Fix invalid suggestions on unit range bindings

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -2010,7 +2010,18 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             {
                 if let ObligationCauseCode::Pattern { span: Some(span), .. } = *cause.code() {
                     if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
-                        let suggestion = if expected_def.is_struct() {
+                        let suggestion = if exp_found.found.is_unit() {
+                            let exp_str = format!("{:?}", expected_def);
+
+                            // `RangeInclusive`'s fields are private, we have to call methods instead.
+                            if exp_str.contains("core::ops::RangeInclusive")
+                                || exp_str.contains("std::ops::RangeInclusive")
+                            {
+                                format!("({}).{}()", snippet, name)
+                            } else {
+                                format!("({}).{}", snippet, name)
+                            }
+                        } else if expected_def.is_struct() {
                             format!("{}.{}", snippet, name)
                         } else if expected_def.is_union() {
                             format!("unsafe {{ {}.{} }}", snippet, name)

src/test/ui/range/issue-93122-bind-unit-range-to-unit.fixed

@@ -0,0 +1,9 @@
+// run-rustfix
+
+fn main() {
+    let () = (()..).start; //~ ERROR: mismatched types
+    let () = (..()).end; //~ ERROR: mismatched types
+    let () = (..=()).end; //~ ERROR: mismatched types
+    let () = (()..()).start; //~ ERROR: mismatched types
+    let () = (()..=()).start(); //~ ERROR: mismatched types
+}

src/test/ui/range/issue-93122-bind-unit-range-to-unit.rs

@@ -0,0 +1,9 @@
+// run-rustfix
+
+fn main() {
+    let () = ()..; //~ ERROR: mismatched types
+    let () = ..(); //~ ERROR: mismatched types
+    let () = ..=(); //~ ERROR: mismatched types
+    let () = ()..(); //~ ERROR: mismatched types
+    let () = ()..=(); //~ ERROR: mismatched types
+}

src/test/ui/range/issue-93122-bind-unit-range-to-unit.stderr

@@ -0,0 +1,78 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-93122-bind-unit-range-to-unit.rs:4:9
+   |
+LL |     let () = ()..;
+   |         ^^   ---- this expression has type `RangeFrom<()>`
+   |         |
+   |         expected struct `RangeFrom`, found `()`
+   |
+   = note: expected struct `RangeFrom<()>`
+           found unit type `()`
+help: you might have meant to use field `start` whose type is `()`
+   |
+LL |     let () = (()..).start;
+   |              ~~~~~~~~~~~~
+
+error[E0308]: mismatched types
+  --> $DIR/issue-93122-bind-unit-range-to-unit.rs:5:9
+   |
+LL |     let () = ..();
+   |         ^^   ---- this expression has type `RangeTo<()>`
+   |         |
+   |         expected struct `RangeTo`, found `()`
+   |
+   = note: expected struct `RangeTo<()>`
+           found unit type `()`
+help: you might have meant to use field `end` whose type is `()`
+   |
+LL |     let () = (..()).end;
+   |              ~~~~~~~~~~
+
+error[E0308]: mismatched types
+  --> $DIR/issue-93122-bind-unit-range-to-unit.rs:6:9
+   |
+LL |     let () = ..=();
+   |         ^^   ----- this expression has type `RangeToInclusive<()>`
+   |         |
+   |         expected struct `RangeToInclusive`, found `()`
+   |
+   = note: expected struct `RangeToInclusive<()>`
+           found unit type `()`
+help: you might have meant to use field `end` whose type is `()`
+   |
+LL |     let () = (..=()).end;
+   |              ~~~~~~~~~~~
+
+error[E0308]: mismatched types
+  --> $DIR/issue-93122-bind-unit-range-to-unit.rs:7:9
+   |
+LL |     let () = ()..();
+   |         ^^   ------ this expression has type `std::ops::Range<()>`
+   |         |
+   |         expected struct `std::ops::Range`, found `()`
+   |
+   = note: expected struct `std::ops::Range<()>`
+           found unit type `()`
+help: you might have meant to use field `start` whose type is `()`
+   |
+LL |     let () = (()..()).start;
+   |              ~~~~~~~~~~~~~~
+
+error[E0308]: mismatched types
+  --> $DIR/issue-93122-bind-unit-range-to-unit.rs:8:9
+   |
+LL |     let () = ()..=();
+   |         ^^   ------- this expression has type `RangeInclusive<()>`
+   |         |
+   |         expected struct `RangeInclusive`, found `()`
+   |
+   = note: expected struct `RangeInclusive<()>`
+           found unit type `()`
+help: you might have meant to use field `start` whose type is `()`
+   |
+LL |     let () = (()..=()).start();
+   |              ~~~~~~~~~~~~~~~~~
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0308`.