Rollup merge of #68108 - varkor:chained-comparison-suggestions, r=Centril

Add suggestions when encountering chained comparisons

Ideally, we'd also prevent the type error, which is just extra noise, but that will require moving the error from the parser, and I think the suggestion makes things clear enough for now.

Fixes https://github.com/rust-lang/rust/issues/65659.

4 files changed, 220 insertions, 11 deletions

diff --git a/src/test/ui/parser/chained-comparison-suggestion.rs b/src/test/ui/parser/chained-comparison-suggestion.rs
new file mode 100644
index 00000000000..0431196f174
--- /dev/null
+++ b/src/test/ui/parser/chained-comparison-suggestion.rs
@@ -0,0 +1,40 @@
+// Check that we get nice suggestions when attempting a chained comparison.
+
+fn comp1() {
+    1 < 2 <= 3; //~ ERROR comparison operators cannot be chained
+    //~^ ERROR mismatched types
+}
+
+fn comp2() {
+    1 < 2 < 3; //~ ERROR comparison operators cannot be chained
+}
+
+fn comp3() {
+    1 <= 2 < 3; //~ ERROR comparison operators cannot be chained
+    //~^ ERROR mismatched types
+}
+
+fn comp4() {
+    1 <= 2 <= 3; //~ ERROR comparison operators cannot be chained
+    //~^ ERROR mismatched types
+}
+
+fn comp5() {
+    1 > 2 >= 3; //~ ERROR comparison operators cannot be chained
+    //~^ ERROR mismatched types
+}
+
+fn comp6() {
+    1 > 2 > 3; //~ ERROR comparison operators cannot be chained
+}
+
+fn comp7() {
+    1 >= 2 > 3; //~ ERROR comparison operators cannot be chained
+}
+
+fn comp8() {
+    1 >= 2 >= 3; //~ ERROR comparison operators cannot be chained
+    //~^ ERROR mismatched types
+}
+
+fn main() {}
diff --git a/src/test/ui/parser/chained-comparison-suggestion.stderr b/src/test/ui/parser/chained-comparison-suggestion.stderr
new file mode 100644
index 00000000000..5c10a4599dd
--- /dev/null
+++ b/src/test/ui/parser/chained-comparison-suggestion.stderr
@@ -0,0 +1,159 @@
+error: comparison operators cannot be chained
+  --> $DIR/chained-comparison-suggestion.rs:4:7
+   |
+LL |     1 < 2 <= 3;
+   |       ^^^^^^
+   |
+help: split the comparison into two...
+   |
+LL |     1 < 2 && 2 <= 3;
+   |     ^^^^^^^^^^^^^
+help: ...or parenthesize one of the comparisons
+   |
+LL |     (1 < 2) <= 3;
+   |     ^^^^^^^^^^
+
+error: comparison operators cannot be chained
+  --> $DIR/chained-comparison-suggestion.rs:9:7
+   |
+LL |     1 < 2 < 3;
+   |       ^^^^^
+   |
+   = help: use `::<...>` instead of `<...>` to specify type arguments
+   = help: or use `(...)` if you meant to specify fn arguments
+help: split the comparison into two...
+   |
+LL |     1 < 2 && 2 < 3;
+   |     ^^^^^^^^^^^^
+help: ...or parenthesize one of the comparisons
+   |
+LL |     (1 < 2) < 3;
+   |     ^^^^^^^^^
+
+error: comparison operators cannot be chained
+  --> $DIR/chained-comparison-suggestion.rs:13:7
+   |
+LL |     1 <= 2 < 3;
+   |       ^^^^^^
+   |
+help: split the comparison into two...
+   |
+LL |     1 <= 2 && 2 < 3;
+   |     ^^^^^^^^^^^^^
+help: ...or parenthesize one of the comparisons
+   |
+LL |     (1 <= 2) < 3;
+   |     ^^^^^^^^^^
+
+error: comparison operators cannot be chained
+  --> $DIR/chained-comparison-suggestion.rs:18:7
