Check for missing space between fat arrow and range pattern

Author: clubby789

compiler/rustc_parse/src/parser/expr.rs

@@ -3168,7 +3168,7 @@ impl<'a> Parser<'a> {
         limits: RangeLimits,
     ) -> ExprKind {
         if end.is_none() && limits == RangeLimits::Closed {
-            self.inclusive_range_with_incorrect_end(self.prev_token.span);
+            self.inclusive_range_with_incorrect_end();
             ExprKind::Err
         } else {
             ExprKind::Range(start, end, limits)

compiler/rustc_parse/src/parser/pat.rs

@@ -746,32 +746,38 @@ impl<'a> Parser<'a> {
             // Parsing e.g. `X..`.
             if let RangeEnd::Included(_) = re.node {
                 // FIXME(Centril): Consider semantic errors instead in `ast_validation`.
-                self.inclusive_range_with_incorrect_end(re.span);
+                self.inclusive_range_with_incorrect_end();
             }
             None
         };
         Ok(PatKind::Range(Some(begin), end, re))
     }
 
-    pub(super) fn inclusive_range_with_incorrect_end(&mut self, span: Span) {
+    pub(super) fn inclusive_range_with_incorrect_end(&mut self) {
         let tok = &self.token;
-
+        let span = self.prev_token.span;
         // If the user typed "..==" instead of "..=", we want to give them
         // a specific error message telling them to use "..=".
+        // If they typed "..=>", suggest they use ".. =>".
         // Otherwise, we assume that they meant to type a half open exclusive
         // range and give them an error telling them to do that instead.
-        if matches!(tok.kind, token::Eq) && tok.span.lo() == span.hi() {
-            let span_with_eq = span.to(tok.span);
+        let no_space = tok.span.lo() == span.hi();
+        match tok.kind {
+            token::Eq if no_space => {
+                let span_with_eq = span.to(tok.span);
 
-            // Ensure the user doesn't receive unhelpful unexpected token errors
-            self.bump();
-            if self.is_pat_range_end_start(0) {
-                let _ = self.parse_pat_range_end().map_err(|e| e.cancel());
-            }
+                // Ensure the user doesn't receive unhelpful unexpected token errors
+                self.bump();
+                if self.is_pat_range_end_start(0) {
+                    let _ = self.parse_pat_range_end().map_err(|e| e.cancel());
+                }
 
-            self.error_inclusive_range_with_extra_equals(span_with_eq);
-        } else {
-            self.error_inclusive_range_with_no_end(span);
+                self.error_inclusive_range_with_extra_equals(span_with_eq);
+            }
+            token::Gt if no_space => {
+                self.error_inclusive_range_match_arrow(span);
+            }
+            _ => self.error_inclusive_range_with_no_end(span),
         }
     }
 
@@ -782,6 +788,18 @@ impl<'a> Parser<'a> {
             .emit();
     }
 
+    fn error_inclusive_range_match_arrow(&self, span: Span) {
+        let without_eq = span.with_hi(span.hi() - rustc_span::BytePos(1));
+        self.struct_span_err(span, "unexpected `=>` after open range")
+            .span_suggestion_verbose(
+                without_eq.shrink_to_hi(),
+                "add a space between the pattern and `=>`",
+                " ",
+                Applicability::MachineApplicable,
+            )
+            .emit();
+    }
+
     fn error_inclusive_range_with_no_end(&self, span: Span) {
         struct_span_err!(self.sess.span_diagnostic, span, E0586, "inclusive range with no end")
             .span_suggestion_short(span, "use `..` instead", "..", Applicability::MachineApplicable)

tests/ui/half-open-range-patterns/half-open-range-pats-inclusive-match-arrow.rs

@@ -0,0 +1,8 @@
+fn main() {
+    let x = 42;
+    match x {
+        0..=73 => {},
+        74..=> {},   //~ ERROR unexpected `=>` after open range
+                     //~^ ERROR expected one of `=>`, `if`, or `|`, found `>`
+    }
+}

tests/ui/half-open-range-patterns/half-open-range-pats-inclusive-match-arrow.stderr

@@ -0,0 +1,19 @@
+error: unexpected `=>` after open range
+  --> $DIR/half-open-range-pats-inclusive-match-arrow.rs:5:11
+   |
+LL |         74..=> {},
+   |           ^^^
+   |
+help: add a space between the pattern and `=>`
+   |
+LL |         74.. => {},
+   |             +
+
+error: expected one of `=>`, `if`, or `|`, found `>`
+  --> $DIR/half-open-range-pats-inclusive-match-arrow.rs:5:14
+   |
+LL |         74..=> {},
+   |              ^ expected one of `=>`, `if`, or `|`
+
+error: aborting due to 2 previous errors
+