Diagnostic codes

Every problem xo check reports has a stable code. A code never changes meaning; new problems get new codes. xo check --json reports them as JSON with notes and machine applicable fixes, and xo explain <code> prints the explanation below in a terminal.

Code	Severity	Phase	Meaning
`XO0101`	error	checker	name breaks the enforced naming rules (core 1.5)
`XO0110`	error	checker	unused local binding or import (core 3.1)
`XO0150`	error	lexer	character that cannot start any token
`XO0151`	error	lexer	string, raw string, bytes, or triple quoted string not closed
`XO0152`	error	lexer	rune literal that is empty, unterminated, or holds more than one character
`XO0153`	error	lexer	unknown or malformed escape sequence
`XO0154`	error	lexer	bad digit, misplaced `_`, missing digits, or out of range integer
`XO0155`	error	lexer	unknown duration unit, fractional or non decimal duration
`XO0156`	error	lexer	`${` without a matching `}` in the same string
`XO0157`	error	lexer	`${}` with no expression
`XO0158`	error	lexer	`use` not followed by a valid import path
`XO0160`	error	parser	a token that does not fit the grammar here
`XO0161`	error	parser	list element followed by a newline instead of a comma (core 1.3)
`XO0162`	error	parser	`else` must be on the line of the closing brace (core 1.3)
`XO0163`	error	parser	unparenthesized struct literal in a condition (core 1.4)
`XO0164`	error	parser	`(`, `[`, or `{` not closed before end of file
`XO0165`	error	parser	syntax that is not a pattern (core 3.5)
`XO0166`	error	parser	`while cond` used as an expression; only `while { }` is one
`XO0167`	error	parser	top level declaration (fn, struct, const, …) inside a block
`XO0168`	error	parser	statement (let, expression, …) at top level
`XO0169`	error	parser	match or select arms separated by `,` instead of newlines
`XO0170`	error	parser	several effects in a function type must be parenthesized (core 15)
`XO0171`	error	parser	`..` used more than once or not last in pattern fields
`XO0172`	error	parser	function declaration without a body
`XO0173`	error	parser	`use` after the first declaration (core 15: File)
`XO0174`	error	parser	bare `self` parameter outside an interface method signature
`XO0175`	error	parser	two statements or arms on one line
`XO0176`	error	parser	`pub` before something that cannot be exported
`XO0177`	error	parser	syntax from another language (Go, Rust, TypeScript, Python, Swift, Kotlin); names the habit and the Xo form (decision 0087)
`XO0178`	error	parser	a line starts with a binary operator (`\|\|`, `&&`, `+`, `==`, …); end the previous line with it (core 1.3, decision 0136)
`XO0201`	error	checker	Result value is never used (core 2.6)
`XO0202`	error	checker	error type does not convert to the enclosing error type (core 2.6)
`XO0203`	error	checker	success of a Result typed T must be written Ok(x) (core 2.6)
`XO0204`	error	checker	`?` on an Option in a function returning a Result (core 2.5)
`XO0205`	error	checker	`?` where the enclosing function cannot return the failure (core 2.5, 2.6)
`XO0206`	error	checker	`let _ =` discards a Result; use `.ignore_err("reason")` (core 2.6)
`XO0207`	warning	vet	method chain on the right of `??` starts at a literal, so it applies only to the default (core 15 precedence)
`XO0208`	note	vet	`.ignore_err("reason")` discards an error; listed with its reason for review (core 2.6)
`XO0301`	error	checker	match does not cover every case (core 2.3)
`XO0302`	error	checker	pattern does not fit the type of the matched value (core 3.5)
`XO0303`	error	checker	refutable pattern where an irrefutable one is required (core 3.5)
`XO0304`	error	checker	alternatives of an or pattern bind different names or types (core 3.5)
`XO0305`	warning	checker	`_` arm hides variants of an enum declared in this module (core 3.5)
`XO0401`	error	checker	value of the wrong type; only three implicit conversions exist (core 2.10)
`XO0402`	error	checker	type arguments cannot be inferred (core 2.7)
`XO0403`	error	checker	wrong arguments or fields: count, names, missing, or duplicates
`XO0404`	error	checker	type does not implement a required interface or constraint (core 2.4)
`XO0405`	error	checker	type cannot be inferred here; annotate it
`XO0406`	error	checker	operator, index, slice, or iteration not defined for the type
`XO0407`	error	checker	name used as the wrong kind of thing (a type as a value, a value that is not callable)
`XO0408`	error	checker	const initializer is not a compile time value (core 3.1)
`XO0409`	error	checker	invalid explicit conversion (core 2.1)
`XO0410`	error	checker	reading a Map by index; use `get` or `get_or_fault` (core 3.6)
`XO0420`	error	checker	constant std/time layout is invalid: unknown field, bad field form, or stray brace (std.md 13.3)
`XO0501`	error	checker	var method called on a non var binding (core 3.4)
`XO0502`	error	checker	assignment to an immutable binding or to a captured variable (core 5.2, 3.3)
