Diagnostics

The compiler speaks at four severities. An error rejects the program. A warning means the program compiles and something about it deserves fixing. An info is neutral information, and a hint is a suggestion: the program is fine, a clearer form exists.

Every diagnostic carries a stable code — LYR-SEM0071 and its kind. A code keeps its meaning and its severity forever: a retired code is never reused, and no flag turns a warning into something else. What changes with a flag is at most the exit code.

Warnings

import std.io.console { println };

fn main(): int {
    let unused = 42;
    println("the program runs all the same");
    return 0;
}

main.lyr:4:9: warning[LYR-SEM0071]: 'unused' is never used
    let unused = 42;
        ^
  note: name it '_' when the value is deliberately unused

What warns today: a local binding, loop variable, catch binding or pattern binding that is never referenced (LYR-SEM0071); an imported name nobody in the file uses (LYR-SEM0072); a statement that can never run because control flow always leaves the block before it (LYR-SEM0073); and every use of something marked @Deprecated (LYR-SEM0076), with the attribute's message naming the way forward.

Calling a static extension method through an instance (LYR-SEM0074) was on that list until 2.0 and is an error since: a static member belongs to the type, so Type.method(…) is the call. The message says so where the old form is written.

Naming a binder _ says the value is deliberately unused, and no warning fires. Parameters never warn — a signature is often fixed by an interface — and neither does the shorthand field pattern Rect { w, h }, whose names belong to the fields rather than to you.

Warnings never fail a build by themselves. In CI you can make them:

lyric check src/main.lyr --deny-warnings

The warnings keep their severity in the output; one closing error carries the policy into the exit code, and a denied build writes no file.

Hints

LYR-SEM0075 is a hint: a var through which nothing is ever changed — no reassignment, no field or element write, no mut method call — could be a let. A var that documents mutation keeps its var; the hint only speaks when the mutability says nothing.

Notes

A diagnostic may carry notes: other places that belong to the same finding. A duplicate declaration points back at the first one, a missing interface method points at the member it fails — in whatever file that member lives — and an unknown name suggests the closest one in scope when there is exactly one close candidate:

main.lyr:3:12: error[LYR-SEM0002]: unknown identifier 'coutn'
    return coutn;
           ^^^^^
  note: did you mean 'count'?

In an editor, notes appear under the message and jump to their place on click; unused and unreachable code renders faded, and the deprecated instance form is struck through.