Dart VM-Specific Pragma Annotations

Source: https://mrale.ph/dartvm/pragmas.html

Tools that work with Dart programs may accept hints to guide their behavior as pragma annotations on declarations. Each tool decides which hints it accepts, what they mean, and whether and how they apply to sub-parts of the annotated entity.

Tools that recognize pragma hints should pick a pragma prefix to identify the tool. They should recognize any hint with a name starting with their prefix followed by : as if it was intended for that tool. A hint with a prefix for another tool should be ignored (unless compatibility with that other tool is a goal).

A tool may recognize unprefixed names as well, if they would recognize that name with their own prefix in front.

If the hint can be parameterized, an extra options object can be added as well.

For example:

@pragma('Tool:pragma-name', [param1, param2, ...])
class Foo { }

@pragma('OtherTool:other-pragma')
void foo() { }

Here class Foo is annotated with a Tool specific pragma 'pragma-name' and function foo is annotated with a pragma 'other-pragma' specific to OtherTool.

Pragmas for general use

These pragmas are part of the VM's API and are safe for use in external code.

Pragma	Meaning
`vm:entry-point`	Defining entry-points into Dart code for an embedder or native methods
`vm:never-inline`	Never inline a function or method
`vm:prefer-inline`	Inline a function or method when possible
`vm:notify-debugger-on-exception`	Marks a function that catches exceptions, making the VM treat any caught exception as if they were uncaught. This can be used to notify an attached debugger during debugging, without pausing the app during regular execution.
`vm:external-name`	Allows to specify an external (native) name for an `external` function. This name is used to lookup native implementation via native resolver associated with the current library through embedding APIs. This is a replacement for legacy VM specific `native "name"` syntax.
`vm:invisible`	Allows to mark a function as invisible so it will not appear on stack traces.
`vm:always-consider-inlining`	Marks a function which particularly benefits from inlining and specialization in context of the caller (for example, when concrete types of arguments are known). Inliner will not give up after one failed inlining attempt and will continue trying to inline this function.

Unsafe pragmas for general use

These pragmas are available for use in third-party code but are potentially unsafe. The use of these pragmas is discouraged unless the developer fully understands potential repercussions.

Pragma	Meaning
`vm:unsafe:no-interrupts`	Removes all `CheckStackOverflow` instructions from the optimized version of the marked function, which disables stack overflow checking and interruption within that function. This pragma exists mainly for performance evaluation and should not be used in a general-purpose code, because VM relies on these checks for OOB message delivery and GC scheduling.

Pragmas for internal use

These pragmas can cause unsound behavior if used incorrectly and therefore are only allowed within the core SDK libraries.

Pragma	Meaning
`vm:exact-result-type`	Declaring an exact result type of a method
`vm:recognized`	Marking this as a recognized method

Pragmas for internal testing

These pragmas are used for inspecting or modifying internal VM state and should be used exclusively by SDK tests. They must be enabled with the --enable-testing-pragmas flag. The names of these pragmas are prefixed with "testing". Additionally, they are categorized into "safe" and "unsafe" forms: "safe" pragmas should not affect the behavior of the program and can be safely added anywhere, whereas "unsafe" pragmas may change the code's behavior or may cause the VM to crash if used improperly.

Pragma	Meaning
`vm:testing.unsafe.trace-entrypoints-fn`	Observing which flow-graph-level entry-point was used when a function was called

Flutter toString transformer pragmas

These pragmas are useful to exclude certain toString methods from toString transformation, which is enabled with --delete-tostring-package-uri option in kernel compilers and used by Flutter to remove certain toString methods in release mode to reduce size.

Pragma	Meaning
`flutter:keep-to-string`	Avoid transforming the annotated toString method.
`flutter:keep-to-string-in-subtypes`	Avoid transforming toString methods in all subtypes of the annotated class.