+   |
+LL |     1 <= 2 <= 3;
+   |       ^^^^^^^
+   |
+help: split the comparison into two...
+   |
+LL |     1 <= 2 && 2 <= 3;
+   |     ^^^^^^^^^^^^^^
+help: ...or parenthesize one of the comparisons
+   |
+LL |     (1 <= 2) <= 3;
+   |     ^^^^^^^^^^^
+
+error: comparison operators cannot be chained
+  --> $DIR/chained-comparison-suggestion.rs:23:7
+   |
+LL |     1 > 2 >= 3;
+   |       ^^^^^^
+   |
+help: split the comparison into two...
+   |
+LL |     1 > 2 && 2 >= 3;
+   |     ^^^^^^^^^^^^^
+help: ...or parenthesize one of the comparisons
+   |
+LL |     (1 > 2) >= 3;
+   |     ^^^^^^^^^^
+
+error: comparison operators cannot be chained
+  --> $DIR/chained-comparison-suggestion.rs:28:7
+   |
+LL |     1 > 2 > 3;
+   |       ^^^^^
+   |
+   = help: use `::<...>` instead of `<...>` to specify type arguments
+   = help: or use `(...)` if you meant to specify fn arguments
+help: split the comparison into two...
+   |
+LL |     1 > 2 && 2 > 3;
+   |     ^^^^^^^^^^^^
+help: ...or parenthesize one of the comparisons
+   |
+LL |     (1 > 2) > 3;
+   |     ^^^^^^^^^
+
+error: comparison operators cannot be chained
+  --> $DIR/chained-comparison-suggestion.rs:32:7
+   |
+LL |     1 >= 2 > 3;
+   |       ^^^^^^
+   |
+   = help: use `::<...>` instead of `<...>` to specify type arguments
+   = help: or use `(...)` if you meant to specify fn arguments
+help: split the comparison into two...
+   |
+LL |     1 >= 2 && 2 > 3;
+   |     ^^^^^^^^^^^^^
+help: ...or parenthesize one of the comparisons
+   |
+LL |     (1 >= 2) > 3;
+   |     ^^^^^^^^^^
+
+error: comparison operators cannot be chained
+  --> $DIR/chained-comparison-suggestion.rs:36:7
+   |
+LL |     1 >= 2 >= 3;
+   |       ^^^^^^^
+   |
+help: split the comparison into two...
+   |
+LL |     1 >= 2 && 2 >= 3;
+   |     ^^^^^^^^^^^^^^
+help: ...or parenthesize one of the comparisons
+   |
+LL |     (1 >= 2) >= 3;
+   |     ^^^^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/chained-comparison-suggestion.rs:4:14
+   |
+LL |     1 < 2 <= 3;
+   |              ^ expected `bool`, found integer
+
+error[E0308]: mismatched types
+  --> $DIR/chained-comparison-suggestion.rs:13:14
+   |
+LL |     1 <= 2 < 3;
+   |              ^ expected `bool`, found integer
+
+error[E0308]: mismatched types
+  --> $DIR/chained-comparison-suggestion.rs:18:15
+   |
+LL |     1 <= 2 <= 3;
+   |               ^ expected `bool`, found integer
+
+error[E0308]: mismatched types
+  --> $DIR/chained-comparison-suggestion.rs:23:14
+   |
+LL |     1 > 2 >= 3;
+   |              ^ expected `bool`, found integer
+
+error[E0308]: mismatched types
+  --> $DIR/chained-comparison-suggestion.rs:36:15
+   |
+LL |     1 >= 2 >= 3;
+   |               ^ expected `bool`, found integer
+
+error: aborting due to 13 previous errors
+
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/parser/require-parens-for-chained-comparison.rs b/src/test/ui/parser/require-parens-for-chained-comparison.rs
index 9c7a25d589a..e27b03dddc5 100644
--- a/src/test/ui/parser/require-parens-for-chained-comparison.rs
+++ b/src/test/ui/parser/require-parens-for-chained-comparison.rs
@@ -3,24 +3,26 @@ struct X;
 
