astgen: forbid trailing whitespace in multiline strings

Trailing whitespace in multiline strings is annoying for all the usual
reasons trailing whitespace is annoying (unrelated line changes).

However, in this case it even can leak into whatever the string literal
is used for. For example, in the previous commit there is trailing
whitespace leaking into the help messages.

So it makes sense to forbid this outright.

`zig fmt` intentionally doesn't try to clean up here --- if there's a
trailing whitespace in a multiline literal, it is _ambiguous_ whether
the user added that by accident, or whether they _intended_ to have a
space there.

If you need to have trailing whitespace, you can contatenate `\\` and
`"` strings with `++`:

    const S =
        \\hello
    ++ "  \n" ++
        \\world
        \\
    ;

Closes: ziglang#19299

lib/std/zig/AstGen.zig

@@ -11679,6 +11679,7 @@ fn strLitNodeAsString(astgen: *AstGen, node: Ast.Node.Index) !IndexSlice {
         const slice = tree.tokenSlice(tok_i);
         const carriage_return_ending: usize = if (slice[slice.len - 2] == '\r') 2 else 1;
         const line_bytes = slice[2 .. slice.len - carriage_return_ending];
+        try astgen.strLitNodeCheckTrailingWhitespace(tok_i, line_bytes);
         try string_bytes.appendSlice(gpa, line_bytes);
         tok_i += 1;
     }
@@ -11687,6 +11688,7 @@ fn strLitNodeAsString(astgen: *AstGen, node: Ast.Node.Index) !IndexSlice {
         const slice = tree.tokenSlice(tok_i);
         const carriage_return_ending: usize = if (slice[slice.len - 2] == '\r') 2 else 1;
         const line_bytes = slice[2 .. slice.len - carriage_return_ending];
+        try astgen.strLitNodeCheckTrailingWhitespace(tok_i, line_bytes);
         try string_bytes.ensureUnusedCapacity(gpa, line_bytes.len + 1);
         string_bytes.appendAssumeCapacity('\n');
         string_bytes.appendSliceAssumeCapacity(line_bytes);
@@ -11699,6 +11701,19 @@ fn strLitNodeAsString(astgen: *AstGen, node: Ast.Node.Index) !IndexSlice {
     };
 }
 
+fn strLitNodeCheckTrailingWhitespace(
+    astgen: *AstGen,
+    tok: Ast.TokenIndex,
+    line_bytes: []const u8,
+) !void {
+    switch (line_bytes[line_bytes.len - 1]) {
+        '\t', ' ' => {},
+        '\r', '\n' => unreachable,
+        else => return,
+    }
+    return astgen.failTok(tok, "multiline string cannot contain trailing whitespace", .{});
+}
+
 fn testNameString(astgen: *AstGen, str_lit_token: Ast.TokenIndex) !Zir.NullTerminatedString {
     const gpa = astgen.gpa;
     const string_bytes = &astgen.string_bytes;

test/cases/compile_errors/multiline_string_with_trailing_whitespace.zig

@@ -0,0 +1,9 @@
+const S =
+    \\Hello, world    
+; //              ^^^^^ trailing whitespace here
+// error
+// backend=stage2
+// target=native
+//
+// :1:5: error: multiline string cannot contain trailing whitespac
+//