#265 Warn on redundant semicolon after else #290

someoneigna · 2021-05-13T03:44:03Z

Summary

Warn when a semicolon is found after else.

After this change a warning is reported for the semicolons at 7:7 12:15 and 16:2

let x = 0;
function f() {}
function g() {}

if (x) {
    f();
} else; {
    g();
}


if (x) {} else;
{ }

if (x) {} else
;{ }

Output in VIM plugin format is:

{"qflist": [{"col": 7, "lnum": 7, "end_col": 7, "end_lnum": 7, "type": "W", "nr": "E202", "vcol": 0, "text": "redundant semicolon after else", "filename": "invalid.js"},
{"col": 15, "lnum": 12, "end_col": 15, "end_lnum": 12, "type": "W", "nr": "E202", "vcol": 0, "text": "redundant semicolon after else", "filename": "invalid.js"},
{"col": 1, "lnum": 16, "end_col": 1, "end_lnum": 16, "type": "W", "nr": "E202", "vcol": 0, "text": "redundant semicolon after else", "filename": "invalid.js"}]}

else;;; {} is also valid code but was left out for simplicity. (loop of peek and skip required).

github-actions · 2021-05-13T03:44:47Z

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someoneigna · 2021-05-13T03:45:44Z

I have read and hereby agree to quick-lint-js' Contributor License Agreement (CLA-v1.md).

src/quick-lint-js/parse.h

src/quick-lint-js/error.h

strager

Minor: It'd also be great if you could add documentation to docs/errors/ for this new error type. Not necessary, though.

src/quick-lint-js/error.h

src/quick-lint-js/parse.h

strager · 2021-05-13T08:26:15Z

src/quick-lint-js/error.h

+  QLJS_ERROR_TYPE(                                                             \
+      error_redundant_semicolon_after_else, "E202",                            \
+      { source_code_span semicolon; },                                         \
+      .warning(QLJS_TRANSLATABLE("redundant semicolon after else"),            \


Blocking: This error message is incorrect. The extra semicolon is not redundant; it changes behavior.

Example:

for (let b of [true, false]) { if (b) { console.log('t'); } else /* Maybe add a semicolon here */ { console.log('f'); } console.log('---'); }

Without semicolon:

t --- f ---

With semicolon:

t f --- f ---

We should communicate that the semicolon is likely unintended, but also imply that removing the semicolon would change behavior.

Perhaps this message is better: "else has empty body; consider removing the semicolon or removing the 'else'" The more I think about it, the more I think we should have two different error messages:

if (b) { } else; { // else has empty body; consider removing the semicolon to make it not empty }

if (b) { } else; // else has empty body; consider removing the redundant 'else' { }

(The first example has no line break between ; and {. The second example does.)

(Perhaps the second case doesn't deserve a diagnostic at all though. Or perhaps it should be of a lesser severity than "warning".)

src/quick-lint-js/parse.h

strager · 2021-05-13T08:29:08Z

test/test-parse-statement.cpp

+    EXPECT_THAT(v.visits, ElementsAre("visit_variable_use",      // cond
+                                      "visit_enter_block_scope",  // (if)
+                                      "visit_variable_use",       // body
+                                      "visit_exit_block_scope")); // (else)


Nit: The comment is misleading. The visit_exit_block_scope is for the if body, not the else body.

Suggested change

"visit_exit_block_scope")); // (else)

"visit_exit_block_scope")); // (if)

src/quick-lint-js/parse.h

docs/errors/E220.md

strager · 2021-05-16T21:50:34Z

docs/errors/E220.md

+    > true
+    > & false
+
+To avoid this behavior, remove the semicolon between the else keyword and the opening brace of the else block.


Include an example of what working code looks like.

docs/errors/E220.md

src/quick-lint-js/parse.h

test/test-parse-statement.cpp

strager · 2021-05-16T21:53:53Z

test/test-parse-statement.cpp

+    EXPECT_THAT(v.errors,
+                ElementsAre(ERROR_TYPE_FIELD(
+                    error_unexpected_semicolon_after_else, semicolon,
+                    offsets_matcher(&code, strlen(u8"if (cond) { body; } else"), u8";"))));


src/quick-lint-js/parse.h

someoneigna · 2021-05-27T00:45:03Z

docs/errors/E202.md

+> else
+```
+
+To avoid this behavior, remove the semicolon between the else keyword and the opening brace of the else block.


Maybe:
In case this is impacting your code, you should remove the semicolon next to the else keyword

someoneigna · 2021-05-27T00:50:30Z

src/quick-lint-js/parse.h

+      if (this->peek().type == token_type::right_curly) {
+        this->error_reporter_->report(error_else_has_empty_body{
+            .where = source_code_span(expect_left_curly.begin, this->peek().end)
+        });
+      } else if (token_after_else.type == token_type::semicolon &&


This is the check for an empty block on the else case. Now that I read it again, I see it should only be triggered if the else has a semicolon 🤦‍♂️
#290 (comment)

Is creating a transaction on the lexer and then rolling it back the correct way to peek forward?