`XO0503`	error	checker	one variable or overlapping places passed to two `var` parameters of one call (core 5.2, decision 0111)
`XO0510`	error	checker	use of a moved Buf, StrBuilder, or BytesBuilder (core 5.3)
`XO0520`	error	checker	mutation of a copied or stored collection element is never written back (core 5.2)
`XO0601`	error	checker	name does not resolve (core 4.3)
`XO0602`	error	checker	import path does not yield a valid local name (core 4.1)
`XO0603`	error	checker	name declared twice in one scope, or a predeclared name redefined (core 3.1, 1.4)
`XO0604`	error	checker	cyclic module imports (core 4.1)
`XO0605`	error	checker	name is not `pub` in its module, or the std type is an opaque handle (core 1.5)
`XO0606`	error	checker	method declared outside the module of its type (core 3.4)
`XO0607`	error	checker	`break` or `continue` outside a loop, or an unknown label (core 3.5)
`XO0701`	error	checker	effect performed outside the declared set (core 6.2)
`XO0702`	error	checker	unknown effect label (core 6.1)
`XO0703`	warning	vet	effect declared in `uses` is never performed (core 6.2)
`XO0801`	error	checker	child or group task error does not convert to the enclosing error type (core 7.1)
`XO0802`	error	checker	scope handle, or a value containing a task group, escapes its scope (core 7.1)
`XO0803`	error	checker	task started without a scope handle (core 7.1)
`XO0804`	warning	checker	value read in one `Mutex.with` is written back in a later one (core 5.4)
`XO0900`	warning	checker	typed hole report (core 3.7)
`XO0901`	warning	vet	`dbg(...)` left in code (core 11.4)
`XO1001`	error	reserved	`std/sys/<os>` used outside the matching target arm (core 12.2)
`XO1101`	error	checker	Go package cannot be bound (not found, does not build, main package, or binding error)
`XO1102`	error	checker	Go name exists but its signature does not map to Xo, so the binding skipped it
`XO1103`	note	vet	per module report of `use go` packages and the functions that perform `go` (decision 0041)
`XO1104`	error	checker	C header cannot be bound (not found, clang cannot read it, a `link` file is missing, or an unknown effect in its `uses`)
`XO1105`	error	checker	C function exists in the header but its signature does not map to Xo, so the binding skipped it
`XO1201`	error	no GC check, checker (`use c` without `--no-gc`)	construct, std API, or effect not available without a garbage collector (`--no-gc`); planned or impossible
`XO1301`	error	modules	malformed xo.mod require or deny line, xo.lock, or xo.work line, or a deny rule naming a package that does not exist (core 4.2, 4.3)
`XO1302`	error	modules	module version cannot be fetched from git or a module proxy (XO_PROXY): repository, tag, or version missing, or its xo.mod declares another path
`XO1303`	error	modules	module version is not in the module cache and the build is offline (XO_OFFLINE, –offline)
`XO1304`	error	modules	module tree does not match its xo.lock content hash (core 4.3)
`XO1305`	error	modules	xo.lock has no hash for a module version the build uses; run `xo mod tidy`
`XO1306`	error	checker	import of a package from a module that xo.mod does not require
`XO1307`	error	modules	vendor/ does not match xo.mod; run `xo vendor`
`XO1308`	error	build	release build (`xo build --release`, or `--ci`) while an xo.work workspace overrides required modules
`XO1309`	error	checker	call into a dependency performs an effect above its ceiling (`require ... uses ...` in xo.mod, `use ... uses ...` in xo.work)
`XO1310`	error	publish	`xo publish`: a minor or patch release removes or changes an export, or adds an effect to one
`XO1311`	error	publish	`xo publish`: the release cannot be made (not in a git repository, a subdirectory module path not ending in its directory, uncommitted changes, tag exists, version not above the last, wrong /vN path)
`XO1312`	error	checker	`use` of a package that a `deny` rule of xo.mod forbids (decision 0095)
`XO1313`	error	modules	a fetched module version disagrees with the checksum log (XO_SUMDB), or the log’s signature or proofs do not verify (4.3, decision 0124)
`XO1314`	error	modules	the checksum log cannot be consulted, or XO_PROXY, XO_SUMDB, XO_NOPROXY, or XO_NOSUMDB is malformed (4.2, decisions 0124, 0126)
`XO1401`	error	checker	allocation in a real-time function (literal, interpolation, `+` on Str or List, closure, growth method)
`XO1402`	error	checker	loop without a provable bound in a real-time function, or a `bound N` that is not a constant or is exceeded by a constant range
`XO1403`	error	checker	call in a real-time function of a function that is not real-time, a function value, or an interface method
`XO1404`	error	checker	recursion among real-time functions
`XO1405`	error	checker	blocking operation (channel, Mutex, scope, select, task) or capability effect in a real-time function

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