vitessio/vitess · error

unsupported escape %q

Error message

unsupported escape %q

What it means

astfmtgen is a code-generation tool that rewrites Printf-style calls to work with AST nodes. rewriteAstPrintf panics with 'unsupported escape %q' when it encounters a format-verb escape/flag token its rewriter does not know how to translate for AST-aware printing. This is a build-time tool failure, not a runtime library error — it stops `go generate` from producing the astfmt wrappers.

Source

Thrown at go/tools/astfmtgen/main.go:271

			sliceType, ok := inputType.(*types.Slice)
			if !ok {
				panic("'%n' directive requires a slice")
			}
			if types.Implements(sliceType.Elem(), r.astExpr) {
				// Fast path: input is []Expr
				call := &ast.CallExpr{
					Fun: &ast.SelectorExpr{
						X:   callexpr.X,
						Sel: &ast.Ident{Name: "formatExprs"},
					},
					Args: []ast.Expr{inputExpr},
				}
				cursor.InsertBefore(&ast.ExprStmt{X: call})
				break
			}
			panic("slow path for `n` directive for slice of type other than Expr")
		default:
			panic(fmt.Sprintf("unsupported escape %q", token))
		}
		fieldnum++
		i++
	}

	cursor.Delete()
	return true
}

var noQualifier = func(p *types.Package) string {
	return ""
}

View on GitHub (pinned to 01a25a7d17)

Solutions

1. Identify the offending format string in the file being processed and simplify it to a verb astfmtgen supports (e.g. %s / %v on AST nodes)
2. If a genuinely new verb is needed, add a case for it in the token switch in rewriteAstPrintf (go/tools/astfmtgen/main.go) and regenerate
3. Move the untranslatable formatting out of the generated path (pre-format to a string before passing to the printf-style call)

Example fix

// before
log.Errorf("nodes: %+v with %#v", nodes, n)
// after
log.Errorf("nodes: %v with %s", nodes, String(n))

// Before running codegen, check the format strings fed to astfmtgen
for _, verb := range extractVerbs(formatStr) {
    if !supportedVerbs[verb] {
        log.Fatalf("unsupported verb %q for astfmtgen", verb)
    }
}

When it happens

Trigger: Editing format strings processed by astfmtgen (e.g. in log/Errorf calls over AST nodes) with an escape sequence or verb flag the generator has no special case for — typically a new or unusual %verb, %% handling, or width/precision modifier on a slice-of-Expr directive.

Common situations: A developer adds a new log message with an unusual format verb inside sqlparser and runs make codegen; a Go version or fmt change introduces a verb token the tool predates.

Related errors

• package '%s' does not contain 'ast_format.go'
• bad literal argument
• '%n' directive requires a slice
• slow path for `n` directive for slice of type other than Exp
• unknown type %T %v

AI-assisted analysis of vitessio/vitess@01a25a7d17 (2026-09-01). Data as JSON: /api/errors/d0702a368416a7c6. Report an issue: GitHub.