Add a nicer error message for missing in for loop, fixes #40782. #45639

LaurentMazare · 2017-10-30T22:51:12Z

As suggested by @estebank in issue #40782, this works in the same way as #42578: if the in keyword is missing, we continue parsing the expression and if this works correctly an adapted error message is produced. Otherwise we return the old error.

A specific test case has also been added.
This is my first PR on rust-lang/rust so any feedback is very welcome.

.

rust-highfive · 2017-10-30T22:51:25Z

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @pnkfelix (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

estebank

There are some minor changes I'd like, but overall it looks like great! Thank you for taking the time to contribute to the project.

estebank · 2017-10-30T23:18:33Z

src/libsyntax/parse/parser.rs

+                            s.print_pat(&pat)?;
+                            s.s.word(" in ")?;
+                            s.print_expr(&expr)
+                        });


Can't all of this be format!("for {} in {}", path, expr)?

Also, it might be nice to try using tcx.sess.codemap().span_to_snippet(expr.span) and only use s.print_expr(&expr) if it couldn't be found. This way the expr is exactly as it was written in the original code, instead of as whatever print_expr prefers to output (think of cases like for i 0 .. 2 {, we want to keep the user's formatting).

I went the easy way and replaced all this with a span_suggestion_short as mentioned below so all this should not apply anymore.

estebank · 2017-10-30T23:25:51Z

src/libsyntax/parse/parser.rs

+                            s.print_expr(&expr)
+                        });
+                        err.span_suggestion(
+                            in_span,


This span has to cover from the entire suggestion. In this case, you're suggesting for {pat} in {expr}, so the span should cover from the beginning of the for span to the end of the expr.span. This is because suggestions should be able to be applied blindly by other tools to the code, so the replacement should always make sense and have a high degree of confidence (as you do here, because the parsing continued successfully).

Otherwise, instead of pointing at the entire for {pat} {expr} span, use a span_suggestion_short, which hides the actual suggestion in the cli output but is still available to code fixing tools like RLS, and suggest only the in in the span between the end of pat.span to the start of expr.span, which would give you the following output:

12 | for i 0..2 { | ^ | | | expected `in` here | help: try adding `in` here

Switched to using span_suggestion_short.

estebank · 2017-10-30T23:29:01Z

src/libsyntax/parse/parser.rs

+                    Ok(expr) => {
+                        // Successfully parsed the expr, print a nice error message.
+                        in_err.cancel();
+                        let in_span = parser_snapshot_after_in.span;


in_span should probably be parser_snapshot_after_in.prev_span.end().to(parser_snapshot_after_in.prev_span.span.start()) to point at the empty space between the pat and the expr, regardless of how long it is:

12 | for i 0..2 { | ^^^ | | | expected `in` here

Ah, my understanding of spans was indeed pretty poor, hopefully it's getting better. I used between rather than exactly what you suggested but it seems to do the trick.

estebank · 2017-10-30T23:31:35Z

src/libsyntax/parse/parser.rs

+
+                match self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL, None) {
+                    Ok(expr) => {
+                        // Successfully parsed the expr, print a nice error message.


Expand the comment with more explanation, stating the reasoning. Something along the lines of "successfully parsed the expr, which means that the in kw is missing" and even a simple code sample like for i 0..2 {.

I've added the example you suggested.

LaurentMazare · 2017-10-31T19:50:23Z

Thanks a lot for the feedback @estebank I just pushed a second commit to address your issues.

estebank

r=me once the suggestion comment has been addressed.

estebank · 2017-10-31T20:31:35Z

src/libsyntax/parse/parser.rs

+                        let mut err = self.sess.span_diagnostic
+                            .struct_span_err(in_span, "missing `in` in `for` loop");
+                        err.span_label(in_span, "expected `in` here");
+                        err.span_suggestion_short(in_span, "try adding `in` here", "in".into());


One more thing, make the suggestion " in ".into(), otherwise when applying the suggestion the resulting code would be for iin0..2 {.

