improve parse item fallback

Author: Orion Gonzalez

compiler/rustc_ast_pretty/src/pprust/mod.rs

@@ -63,6 +63,10 @@ pub fn path_segment_to_string(p: &ast::PathSegment) -> String {
     State::new().path_segment_to_string(p)
 }
 
+pub fn stmt_to_string(p: &ast::Stmt) -> String {
+    State::new().stmt_to_string(p)
+}
+
 pub fn vis_to_string(v: &ast::Visibility) -> String {
     State::new().vis_to_string(v)
 }

compiler/rustc_parse/src/parser/item.rs

@@ -21,7 +21,8 @@ use tracing::debug;
 use super::diagnostics::{ConsumeClosingDelim, dummy_arg};
 use super::ty::{AllowPlus, RecoverQPath, RecoverReturnSign};
 use super::{
-    AttrWrapper, FollowedByType, ForceCollect, Parser, PathStyle, Trailing, UsePreAttrPos,
+    AttemptLocalParseRecovery, AttrWrapper, FollowedByType, ForceCollect, Parser, PathStyle,
+    Trailing, UsePreAttrPos,
 };
 use crate::errors::{self, MacroExpandsToAdtField};
 use crate::{fluent_generated as fluent, maybe_whole};
@@ -74,21 +75,11 @@ impl<'a> Parser<'a> {
             items.push(item);
         }
 
+        // The last token should be `term`: either EOF or `}`. If it's not that means that we've had an error
+        // parsing an item
         if !self.eat(term) {
-            let token_str = super::token_descr(&self.token);
             if !self.maybe_consume_incorrect_semicolon(items.last().map(|x| &**x)) {
-                let msg = format!("expected item, found {token_str}");
-                let mut err = self.dcx().struct_span_err(self.token.span, msg);
-                let span = self.token.span;
-                if self.is_kw_followed_by_ident(kw::Let) {
-                    err.span_label(
-                        span,
-                        "consider using `const` or `static` instead of `let` for global variables",
-                    );
-                } else {
-                    err.span_label(span, "expected item")
-                        .note("for a full list of items that can appear in modules, see <https://doc.rust-lang.org/reference/items.html>");
-                };
+                let err = self.fallback_incorrect_item();
                 return Err(err);
             }
         }
@@ -97,6 +88,64 @@ impl<'a> Parser<'a> {
         let mod_spans = ModSpans { inner_span: lo.to(self.prev_token.span), inject_use_span };
         Ok((attrs, items, mod_spans))
     }
+
+    /// Tries to parse the item as a statement to provide further diagnostics.
+    fn fallback_incorrect_item(&mut self) -> rustc_errors::Diag<'a> {
+        let token_str = super::token_descr(&self.token);
+        let token_span = self.token.span;
+        let mut err =
+            self.dcx().struct_span_err(token_span, format!("expected item, found {token_str}"));
+
+        let mut do_default_diag = true;
+
+        match self.parse_full_stmt(AttemptLocalParseRecovery::No) {
+            Ok(Some(stmt)) => {
+                do_default_diag = false;
+                let span = stmt.span;
+                match &stmt.kind {
+                    StmtKind::Let(_) => {
+                        err.span_label(span, "unexpected `let` binding outside of a function")
+                            .help(format!("consider using `const` or `static` instead of `let` for global variables, or put it inside of a function:  fn foo() {{ {} }}",
+                            pprust::stmt_to_string(&stmt)));
+                    }
+                    StmtKind::Semi(expr) => {
+                        err.span_label(span, "unexpected expression").help(format!(
+                            "consider putting it inside a function: fn foo() {{ {}; }}",
+                            pprust::expr_to_string(expr)
+                        ));
+                    }
+                    StmtKind::Expr(expr) => {
+                        err.span_label(span, "unexpected expression").help(format!(
+                            "consider putting it inside a function: fn foo() {{ {} }}",
+                            pprust::expr_to_string(expr)
+                        ));
+                    }
+                    StmtKind::Empty => {
+                        unreachable!(
+                            "Should have been handled by maybe_consume_incorrect_semicolon"
+                        );
+                    }
+                    StmtKind::Item(_) | StmtKind::MacCall(_) => {
+                        unreachable!("These should be valid items!")
+                    }
+                };
+            }
+            // It's not a statement, we can't do much recovery.
+            Ok(None) => {}
+            Err(e) => {
+                // We don't really care about an error parsing this statement.
+                e.cancel();
+            }
+        }
+
+        if do_default_diag {
+            err.span_label(token_span, "expected item");
+        }
+
+        err.note("for a full list of items that can appear in modules, see <https://doc.rust-lang.org/reference/items.html>");
+
+        err
+    }
 }
 
