Add a lint to warn on `T: Drop` bounds
**What it does:** Checks for generics with `std::ops::Drop` as bounds.
**Why is this bad?** `Drop` bounds do not really accomplish anything.
A type may have compiler-generated drop glue without implementing the
`Drop` trait itself. The `Drop` trait also only has one method,
`Drop::drop`, and that function is by fiat not callable in user code.
So there is really no use case for using `Drop` in trait bounds.
**Known problems:** None.
**Example:**
```rust
fn foo<T: Drop>() {}
```
Fixes#3773
**What it does:** Checks for generics with `std::ops::Drop` as bounds.
**Why is this bad?** `Drop` bounds do not really accomplish anything.
A type may have compiler-generated drop glue without implementing the
`Drop` trait itself. The `Drop` trait also only has one method,
`Drop::drop`, and that function is by fiat not callable in user code.
So there is really no use case for using `Drop` in trait bounds.
**Known problems:** None.
**Example:**
```rust
fn foo<T: Drop>() {}
```
Fix ICE in needless_pass_by_value lint
If I understand it correctly, we were first creating a type with a
`RegionKind::ReErased` region and then deleted it again in
`util::implements_trait` with:
cx.tcx.erase_regions(&ty);
causing the type query to fail.
It looks like using `ReEmpty` works around that deletion.
Fixes#3144
Macro check for assertion_on_constants lint
The `assertion_on_constants` lint currently has following output for this code [Playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=6f2c9df6fc50baf847212d3b5136ee97):
```rust
macro_rules! assert_const {
($len:expr) => {
assert!($len > 0);
}
}
fn main() {
assert_const!(3);
assert_const!(-1);
}
```
```
warning: assert!(const: true) will be optimized out by the compiler
--> src/main.rs:3:9
|
3 | assert!($len > 0);
| ^^^^^^^^^^^^^^^^^^
...
8 | assert_const!(3);
| ---------------- in this macro invocation
|
= note: #[warn(clippy::assertions_on_constants)] on by default
= help: remove it
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#assertions_on_constants
warning: assert!(const: false) should probably be replaced
--> src/main.rs:3:9
|
3 | assert!($len > 0);
| ^^^^^^^^^^^^^^^^^^
...
9 | assert_const!(-1);
| ----------------- in this macro invocation
|
= help: use panic!() or unreachable!()
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#assertions_on_constants
```
This is contradictory. This lint should not trigger if the `assert!` is in a macro itself.
If I understand it correctly, we were first creating a type with a
`RegionKind::ReErased` region and then deleted it again in
`util::implements_trait` with:
cx.tcx.erase_regions(&ty);
causing the type query to fail.
It looks like using `ReEmpty` works around that deletion.
Fix `cast_sign_loss` false positive
This checks if the value is a non-negative constant before linting about
losing the sign.
Because the `constant` function doesn't handle const functions, we check if
the value is from a call to a `max_value` function directly. A utility method
called `get_def_path` was added to make checking for the function paths
easier.
Fixes#2728
Adding lint test for excessive LOC.
This is a WIP for #2377. Just wanted to pull in because I had a few questions:
1. Is it okay that I'm approaching this via counting by looking at each line in the snippet instead of looking at the AST tree? If there's another way to do it, I want to make sure I'm doing the correct way, but I wasn't sure since the output AST JSON doesn't seem to contain whitespace.
2. My function is definitely going to trigger the lint, so also wanted to see if there was something obvious I could do to reduce it.
3. Are the two tests fine, or is there something obvious I'm missing?
4. Obviously bigger question - am I approaching the line count correctly. Current strategy is count a line if it contains some code, so skip if it's just comments or empty.
`hir::Ty` doesn't seem to know anything about type bounds and
`cx.tcx.type_of(def_id)` caused an ICE when it was passed a generic type
with a bound:
```
src/librustc_typeck/collect.rs:1311: unexpected non-type Node::GenericParam: Type { default: None, synthetic: None }
```
Converting it to a proper `Ty` fixes the ICE and catches a few more
places where the lint applies.
This checks if the value is a non-negative constant before linting about
losing the sign.
Because the `constant` function doesn't handle const functions, we check if
the value is from a call to a `max_value` function directly. A utility method
called `get_def_path` was added to make checking for the function paths
easier.
Fixes#2728
Add initial version of const_fn lint
This adds an initial version of a lint that can tell if a function could be `const`.
TODO:
- [x] Finish up the docs
- [x] Fix the ICE
cc #2440
