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
+                    err.span_suggestion(
+                        span,
+                        "you might want to use `if let` to ignore the variant that isn't matched",
+                        format!("if {} {{ /* */ }}", &snippet[..snippet.len() - 1]),
+                        Applicability::HasPlaceholders,
+                    );
                 }
+                err.note("for more information, visit \
+                          https://doc.rust-lang.org/book/ch18-02-refutability.html");
             }
 
             adt_defined_here(cx, &mut err, pattern_ty, &witnesses);

src/test/ui/consts/const-match-check.eval1.stderr

@@ -3,6 +3,13 @@ error[E0005]: refutable pattern in local binding: `std::i32::MIN..=-1i32` and `1
    |
 LL |     A = { let 0 = 0; 0 },
    |               ^ patterns `std::i32::MIN..=-1i32` and `1i32..=std::i32::MAX` 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
+help: you might want to use `if let` to ignore the variant that isn't matched
+   |
+LL |     A = { if let 0 = 0 { /* */ } 0 },
+   |           ^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/consts/const-match-check.eval2.stderr

@@ -3,6 +3,13 @@ error[E0005]: refutable pattern in local binding: `std::i32::MIN..=-1i32` and `1
    |
 LL |     let x: [i32; { let 0 = 0; 0 }] = [];
    |                        ^ patterns `std::i32::MIN..=-1i32` and `1i32..=std::i32::MAX` 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
+help: you might want to use `if let` to ignore the variant that isn't matched
+   |
+LL |     let x: [i32; { if let 0 = 0 { /* */ } 0 }] = [];
+   |                    ^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/consts/const-match-check.matchck.stderr

@@ -3,24 +3,52 @@ error[E0005]: refutable pattern in local binding: `std::i32::MIN..=-1i32` and `1
    |
 LL | const X: i32 = { let 0 = 0; 0 };
    |                      ^ patterns `std::i32::MIN..=-1i32` and `1i32..=std::i32::MAX` 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
+help: you might want to use `if let` to ignore the variant that isn't matched
+   |
+LL | const X: i32 = { if let 0 = 0 { /* */ } 0 };
+   |                  ^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0005]: refutable pattern in local binding: `std::i32::MIN..=-1i32` and `1i32..=std::i32::MAX` not covered
   --> $DIR/const-match-check.rs:8:23
    |
 LL | static Y: i32 = { let 0 = 0; 0 };
    |                       ^ patterns `std::i32::MIN..=-1i32` and `1i32..=std::i32::MAX` 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
+help: you might want to use `if let` to ignore the variant that isn't matched
+   |
+LL | static Y: i32 = { if let 0 = 0 { /* */ } 0 };
+   |                   ^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0005]: refutable pattern in local binding: `std::i32::MIN..=-1i32` and `1i32..=std::i32::MAX` not covered
   --> $DIR/const-match-check.rs:13:26
    |
 LL |     const X: i32 = { let 0 = 0; 0 };
    |                          ^ patterns `std::i32::MIN..=-1i32` and `1i32..=std::i32::MAX` 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
+help: you might want to use `if let` to ignore the variant that isn't matched
+   |
+LL |     const X: i32 = { if let 0 = 0 { /* */ } 0 };
+   |                      ^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0005]: refutable pattern in local binding: `std::i32::MIN..=-1i32` and `1i32..=std::i32::MAX` not covered
   --> $DIR/const-match-check.rs:19:26
    |
 LL |     const X: i32 = { let 0 = 0; 0 };
    |                          ^ patterns `std::i32::MIN..=-1i32` and `1i32..=std::i32::MAX` 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
+help: you might want to use `if let` to ignore the variant that isn't matched
+   |
+LL |     const X: i32 = { if let 0 = 0 { /* */ } 0 };
+   |                      ^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 4 previous errors
 

src/test/ui/empty/empty-never-array.stderr

@@ -11,6 +11,13 @@ LL | | }
 ...
 LL |       let Helper::U(u) = Helper::T(t, []);
    |           ^^^^^^^^^^^^ pattern `T(_, _)` 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
+help: you might want to use `if let` to ignore the variant that isn't matched
+   |
+LL |     if let Helper::U(u) = Helper::T(t, []) { /* */ }
+   |
 
 error[E0381]: use of possibly-uninitialized variable: `u`
   --> $DIR/empty-never-array.rs:12:5

src/test/ui/error-codes/E0005.stderr

@@ -3,6 +3,13 @@ error[E0005]: refutable pattern in local binding: `None` not covered
    |
 LL |     let Some(y) = x;
    |         ^^^^^^^ pattern `None` 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
+help: you might want to use `if let` to ignore the variant that isn't matched
+   |
+LL |     if let Some(y) = x { /* */ }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/feature-gates/feature-gate-exhaustive-patterns.nll.stderr

