Suggest => --> >= in conditions

Author: sjwang05

compiler/rustc_parse/src/parser/expr.rs

@@ -2425,6 +2425,7 @@ impl<'a> Parser<'a> {
             }
         } else {
             let attrs = self.parse_outer_attributes()?; // For recovery.
+            let maybe_fatarrow = self.token.clone();
             let block = if self.check(&token::OpenDelim(Delimiter::Brace)) {
                 self.parse_block()?
             } else {
@@ -2434,10 +2435,20 @@ impl<'a> Parser<'a> {
                     self.error_on_extra_if(&cond)?;
                     // Parse block, which will always fail, but we can add a nice note to the error
                     self.parse_block().map_err(|mut err| {
-                        err.span_note(
-                            cond_span,
-                            "the `if` expression is missing a block after this condition",
-                        );
+                        // Look for usages of '=>' where '>=' was probably intended.
+                        if maybe_fatarrow.kind == token::FatArrow {
+                            err.span_suggestion(
+                                maybe_fatarrow.span,
+                                "you probably meant to write a \"greater than or equal to\" comparison",
+                                ">=",
+                                Applicability::MachineApplicable,
+                            );
+                        } else {
+                            err.span_note(
+                                cond_span,
+                                "the `if` expression is missing a block after this condition",
+                            );
+                        }
                         err
                     })?
                 }

tests/ui/parser/eq-gt-to-gt-eq.rs

@@ -0,0 +1,27 @@
+// Check that we try to correct `=>` to `>=` in conditions.
+
+fn foo() {
+    let a = 0;
+    let b = 4;
+    if a => b { //~ERROR
+        println!("yay!");
+    }
+}
+
+fn bar() {
+    let a = 0;
+    let b = 4;
+    if a = >b { //~ERROR
+        println!("yay!");
+    }
+}
+
+fn baz() {
+    let a = 0;
+    let b = 4;
+    if a = > b { //~ERROR
+        println!("yay!");
+    }
+}
+
+fn main() {}

tests/ui/parser/eq-gt-to-gt-eq.stderr

@@ -0,0 +1,25 @@
+error: expected `{`, found `=>`
+  --> $DIR/eq-gt-to-gt-eq.rs:6:10
+   |
+LL |     if a => b {
+   |          ^^ expected `{`
+   |
+help: you probably meant to write a "greater than or equal to" comparison
+   |
+LL |     if a >= b {
+   |          ~~
+
+error: expected expression, found `>`
+  --> $DIR/eq-gt-to-gt-eq.rs:14:12
+   |
+LL |     if a = >b {
+   |            ^ expected expression
+
+error: expected expression, found `>`
+  --> $DIR/eq-gt-to-gt-eq.rs:22:12
+   |
+LL |     if a = > b {
+   |            ^ expected expression
+
+error: aborting due to 3 previous errors
+