|
330 | 330 |
|
331 | 331 | This feature is used heavily by Cargo; it will pass `--cap-lints allow` when |
332 | 332 | compiling your dependencies, so that if they have any warnings, they do not |
333 | | -pollute the output of your build. |
| 333 | +pollute the output of your build. However, note that `--cap-lints allow` does **not** override lints marked as `force-warn`. |
| 334 | +
|
| 335 | +## Priority of lint level sources |
| 336 | +
|
| 337 | +Rust allows setting lint levels (`allow`, `warn`, `deny`, `forbid`, `force-warn`) through various sources: |
| 338 | +
|
| 339 | +- **Attributes**: `#[allow(...)]`, `#![deny(...)]`, etc. |
| 340 | +- **Command-line options**: `--cap-lints`, `--force-warn`, `-A`, `-W`, `-D`, `-F` |
| 341 | +
|
| 342 | +Here’s how these different lint controls interact: |
| 343 | +
|
| 344 | +1. [`--force-warn`](#force-warn) forces a lint to warning level, and takes precedence over attributes and all other CLI flags. |
| 345 | +
|
| 346 | + ```rust,compile_fail |
| 347 | + #[forbid(unused_variables)] |
| 348 | + fn main() { |
| 349 | + let x = 42; |
| 350 | + } |
| 351 | + ``` |
| 352 | +
|
| 353 | + Compiled with: |
| 354 | +
|
| 355 | + ```bash |
| 356 | + $ rustc --force-warn unused_variables lib.rs |
| 357 | + warning: unused variable: `x` |
| 358 | + --> lib.rs:3:9 |
| 359 | + | |
| 360 | + 3 | let x = 42; |
| 361 | + | ^ help: if this is intentional, prefix it with an underscore: `_x` |
| 362 | + | |
| 363 | + = note: requested on the command line with `--force-warn unused-variables` |
| 364 | +
|
| 365 | + warning: 1 warning emitted |
| 366 | + ``` |
| 367 | +
|
| 368 | +2. [`--cap-lints`](#capping-lints) sets the maximum level of a lint, and takes precedence over attributes as well as the `-D`, `-W`, and `-F` CLI flags. |
| 369 | +
|
| 370 | + ```rust,compile_fail |
| 371 | + #[deny(unused_variables)] |
| 372 | + fn main() { |
| 373 | + let x = 42; |
| 374 | + } |
| 375 | + ``` |
| 376 | +
|
| 377 | + Compiled with: |
| 378 | +
|
| 379 | + ```bash |
| 380 | + $ rustc --cap-lints=warn lib.rs |
| 381 | + warning: unused variable: `x` |
| 382 | + --> test1.rs:3:9 |
| 383 | + | |
| 384 | + 3 | let x = 42; |
| 385 | + | ^ help: if this is intentional, prefix it with an underscore: `_x` |
| 386 | + | |
| 387 | + note: the lint level is defined here |
| 388 | + --> test1.rs:1:8 |
| 389 | + | |
| 390 | + 1 | #[deny(unused_variables)] |
| 391 | + | ^^^^^^^^^^^^^^^^ |
| 392 | +
|
| 393 | + warning: 1 warning emitted |
| 394 | + ``` |
| 395 | +
|
| 396 | +3. [CLI level flags](#via-compiler-flag) take precedence over attributes. |
| 397 | +
|
| 398 | + The order of the flags matter; flags on the right take precedence over earlier flags. |
| 399 | +
|
| 400 | + ```rust |
| 401 | + fn main() { |
| 402 | + let x = 42; |
| 403 | + } |
| 404 | + ``` |
| 405 | +
|
| 406 | + Compiled with: |
| 407 | +
|
| 408 | + ```bash |
| 409 | + $ rustc -A unused_variables -D unused_variables lib.rs |
| 410 | + error: unused variable: `x` |
| 411 | + --> test1.rs:2:9 |
| 412 | + | |
| 413 | + 2 | let x = 42; |
| 414 | + | ^ help: if this is intentional, prefix it with an underscore: `_x` |
| 415 | + | |
| 416 | + = note: requested on the command line with `-D unused-variables` |
| 417 | +
|
| 418 | + error: aborting due to 1 previous error |
| 419 | + ``` |
| 420 | +
|
| 421 | +4. Within the source, [attributes](#via-an-attribute) at a lower-level in the syntax tree take precedence over attributes at a higher level, or from a previous attribute on the same entity as listed in left-to-right source order. |
| 422 | +
|
| 423 | + ```rust |
| 424 | + #![deny(unused_variables)] |
| 425 | +
|
| 426 | + #[allow(unused_variables)] |
| 427 | + fn main() { |
| 428 | + let x = 42; // Allow wins |
| 429 | + } |
| 430 | + ``` |
| 431 | +
|
| 432 | + - The exception is once a lint is set to "forbid", it is an error to try to change its level except for `deny`, which is allowed inside a forbid context, but is ignored. |
| 433 | +
|
| 434 | +In terms of priority, [lint groups](groups.md) are treated as-if they are expanded to a list of all of the lints they contain. The exception is the `warnings` group which ignores attribute and CLI order and applies to all lints that would otherwise warn within the entity. |
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