@@ -0,0 +1,16 @@
+error[E0005]: refutable pattern in local binding: `Err(_)` not covered
+  --> $DIR/feature-gate-exhaustive-patterns.rs:7:9
+   |
+LL |     let Ok(_x) = foo();
+   |         ^^^^^^ 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
+help: you might want to use `if let` to ignore the variant that isn't matched
+   |
+LL |     if let Ok(_x) = foo() { /* */ }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0005`.

src/test/ui/feature-gates/feature-gate-exhaustive-patterns.stderr

@@ -3,6 +3,13 @@ error[E0005]: refutable pattern in local binding: `Err(_)` not covered
    |
 LL |     let Ok(_x) = foo();
    |         ^^^^^^ 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
+help: you might want to use `if let` to ignore the variant that isn't matched
+   |
+LL |     if let Ok(_x) = foo() { /* */ }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-31561.stderr

@@ -12,6 +12,13 @@ LL | | }
 ...
 LL |       let Thing::Foo(y) = Thing::Foo(1);
    |           ^^^^^^^^^^^^^ patterns `Bar` and `Baz` 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
+help: you might want to use `if let` to ignore the variant that isn't matched
+   |
+LL |     if let Thing::Foo(y) = Thing::Foo(1) { /* */ }
+   |
 
 error: aborting due to previous error
 

src/test/ui/match/non-exhaustive-defined-here.stderr

@@ -41,6 +41,13 @@ LL | | }
 ...
 LL |       let E::A = e;
    |           ^^^^ patterns `B` and `C` 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
+help: you might want to use `if let` to ignore the variant that isn't matched
+   |
+LL |     if let E::A = e { /* */ }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0004]: non-exhaustive patterns: `&B` and `&C` not covered
   --> $DIR/non-exhaustive-defined-here.rs:40:11
@@ -85,6 +92,13 @@ LL | | }
 ...
 LL |       let E::A = e;
    |           ^^^^ patterns `&B` and `&C` 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
+help: you might want to use `if let` to ignore the variant that isn't matched
+   |
+LL |     if let E::A = e { /* */ }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0004]: non-exhaustive patterns: `&&mut &B` and `&&mut &C` not covered
   --> $DIR/non-exhaustive-defined-here.rs:48:11
@@ -129,6 +143,13 @@ LL | | }
 ...
 LL |       let E::A = e;
    |           ^^^^ patterns `&&mut &B` and `&&mut &C` 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
+help: you might want to use `if let` to ignore the variant that isn't matched
+   |
+LL |     if let E::A = e { /* */ }
+   |
 
 error[E0004]: non-exhaustive patterns: `None` not covered
   --> $DIR/non-exhaustive-defined-here.rs:65:11
@@ -163,6 +184,13 @@ LL | | }
 ...
 LL |       let Opt::Some(ref _x) = e;
    |           ^^^^^^^^^^^^^^^^^ pattern `None` 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
+help: you might want to use `if let` to ignore the variant that isn't matched
+   |
+LL |     if let Opt::Some(ref _x) = e { /* */ }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 8 previous errors
 

src/test/ui/recursion/recursive-types-are-not-uninhabited.stderr

@@ -3,6 +3,13 @@ error[E0005]: refutable pattern in local binding: `Err(_)` not covered
    |
 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
+help: you might want to use `if let` to ignore the variant that isn't matched
+   |
+LL |     if let Ok(x) = res { /* */ }
+   |
 
 error[E0381]: use of possibly-uninitialized variable: `x`
   --> $DIR/recursive-types-are-not-uninhabited.rs:8:5

src/test/ui/refutable-pattern-errors.stderr

@@ -9,6 +9,13 @@ error[E0005]: refutable pattern in local binding: `(std::i32::MIN..=0i32, _)` an
    |
 LL |     let (1, (Some(1), 2..=3)) = (1, (None, 2));
    |         ^^^^^^^^^^^^^^^^^^^^^ patterns `(std::i32::MIN..=0i32, _)` and `(2i32..=std::i32::MAX, _)` 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
+help: you might want to use `if let` to ignore the variant that isn't matched
+   |
+LL |     if let (1, (Some(1), 2..=3)) = (1, (None, 2)) { /* */ }
+   |
 
 error: aborting due to 2 previous errors
 

src/test/ui/uninhabited/uninhabited-irrefutable.stderr

@@ -12,6 +12,13 @@ LL | | }
 ...
 LL |       let Foo::D(_y) = x;
    |           ^^^^^^^^^^ pattern `A(_)` 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
+help: you might want to use `if let` to ignore the variant that isn't matched
+   |
+LL |     if let Foo::D(_y) = x { /* */ }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/uninhabited/uninhabited-matches-feature-gated.stderr

@@ -51,6 +51,13 @@ error[E0005]: refutable pattern in local binding: `Err(_)` not covered
    |
 LL |     let Ok(x) = x;
    |         ^^^^^ 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
+help: you might want to use `if let` to ignore the variant that isn't matched
+   |
+LL |     if let Ok(x) = x { /* */ }
+   |
 
 error: aborting due to 7 previous errors