FEATURE_NAME

• Proposed
• Prototype: Not Started
• Implementation: Not Started
• Specification: Not Started

Summary

Labels currently need to be part of statements but can't be statements on their own. We should consider making "Label:" a statement on its own.

Motivation

Source generators that utilize gotos and labels need to be aware when generating code whether a label being jumped to is at the end of a scope. If it is, emitting "Label:" will cause compilation failures, because the C# spec/compiler currently prohibit a label from being followed by a closing brace. e.g. if code is being generated for the equivalent of:

if (Condition())
{
    Work();
}

as:

if (!Condition()) goto AfterWork;
Work();
AfterWork:

the source generator now needs to either know that there will be additional code being generated after AfterWork:, track it to be able to output a semicolon if there isn't, or just always emit it as:

if (!Condition()) goto AfterWork;
Work();
AfterWork:;

which looks weird and makes for somewhat strange stepping behavior when debugging through the source-generated code. This is all because if it's emitted as:

{
    if (!Condition()) goto AfterWork;
    Work();
    AfterWork:
}

that will fail to compile.

Detailed design

The C# specification currently states:

A labeled_statement permits a statement to be prefixed by a label. Labeled statements are permitted in blocks, but are not permitted as embedded statements.

labeled_statement
    : identifier ':' statement
    ;

such that Label: is only permitted as part of "labeled statement", i.e. there has to be a statement to put the label on. We should consider allowing Label: to be a statement of its own.

Drawbacks

Alternatives

Unresolved questions

Design meetings