Is creating a transaction on the lexer and then rolling it back the correct way to peek forward?

We only need a transaction if you want to look ahead multiple tokens.

Can you give an example JS program where using this->peek() won't work?

I had gone through a rabbit hole of actually checking the body of the next scope to the else. That's why I was advancing the cursor and parsing the next scope after the else using a transaction 🤦‍♂️ . I've rolled the code back to only check for the newline and the opening brace.
Commits are now squashed, let me know what you think.

strager · 2021-05-27T08:00:29Z

Did you want me to review your latest changes, or are you still hacking?

src/quick-lint-js/error.h

strager

Thanks for the update. The logic looks much cleaner now!

strager · 2021-06-10T08:07:40Z

docs/errors/E204.md

+    if (true) {
+        console.log("true");
+    }
+    console.log("always");


Lovely docs!

strager · 2021-06-10T08:12:30Z

test/test-parse-statement.cpp

+            offsets_matcher(&code, strlen(u8"if (cond) { body; } else {"),
+                            u8"}"))));


Blocking: This strlen string doesn't match the code. The u8"}" looks wrong too. Did you mean the following instead?

Suggested change

offsets_matcher(&code, strlen(u8"if (cond) { body; } else {"),

u8"}"))));

offsets_matcher(&code, strlen(u8"if (cond) { body; } else;\n"),

u8"{"))));

If so, the error looks misplaced. It should point at the else token or the ;, not at the following unrelated {.

Right now it detects else;{} (semicolon after else) and else;\n{} (empty body), the following in parse.h:2534 should change to only trigger else has empty body when it is not preceded by a newline:

- if (token_after_else.type == token_type::semicolon && - this->peek().type == token_type::left_curly) { + if (token_after_else.type == token_type::semicolon && + this->peek().type == token_type::left_curly && + !this->peek().has_leading_newline) { - if (this->peek().has_leading_newline) { + if (body is empty) { this->error_reporter_->report( error_else_has_empty_body{.where = this->peek().span()}); } else { this->error_reporter_->report(error_unexpected_semicolon_after_else{ .semicolon = token_after_else.span()}); } }

Should I omit the else has empty body warning altogether, or should I detect the empty body?
I do not know how to do it without making use of a transaction, advancing the peek and then doing a rollback afterwards.

Should I omit the else has empty body warning altogether

Yeah, let's only emit error_unexpected_semicolon_after_else in this PR.

I do not know how to do it without making use of a transaction, advancing the peek and then doing a rollback afterwards.

Would something like this work? Or am I missing an edge case?

if (token_after_else.type == token_type::semicolon && this->peek().type == token_type::left_curly && !this->peek().has_leading_newline) { this->error_reporter_->report(error_unexpected_semicolon_after_else{ .semicolon = token_after_else.span(), }); }

strager · 2021-06-10T08:13:51Z

test/test-parse-statement.cpp

+TEST(test_parse, else_has_empty_body) {
+  {
+    spy_visitor v;
+    padded_string code(u8"if (cond) { body; } else;\n{ }"_sv);


Nit: I don't think it's worth having a diagnostic for this code. If there's a newline after the else;, I think the empty body is intentional.

strager · 2021-06-25T04:39:07Z

docs/errors/E204.md

@@ -0,0 +1,30 @@
+# E204: semicolon after else may be causing unexpected behavior.


Someone else took error code E204. Choose a different error code.

someoneigna commented May 13, 2021

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strager mentioned this pull request May 13, 2021

9$: Warn on semicolon in if-else chain #265

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9$: Warn on semicolon in if-else chain #265

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someoneigna force-pushed the warn-else-semicolon branch from a4ce1c3 to 82917f4 Compare May 16, 2021 21:37

someoneigna commented May 16, 2021

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someoneigna force-pushed the warn-else-semicolon branch 4 times, most recently from 0b6c261 to 81acb09 Compare June 8, 2021 05:16

Warn for possible unexpected behavior when else followed by a semicolon.

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someoneigna force-pushed the warn-else-semicolon branch from 81acb09 to 79d5928 Compare June 9, 2021 03:02

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#265 Warn on redundant semicolon after else #290

#265 Warn on redundant semicolon after else #290

someoneigna commented May 13, 2021

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	"visit_exit_block_scope")); // (else)
	"visit_exit_block_scope")); // (if)

		offsets_matcher(&code, strlen(u8"if (cond) { body; } else {"),
		u8"}"))));

		@@ -0,0 +1,30 @@
		# E204: semicolon after else may be causing unexpected behavior.

#265 Warn on redundant semicolon after else #290

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#265 Warn on redundant semicolon after else #290

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someoneigna commented May 13, 2021

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strager commented May 27, 2021

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github-actions bot commented May 13, 2021 •

edited