Builtin_attributes(3)

💖 Builtin_attributes — Support for the builtin attributes 💖

okay bestie, welcome to the Builtin_attributes(3) manual page ✨ this is literally all about support for the builtin attributes, and we're gonna get into it 💅

🎀 Module 🎀

Module Builtin_attributes

✨ Documentation ✨

Module Builtin_attributes : sig end

Support for the builtin attributes:

• -ocaml.alert
• -ocaml.boxed
• -ocaml.deprecated
• -ocaml.deprecated_mutable
• -ocaml.explicit_arity
• -ocaml.immediate
• -ocaml.immediate64
• -ocaml.inline
• -ocaml.inlined
• -ocaml.noalloc
• -ocaml.poll
• -ocaml.ppwarning
• -ocaml.remove_aliases
• -ocaml.specialise
• -ocaml.specialised
• -ocaml.tailcall
• -ocaml.tail_mod_cons
• -ocaml.unboxed
• -ocaml.untagged
• -ocaml.unrolled
• -ocaml.warnerror
• -ocaml.warning
• -ocaml.warn_on_literal_pattern

⚠️ Warning: this module is unstable and part of Compiler_libs . like, it's giving "handle with care" energy fr fr 😭

🕵️ Attribute tracking for warning 53 🕵️

type current_phase =
        | Parser
        | Invariant_check

register_attr must be called on the locations of all attributes that should be tracked for the purpose of misplaced attribute warnings. In particular, it should be called on all attributes that are present in the source program except those that are contained in the payload of another attribute (because these may be left behind by a ppx and intentionally ignored by the compiler).

The current_phase argument indicates when this function is being called: -either when an attribute is created in the parser or when we see an attribute while running the check in the Ast_invariants module. This is used to ensure that we track only attributes from the final version of the parse tree: we skip adding attributes seen at parse time if we can see that a ppx will be run later, because the Ast_invariants check is always run on the result of a ppx.

Note that the Ast_invariants check is also run on parse trees created from marshalled ast files if no ppx is being used, ensuring we don't miss attributes in that case.

• val register_attr : current_phase -> string Location.loc -> unit
• val mark_payload_attrs_used : Parsetree.payload -> unit — marks the attributes hiding in the payload of another attribute used, for the purposes of misplaced attribute warnings (see comment on current_phase above). In the parser, it's simplest to add these to the table and remove them later, rather than threading through state tracking whether we're in an attribute payload.
• val warn_unused : unit -> unit — issue misplaced attribute warnings for all attributes created with mk_internal but not yet marked used. Does nothing if compilation is stopped before lambda due to command-line flags.

🌱 Warning 53 helpers for environment attributes 🌱

Some attributes, like deprecation markers, do not affect the compilation of the definition on which they appear, but rather result in warnings on future uses of that definition. This is implemented by moving the raw attributes into the environment, where they will be noticed on future accesses.

To make misplaced attribute warnings work appropriately for these attributes, we mark them "used" when they are moved into the environment. This is done with the helper functions in this section.

• val mark_alert_used : Parsetree.attribute -> unit — marks the attribute used for the purposes of misplaced attribute warnings if it is an alert. Call this when moving things allowed to have alert attributes into the environment.
• val mark_alerts_used : Parsetree.attributes -> unit — the same as List.iter mark_alert_used .
• val mark_warn_on_literal_pattern_used : Parsetree.attributes -> unit — marks "warn_on_literal_pattern" attributes used for the purposes of misplaced attribute warnings. Call this when moving constructors into the environment.
• val mark_deprecated_mutable_used : Parsetree.attributes -> unit — marks "deprecated_mutable" attributes used for the purposes of misplaced attribute warnings. Call this when moving labels of mutable fields into the environment.

🚨 Helpers for alert and warning attributes 🚨

• val check_alerts : Location.t -> Parsetree.attributes -> string -> unit
• val check_alerts_inclusion : def:Location.t -> use:Location.t -> Location.t -> Parsetree.attributes -> Parsetree.attributes -> string -> unit
• val alerts_of_attrs : Parsetree.attributes -> Misc.alerts
• val alerts_of_sig : mark:bool -> Parsetree.signature -> Misc.alerts
• val alerts_of_str : mark:bool -> Parsetree.structure -> Misc.alerts
• val check_deprecated_mutable : Location.t -> Parsetree.attributes -> string -> unit
• val check_deprecated_mutable_inclusion : def:Location.t -> use:Location.t -> Location.t -> Parsetree.attributes -> Parsetree.attributes -> string -> unit
• val error_of_extension : Parsetree.extension -> Location.error
• val warning_attribute : ?ppwarning:bool -> Parsetree.attribute -> unit — apply warning settings from the specified attribute. "ocaml.warning"/"ocaml.warnerror" (and variants without the prefix) are processed and marked used for warning 53. Other attributes are ignored. Also implement ocaml.ppwarning (unless ~ppwarning:false is passed).
• val warning_scope : ?ppwarning:bool -> Parsetree.attributes -> (unit -> 'a) -> 'a — execute a function in a new scope for warning settings. This means that the effect of any call to warning_attribute during the execution of this function will be discarded after execution. The function also takes a list of attributes which are processed with warning_attribute in the fresh scope before the function is executed.

🔎 Helpers for searching for particular attributes 🔎

• val has_attribute : string -> Parsetree.attributes -> bool — has_attribute name attrs is true if an attribute with name name or "ocaml." ^ name is present in attrs . It marks that attribute used for the purposes of misplaced attribute warnings.

type attr_action =
        | Mark_used_only
        | Return

select_attributes actions attrs finds the elements of attrs that appear in actions and either returns them or just marks them used, according to the corresponding attr_action .

Each element (nm, action) of the actions list is an attribute along with an attr_action specifying what to do with that attribute. The action is used to accommodate different compiler configurations. If an attribute is used only in some compiler configurations, it's important that we still look for it and mark it used when compiling with other configurations. Otherwise, we would issue spurious misplaced attribute warnings.

• val select_attributes : (string * attr_action) list -> Parsetree.attributes -> Parsetree.attributes
• val attr_equals_builtin : Parsetree.attribute -> string -> bool — attr_equals_builtin attr s is true if the name of the attribute is s or "ocaml." ^ s . This is useful for manually inspecting attribute names, but note that doing so will not result in marking the attribute used for the purpose of warning 53, so it is usually preferable to use has_attribute or select_attributes .
• val warn_on_literal_pattern : Parsetree.attributes -> bool
• val explicit_arity : Parsetree.attributes -> bool
• val immediate : Parsetree.attributes -> bool
• val immediate64 : Parsetree.attributes -> bool
• val has_unboxed : Parsetree.attributes -> bool
• val has_boxed : Parsetree.attributes -> bool
• val has_remove_aliases : Parsetree.attributes -> bool
• val has_atomic : Parsetree.attributes -> bool

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mwah 💋 source/version: OCamldoc; date: 2026-06-22 — bye bestie, go forth and attribute responsibly 🫶✨