 pub(super) type ItemInfo = (Ident, ItemKind);

tests/ui/parser/issues/issue-113110-non-item-at-module-root.stderr

@@ -2,8 +2,9 @@ error: expected item, found `5`
   --> $DIR/issue-113110-non-item-at-module-root.rs:1:2
    |
 LL |  5
-   |  ^ expected item
+   |  ^ unexpected expression
    |
+   = help: consider putting it inside a function: fn foo() { 5 }
    = note: for a full list of items that can appear in modules, see <https://doc.rust-lang.org/reference/items.html>
 
 error: aborting due to 1 previous error

tests/ui/parser/issues/issue-62913.stderr

@@ -16,8 +16,9 @@ error: expected item, found `"\u\"`
   --> $DIR/issue-62913.rs:1:1
    |
 LL | "\u\"
-   | ^^^^^^ expected item
+   | ^^^^^^ unexpected expression
    |
+   = help: consider putting it inside a function: fn foo() { "\u\" }
    = note: for a full list of items that can appear in modules, see <https://doc.rust-lang.org/reference/items.html>
 
 error: aborting due to 3 previous errors

tests/ui/parser/recover/pub-let-outside-fn.rs

@@ -0,0 +1,5 @@
+mod m {
+    pub let answer = 42;
+    //~^ ERROR visibility `pub` is not followed by an item
+    //~| ERROR expected item, found keyword `let`
+}

tests/ui/parser/recover/pub-let-outside-fn.stderr

@@ -0,0 +1,21 @@
+error: visibility `pub` is not followed by an item
+  --> $DIR/pub-let-outside-fn.rs:2:5
+   |
+LL |     pub let answer = 42;
+   |     ^^^ the visibility
+   |
+   = help: you likely meant to define an item, e.g., `pub fn foo() {}`
+
+error: expected item, found keyword `let`
+  --> $DIR/pub-let-outside-fn.rs:2:9
+   |
+LL |     pub let answer = 42;
+   |         ^^^-------------
+   |         |
+   |         unexpected `let` binding outside of a function
+   |
+   = help: consider using `const` or `static` instead of `let` for global variables, or put it inside of a function:  fn foo() { let answer = 42; }
+   = note: for a full list of items that can appear in modules, see <https://doc.rust-lang.org/reference/items.html>
+
+error: aborting due to 2 previous errors
+

tests/ui/parser/shebang/shebang-doc-comment.stderr

@@ -2,8 +2,11 @@ error: expected item, found `[`
   --> $DIR/shebang-doc-comment.rs:2:1
    |
 LL | [allow(unused_variables)]
-   | ^ expected item
+   | ^------------------------
+   | |
+   | unexpected expression
    |
+   = help: consider putting it inside a function: fn foo() { [allow(unused_variables)] }
    = note: for a full list of items that can appear in modules, see <https://doc.rust-lang.org/reference/items.html>
 
 error: aborting due to 1 previous error

tests/ui/parser/suggest-const-for-global-var.stderr

@@ -2,7 +2,12 @@ error: expected item, found keyword `let`
   --> $DIR/suggest-const-for-global-var.rs:1:1
    |
 LL | let X: i32 = 12;
-   | ^^^ consider using `const` or `static` instead of `let` for global variables
+   | ^^^-------------
+   | |
+   | unexpected `let` binding outside of a function
+   |
+   = help: consider using `const` or `static` instead of `let` for global variables, or put it inside of a function:  fn foo() { let X: i32 = 12; }
+   = note: for a full list of items that can appear in modules, see <https://doc.rust-lang.org/reference/items.html>
 
 error: aborting due to 1 previous error