 fn main() {
     false == false == false;
-    //~^ ERROR chained comparison operators require parentheses
+    //~^ ERROR comparison operators cannot be chained
 
     false == 0 < 2;
-    //~^ ERROR chained comparison operators require parentheses
+    //~^ ERROR comparison operators cannot be chained
     //~| ERROR mismatched types
     //~| ERROR mismatched types
 
     f<X>();
-    //~^ ERROR chained comparison operators require parentheses
+    //~^ ERROR comparison operators cannot be chained
     //~| HELP use `::<...>` instead of `<...>` to specify type arguments
 
     f<Result<Option<X>, Option<Option<X>>>(1, 2);
-    //~^ ERROR chained comparison operators require parentheses
+    //~^ ERROR comparison operators cannot be chained
+    //~| HELP split the comparison into two...
+    //~| ...or parenthesize one of the comparisons
     //~| HELP use `::<...>` instead of `<...>` to specify type arguments
 
     use std::convert::identity;
     let _ = identity<u8>;
-    //~^ ERROR chained comparison operators require parentheses
+    //~^ ERROR comparison operators cannot be chained
     //~| HELP use `::<...>` instead of `<...>` to specify type arguments
     //~| HELP or use `(...)` if you meant to specify fn arguments
 }
diff --git a/src/test/ui/parser/require-parens-for-chained-comparison.stderr b/src/test/ui/parser/require-parens-for-chained-comparison.stderr
index bece9a38800..44edf2de7f8 100644
--- a/src/test/ui/parser/require-parens-for-chained-comparison.stderr
+++ b/src/test/ui/parser/require-parens-for-chained-comparison.stderr
@@ -1,16 +1,16 @@
-error: chained comparison operators require parentheses
+error: comparison operators cannot be chained
   --> $DIR/require-parens-for-chained-comparison.rs:5:11
    |
 LL |     false == false == false;
    |           ^^^^^^^^^^^
 
-error: chained comparison operators require parentheses
+error: comparison operators cannot be chained
   --> $DIR/require-parens-for-chained-comparison.rs:8:11
    |
 LL |     false == 0 < 2;
    |           ^^^^^^
 
-error: chained comparison operators require parentheses
+error: comparison operators cannot be chained
   --> $DIR/require-parens-for-chained-comparison.rs:13:6
    |
 LL |     f<X>();
@@ -21,19 +21,27 @@ help: use `::<...>` instead of `<...>` to specify type arguments
 LL |     f::<X>();
    |      ^^
 
-error: chained comparison operators require parentheses
+error: comparison operators cannot be chained
   --> $DIR/require-parens-for-chained-comparison.rs:17:6
    |
 LL |     f<Result<Option<X>, Option<Option<X>>>(1, 2);
    |      ^^^^^^^^
    |
+help: split the comparison into two...
+   |
+LL |     f < Result && Result <Option<X>, Option<Option<X>>>(1, 2);
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+help: ...or parenthesize one of the comparisons
+   |
+LL |     (f < Result) <Option<X>, Option<Option<X>>>(1, 2);
+   |     ^^^^^^^^^^^^^^
 help: use `::<...>` instead of `<...>` to specify type arguments
    |
 LL |     f::<Result<Option<X>, Option<Option<X>>>(1, 2);
    |      ^^
 
-error: chained comparison operators require parentheses
-  --> $DIR/require-parens-for-chained-comparison.rs:22:21
+error: comparison operators cannot be chained
+  --> $DIR/require-parens-for-chained-comparison.rs:24:21
    |
 LL |     let _ = identity<u8>;
    |                     ^^^^