Add diagnostics for mistyped inclusive range

compiler/rustc_parse/src/parser/expr.rs

@@ -431,7 +431,8 @@ impl<'a> Parser<'a> {
         let span = self.mk_expr_sp(&lhs, lhs.span, rhs_span);
         let limits =
             if op == AssocOp::DotDot { RangeLimits::HalfOpen } else { RangeLimits::Closed };
-        Ok(self.mk_expr(span, self.mk_range(Some(lhs), rhs, limits), AttrVec::new()))
+        let range = self.mk_range(Some(lhs), rhs, limits);
+        Ok(self.mk_expr(span, range, AttrVec::new()))
     }
 
     fn is_at_start_of_range_notation_rhs(&self) -> bool {
@@ -479,7 +480,8 @@ impl<'a> Parser<'a> {
             } else {
                 (lo, None)
             };
-            Ok(this.mk_expr(span, this.mk_range(None, opt_end, limits), attrs.into()))
+            let range = this.mk_range(None, opt_end, limits);
+            Ok(this.mk_expr(span, range, attrs.into()))
         })
     }
 
@@ -2517,13 +2519,13 @@ impl<'a> Parser<'a> {
     }
 
     fn mk_range(
-        &self,
+        &mut self,
         start: Option<P<Expr>>,
         end: Option<P<Expr>>,
         limits: RangeLimits,
     ) -> ExprKind {
         if end.is_none() && limits == RangeLimits::Closed {
-            self.error_inclusive_range_with_no_end(self.prev_token.span);
+            self.inclusive_range_with_incorrect_end(self.prev_token.span);
             ExprKind::Err
         } else {
             ExprKind::Range(start, end, limits)

compiler/rustc_parse/src/parser/pat.rs

@@ -736,15 +736,48 @@ impl<'a> Parser<'a> {
             // Parsing e.g. `X..`.
             if let RangeEnd::Included(_) = re.node {
                 // FIXME(Centril): Consider semantic errors instead in `ast_validation`.
-                // Possibly also do this for `X..=` in *expression* contexts.
-                self.error_inclusive_range_with_no_end(re.span);
+                self.inclusive_range_with_incorrect_end(re.span);
             }
             None
         };
         Ok(PatKind::Range(Some(begin), end, re))
     }
 
-    pub(super) fn error_inclusive_range_with_no_end(&self, span: Span) {
+    pub(super) fn inclusive_range_with_incorrect_end(&mut self, span: Span) {
+        let tok = &self.token;
+
+        // If the user typed "..==" instead of "..=", we want to give them
+        // a specific error message telling them to 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);
+
+            // 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();
+            }
+
+            self.error_inclusive_range_with_extra_equals(span_with_eq);
+        } else {
+            self.error_inclusive_range_with_no_end(span);
+        }
+    }
+
+    fn error_inclusive_range_with_extra_equals(&self, span: Span) {
+        self.struct_span_err(span, "unexpected `=` after inclusive range")
+            .span_suggestion_short(
+                span,
+                "use `..=` instead",
+                "..=".to_string(),
+                Applicability::MaybeIncorrect,
+            )
+            .note("inclusive ranges end with a single equals sign (`..=`)")
+            .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,

src/test/ui/parser/range-inclusive-extra-equals.rs

@@ -0,0 +1,10 @@
+// Makes sure that a helpful message is shown when someone mistypes
+// an inclusive range as `..==` rather than `..=`. This is an
+// easy mistake, because of the resemblance to`==`.
+// See #86395 for a bit of background.
+
+pub fn main() {
+    if let 1..==3 = 1 {} //~ERROR unexpected `=` after inclusive range
+                      //~|HELP use `..=` instead
+                      //~|NOTE inclusive ranges end with a single equals sign
+}

src/test/ui/parser/range-inclusive-extra-equals.stderr

@@ -0,0 +1,10 @@
+error: unexpected `=` after inclusive range
+  --> $DIR/range-inclusive-extra-equals.rs:7:13
+   |
+LL |     if let 1..==3 = 1 {}
+   |             ^^^^ help: use `..=` instead
+   |
+   = note: inclusive ranges end with a single equals sign (`..=`)
+
+error: aborting due to previous error
+