Diagnostics

An error shows the code it’s about, underlines the exact place, labels other places that explain it, and adds notes and suggestions:

error: 'items' was moved earlier (by move, an assignment, an argument or a return), so it can't be used here
   ┌─ app.volt:10:30
   │
 9 │     val n = consume(items);
   │                     ----- moved here
10 │     std::println("{} {}", n, items.len);
   │                              ^^^^^

error: point has no field 'z'
  ┌─ app.volt:5:19
  │
5 │     val d = p.x + p.z;
  │                   ^^^
  │
  = help: did you mean 'x'?

A misspelled name gets the closest real one as a suggestion, and an error inside a generic function’s instance labels the call that made it (twice<bool> is instantiated here). Checking doesn’t stop at the first error: an error ends the function it’s in, and the other functions are still checked. Parsing works the same way: a syntax error ends the item it’s in (a function, a struct), and the parser picks up again at the next item, using the indentation to find it. So one run finds every independent error; it shows the first 20 in source order, and the summary line says how many there were. --error-limit N shows N instead (0: all of them). Warnings (using a @deprecated function, a var on a parameter the function never changes, the reference warnings) print the same way and don’t fail the build.

--message-format picks one:

human (the default): the form above, coloured on a terminal.

short: one line per diagnostic, for editors and grep:

app.volt:5:19: error: point has no field 'z'

json: one JSON object per line, for tools:

{"severity":"error","message":"point has no field 'z'","file":"app.volt","line":5,"column":19,"end_line":5,"end_column":22,"labels":[],"notes":["help: did you mean 'x'?"]}

Each label has its own file, line, column and message. Lines and columns start at 1.

--color auto (the default) colours output on a terminal unless NO_COLOR is set or TERM is dumb; always and never force it.

In colour, a type mismatch highlights where the two types differ: in expected std::vec<i32>, found std::vec<i64>, only i32 and i64 light up, however long the rest of the types is.

The language server reports the same diagnostics as you type, and the VS Code extension’s $volt problem matcher links the human format to the code in build tasks.