Add detection of main under more circumstances

The cases that are intended to be added by this commit are:

- Code with no comments but ends in a semicolon before the declaration
  of `main`.

  No comments was required to prevent `//` being included to comment
  out the line. This was done by not allowing `/` as part of the code
  before the `;` While this is more restrictive than necessary, it
  didn't seem likely that the code before `main` would include `/`
  so didn't seem worthwhile to take the performance hit to make the
  check less restrictive. To prevent long regex scans going multiple
  lines I also disallowed newlines in the code by including `\n` and
  `\r` as disallowed characters. The whole point is code on the same
  line as `main` so this seemed fine and would help prevent
  unnecessarily matching all lines before main when it doesn't
  matter.

- Allow multiline comments within the arguments area of main that both
  start and end within the brackets.

  I'm not expecting a substantial performance cost from this one
  because this case is near the end of the matching.

I used the following test cases

```rust
fn main() { // Should work
    println!("Hello, world!");
}

//;fn main() { //Shouldn't work
    println!("Hello, world!");
}

use std; fn main(){ println!("Hello, world!"); } // Should work

const fn main() { // Should work
    println!("Hello, world!");
}

/* fn main() {} */ // Shouldn't work

fn main(/* comment */) { // Should work
   // snip
}
```

Co-authored-by: Jake Goulding <[email protected]>

ui/frontend/selectors/index.spec.ts

@@ -55,4 +55,12 @@ describe('checking for a main function', () => {
   test('a function with the substring main does not count', () => {
     expect(doMainFunctionSelector('fn mainly()')).toBe(false);
   });
+
+  test('a main function after other items on the same line', () => {
+    expect(doMainFunctionSelector('use std; fn main(){ println!("Hello, world!"); }')).toBe(true);
+  });
+
+  test('a main function with a block comment in the argument list', () => {
+    expect(doMainFunctionSelector('fn main(/* comment */) {')).toBe(true);
+  });
 });

ui/frontend/selectors/index.ts

@@ -20,7 +20,15 @@ export const selectionSelector = (state: State) => state.selection;
 const HAS_TESTS_RE = /^\s*#\s*\[\s*test\s*([^"]*)]/m;
 export const hasTestsSelector = createSelector(codeSelector, code => !!code.match(HAS_TESTS_RE));
 
-const HAS_MAIN_FUNCTION_RE = /^\s*(pub\s+)?\s*(const\s+)?\s*(async\s+)?\s*fn\s+main\s*\(\s*\)/m;
+// https://stackoverflow.com/a/34755045/155423
+const HAS_MAIN_FUNCTION_RE = new RegExp(
+  [
+    /^([^\n\r\/]*;)?/,
+    /\s*(pub\s+)?\s*(const\s+)?\s*(async\s+)?\s*/,
+    /fn\s+main\s*\(\s*(\/\*.*\*\/)?\s*\)/,
+  ].map((r) => r.source).join(''),
+  'm'
+);
 export const hasMainFunctionSelector = createSelector(codeSelector, code => !!code.match(HAS_MAIN_FUNCTION_RE));
 
 const CRATE_TYPE_RE = /^\s*#!\s*\[\s*crate_type\s*=\s*"([^"]*)"\s*]/m;