Recover and suggest use ; to construct array type

Signed-off-by: xizheyin <xizheyin@smail.nju.edu.cn>

Author: xizheyin

compiler/rustc_parse/src/parser/ty.rs

@@ -575,14 +575,84 @@ impl<'a> Parser<'a> {
                 self.expect(exp!(CloseBracket))?;
             }
             TyKind::Array(elt_ty, length)
-        } else {
-            self.expect(exp!(CloseBracket))?;
+        } else if self.eat(exp!(CloseBracket)) {
             TyKind::Slice(elt_ty)
+        } else {
+            self.maybe_recover_array_ty_without_semi(elt_ty)?
         };
 
         Ok(ty)
     }
 
+    /// Recover from malformed array type syntax.
+    ///
+    /// This method attempts to recover from cases like:
+    /// - `[u8, 5]` → suggests using `;`, return a Array type
+    /// - `[u8 5]` → suggests using `;`, return a Array type
+    /// Consider to add more cases in the future.
+    fn maybe_recover_array_ty_without_semi(&mut self, elt_ty: P<Ty>) -> PResult<'a, TyKind> {
+        let span = self.token.span;
+        let token_descr = super::token_descr(&self.token);
+        let mut err =
+            self.dcx().struct_span_err(span, format!("expected `;` or `]`, found {}", token_descr));
+        err.span_label(span, "expected `;` or `]`");
+        err.note("you might have meant to write a slice or array type");
+
+        // If we cannot recover, return the error immediately.
+        if !self.may_recover() {
+            return Err(err);
+        }
+
+        let snapshot = self.create_snapshot_for_diagnostic();
+
+        // we first try to parse pattern like `[u8 5]`
+        let length = match self.parse_expr_anon_const() {
+            Ok(length) => length,
+            Err(e) => {
+                e.cancel();
+                self.bump();
+                match self.parse_expr_anon_const() {
+                    Ok(expr) => {
+                        // if [u8, 5] is parsed successfully, we suggest to add `;` as the separator
+                        err.span_suggestion_verbose(
+                            span,
+                            "you might have meant to use `;` as the separator",
+                            ";",
+                            Applicability::MaybeIncorrect,
+                        );
+                        if let Err(e) = self.expect(exp!(CloseBracket)) {
+                            e.cancel();
+                            self.restore_snapshot(snapshot);
+                            return Err(err);
+                        }
+                        err.emit();
+                        return Ok(TyKind::Array(elt_ty, expr));
+                    }
+                    Err(e) => {
+                        e.cancel();
+                        self.restore_snapshot(snapshot);
+                        return Err(err);
+                    }
+                }
+            }
+        };
+
+        if let Err(e) = self.expect(exp!(CloseBracket)) {
+            e.cancel();
+            self.restore_snapshot(snapshot);
+            return Err(err);
+        }
+
+        err.span_suggestion_verbose(
+            span.shrink_to_lo(),
+            "you might have meant to use `;` as the separator",
+            ";",
+            Applicability::MaybeIncorrect,
+        );
+        err.emit();
+        Ok(TyKind::Array(elt_ty, length))
+    }
+
     fn parse_borrowed_pointee(&mut self) -> PResult<'a, TyKind> {
         let and_span = self.prev_token.span;
         let mut opt_lifetime = self.check_lifetime().then(|| self.expect_lifetime());

src/tools/tidy/src/issues.txt

@@ -2338,7 +2338,6 @@ ui/issues/issue-50415.rs
 ui/issues/issue-50442.rs
 ui/issues/issue-50471.rs
 ui/issues/issue-50518.rs
-ui/issues/issue-50571.rs
 ui/issues/issue-50581.rs
 ui/issues/issue-50582.rs
 ui/issues/issue-50585.rs

src/tools/tidy/src/ui_tests.rs

@@ -17,7 +17,7 @@ use ignore::Walk;
 const ENTRY_LIMIT: u32 = 901;
 // FIXME: The following limits should be reduced eventually.
 
-const ISSUES_ENTRY_LIMIT: u32 = 1619;
+const ISSUES_ENTRY_LIMIT: u32 = 1616;
 
 const EXPECTED_TEST_FILE_EXTENSIONS: &[&str] = &[
     "rs",     // test source files

tests/ui/issues/issue-50571.fixed

@@ -1,3 +1,3 @@
 fn main() {
-    let x: [isize 3]; //~ ERROR expected one of `!`, `(`, `+`, `::`, `;`, `<`, or `]`, found `3`
+    let x: [isize 3]; //~ ERROR expected `;` or `]`, found `3`
 }

tests/ui/issues/issue-50571.rs

@@ -1,10 +1,14 @@
-error: expected one of `!`, `(`, `+`, `::`, `;`, `<`, or `]`, found `3`
+error: expected `;` or `]`, found `3`
   --> $DIR/better-expected.rs:2:19
    |
 LL |     let x: [isize 3];
-   |          -        ^ expected one of 7 possible tokens
-   |          |
-   |          while parsing the type for `x`
+   |                   ^ expected `;` or `]`
+   |
+   = note: you might have meant to write a slice or array type
+help: you might have meant to use `;` as the separator
+   |
+LL |     let x: [isize ;3];
+   |                   +
 
 error: aborting due to 1 previous error
 

tests/ui/issues/issue-50571.stderr

@@ -0,0 +1,11 @@
+// There is a regression introduced for issue #143828
+//@ edition: 2015
+
+#![allow(dead_code)]
+trait Foo {
+    fn foo([a, b]: [i32; 2]) {}
+    //~^ ERROR: expected `;` or `]`, found `,`
+    //~| ERROR: patterns aren't allowed in methods without bodies
+}
+
+fn main() {}

tests/ui/parser/better-expected.rs

@@ -0,0 +1,28 @@
+error: expected `;` or `]`, found `,`
+  --> $DIR/error-pattern-issue-50571.rs:6:14
+   |
+LL |     fn foo([a, b]: [i32; 2]) {}
+   |              ^ expected `;` or `]`
+   |
+   = note: you might have meant to write a slice or array type
+help: you might have meant to use `;` as the separator
+   |
+LL -     fn foo([a, b]: [i32; 2]) {}
+LL +     fn foo([a; b]: [i32; 2]) {}
+   |
+
+error[E0642]: patterns aren't allowed in methods without bodies
+  --> $DIR/error-pattern-issue-50571.rs:6:12
+   |
+LL |     fn foo([a, b]: [i32; 2]) {}
+   |            ^^^^^^
+   |
+help: give this argument a name or use an underscore to ignore it
+   |
+LL -     fn foo([a, b]: [i32; 2]) {}
+LL +     fn foo(_: [i32; 2]) {}
+   |
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0642`.