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Document force-warn

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Co-authored-by: Mark Rousskov <mark.simulacrum@gmail.com>
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inquisitivecrystal 2021-07-23 23:31:09 -07:00 • committed by Mark Rousskov
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39
src/doc/rustc/src/lints/levels.md
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@ -1,11 +1,12 @@
# Lint levels
In `rustc`, lints are divided into four *levels*:
In `rustc`, lints are divided into five *levels*:
1. allow
2. warn
3. deny
4. forbid
3. force-warn
4. deny
5. forbid
Each lint has a default level (explained in the lint listing later in this
chapter), and the compiler has a default warning level. First, let's explain
@ -57,6 +58,14 @@ warning: unused variable: `x`
= note: to avoid this warning, consider using `_x` instead
```
## force-warn
'force-warn' is a special lint level. It's the same as 'warn' in that a lint
at this level will produce a warning, but unlike the 'warn' level, the
'force-warn' level cannot be overridden. If a lint is set to 'force-warn', it
is guaranteed to warn: no more, no less. This is true even if the overall lint
level is capped via cap-lints.
## deny
A 'deny' lint produces an error if you violate it. For example, this code
@ -87,11 +96,12 @@ This lint level gives you that.
## forbid
'forbid' is a special lint level that's stronger than 'deny'. It's the same
as 'deny' in that a lint at this level will produce an error, but unlike the
'deny' level, the 'forbid' level can not be overridden to be anything lower
than an error. However, lint levels may still be capped with `--cap-lints`
(see below) so `rustc --cap-lints warn` will make lints set to 'forbid' just
'forbid' is a special lint level that fills the same role for 'deny' that
'force-warn' does for 'warn'. It's the same as 'deny' in that a lint at this
level will produce an error, but unlike the 'deny' level, the 'forbid' level
can not be overridden to be anything lower than an error. However, lint
levels may still be capped with `--cap-lints` (see below) so `rustc --cap-
lints warn` will make lints set to 'forbid' just
warn.
## Configuring warning levels
@ -113,8 +123,8 @@ certain lint levels. We'll talk about that last.
### Via compiler flag
The `-A`, `-W`, `-D`, and `-F` flags let you turn one or more lints
into allowed, warning, deny, or forbid levels, like this:
The `-A`, `-W`, `--force-warn` `-D`, and `-F` flags let you turn one or more lints
into allowed, warning, force-warn, deny, or forbid levels, like this:
```bash
$ rustc lib.rs --crate-type=lib -W missing-docs
@ -158,7 +168,7 @@ You can also pass each flag more than once for changing multiple lints:
$ rustc lib.rs --crate-type=lib -D missing-docs -D unused-variables
```
And of course, you can mix these four flags together:
And of course, you can mix these five flags together:
```bash
$ rustc lib.rs --crate-type=lib -D missing-docs -A unused-variables
@ -176,6 +186,10 @@ You can make use of this behavior by overriding the level of one specific lint o
$ rustc lib.rs --crate-type=lib -D unused -A unused-variables
```
Since `force-warn` and `forbid` cannot be overridden, setting
one of them will prevent any later level for the same lint from
taking effect.
### Via an attribute
You can also modify the lint level with a crate-wide attribute:
@ -207,7 +221,8 @@ warning: missing documentation for a function
| ^^^^^^^^^^^^
```
All four, `warn`, `allow`, `deny`, and `forbid` all work this way.
`warn`, `allow`, `deny`, and `forbid` all work this way. There is
no way to set a lint to `force-warn` using an attribute.
You can also pass in multiple lints per attribute:

21
src/doc/unstable-book/src/compiler-flags/force-warn.md
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@ -1,21 +0,0 @@
# `force-warn`
The tracking issue for this feature is: [#85512](https://github.com/rust-lang/rust/issues/85512).
------------------------
This feature allows you to cause any lint to produce a warning even if the lint has a different level by default or another level is set somewhere else. For instance, the `force-warn` option can be used to make a lint (e.g., `dead_code`) produce a warning even if that lint is allowed in code with `#![allow(dead_code)]`.
## Example
```rust,ignore (partial-example)
#![allow(dead_code)]
fn dead_function() {}
// This would normally not produce a warning even though the
// function is not used, because dead code is being allowed
fn main() {}
```
We can force a warning to be produced by providing `--force-warn dead_code` to rustc.