Good point, should be fixed now.

estebank · 2017-10-31T21:58:09Z

@bors r+ rollup

bors · 2017-10-31T21:58:10Z

📌 Commit 0d72853 has been approved by estebank

petrochenkov · 2017-10-31T22:53:57Z

src/libsyntax/parse/parser.rs

@@ -3154,13 +3154,47 @@ impl<'a> Parser<'a> {
        // Parse: `for <src_pat> in <src_expr> <src_loop_block>`

        let pat = self.parse_pat()?;
-        self.expect_keyword(keywords::In)?;


This can be done in much simpler way without parser snapshots (please don't overuse the trick from #42578).

if !self.eat_keyword(keywords::In) { span_err(/*missing `in`*/) // Non-fatal error } let expr = self.parse_expr_res(...); // Then parse as usual as if `in` were written

petrochenkov · 2017-10-31T22:54:06Z

@bors r-

LaurentMazare · 2017-11-01T06:50:37Z

Thanks for the feedback @petrochenkov I tried adapting the code although I'm not sure I properly understood what you meant. It seems to me that with the new code, the parser will suggest inserting a in keyword even in the for i {... case but maybe this is not much of an issue.

petrochenkov · 2017-11-01T23:11:06Z

src/libsyntax/parse/parser.rs

+            let in_span = self.prev_span.between(self.span);
+            let mut err = self.sess.span_diagnostic
+                .struct_span_err(in_span, "missing `in` in `for` loop");
+            err.span_label(in_span, "expected `in` here");


Nit: The extra label here creates unnecessary duplication, IMO. The primary message already says that in is missing and the suggestion is already formatted as a label.

petrochenkov · 2017-11-01T23:11:49Z

r=me with label removed (#45639 (comment))

LaurentMazare · 2017-11-01T23:44:23Z

I just pushed a new commit removing this label, thanks!

estebank · 2017-11-01T23:53:29Z

@bors r=petrochenkov rollup

bors · 2017-11-01T23:53:30Z

📌 Commit ed20f3b has been approved by petrochenkov

bors · 2017-11-03T08:04:22Z

⌛ Testing commit ed20f3b with merge 30a4480cb988da9fb1918a13a78a35229e6a0ed6...

bors · 2017-11-03T08:10:10Z

💔 Test failed - status-travis

kennytm · 2017-11-03T14:20:38Z

@bors retry

@estebank

Add a nicer error message for missing in for loop, fixes rust-lang#40782. As suggested by @estebank in issue rust-lang#40782, this works in the same way as rust-lang#42578: if the in keyword is missing, we continue parsing the expression and if this works correctly an adapted error message is produced. Otherwise we return the old error. A specific test case has also been added. This is my first PR on rust-lang/rust so any feedback is very welcome.

Rollup of 9 pull requests - Successful merges: #45548, #45610, #45639, #45669, #45681, #45718, #45722, #45739, #45746 - Failed merges:

Add a nicer error message for missing in for loop, fixes rust-lang#40782

6a62ea6

.

rust-highfive assigned pnkfelix Oct 30, 2017

petrochenkov assigned petrochenkov and unassigned pnkfelix Oct 30, 2017

estebank requested changes Oct 30, 2017

View reviewed changes

kennytm added the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Oct 31, 2017

Fix spans and error messages.

6d060bd

estebank approved these changes Oct 31, 2017

View reviewed changes

LaurentMazare added 2 commits October 31, 2017 21:23

Add some missing spaces.

531b7f2

Formatting tweak.

0d72853

petrochenkov reviewed Oct 31, 2017

View reviewed changes

LaurentMazare added 2 commits November 1, 2017 06:45

Remove the parser snapshot hack.

175cfbf

Preserve original formatting.

d336f02

kennytm added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Nov 1, 2017

petrochenkov reviewed Nov 1, 2017

View reviewed changes

Remove the redundant span_label.

ed20f3b

kennytm added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Nov 2, 2017

kennytm mentioned this pull request Nov 4, 2017

Rollup of 9 pull requests #45755

Merged

bors added a commit that referenced this pull request Nov 4, 2017

Auto merge of #45755 - kennytm:rollup, r=kennytm

9acc333

Rollup of 9 pull requests - Successful merges: #45548, #45610, #45639, #45669, #45681, #45718, #45722, #45739, #45746 - Failed merges:

bors merged commit ed20f3b into rust-lang:master Nov 4, 2017

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Add a nicer error message for missing in for loop, fixes #40782. #45639

Add a nicer error message for missing in for loop, fixes #40782. #45639

LaurentMazare commented Oct 30, 2017

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Add a nicer error message for missing in for loop, fixes #40782. #45639

Add a nicer error message for missing in for loop, fixes #40782. #45639

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