Auto merge of rust-lang#138677 - shepmaster:consistent-elided-lifetime-syntax, r=traviscross,jieyouxu

Add a new `mismatched-lifetime-syntaxes` lint

The lang-team [discussed this](https://hackmd.io/nf4ZUYd7Rp6rq-1svJZSaQ) and I attempted to [summarize](rust-lang#120808 (comment)) their decision. The summary-of-the-summary is:

- Using two different kinds of syntax for elided lifetimes is confusing. In rare cases, it may even [lead to unsound code](rust-lang#48686)! Some examples:

    ```rust
    // Lint will warn about these
    fn(v: ContainsLifetime) -> ContainsLifetime<'_>;
    fn(&'static u8) -> &u8;
    ```

- Matching up references with no lifetime syntax, references with anonymous lifetime syntax, and paths with anonymous lifetime syntax is an exception to the simplest possible rule:

    ```rust
    // Lint will not warn about these
    fn(&u8) -> &'_ u8;
    fn(&'_ u8) -> &u8;
    fn(&u8) -> ContainsLifetime<'_>;
    ```

- Having a lint for consistent syntax of elided lifetimes will make the [future goal](rust-lang#91639) of warning-by-default for paths participating in elision much simpler.

---

This new lint attempts to accomplish the goal of enforcing consistent syntax. In the process, it supersedes and replaces the existing `elided-named-lifetimes` lint, which means it starts out life as warn-by-default.

Author: bors

tests/ui/explicit_iter_loop.fixed

@@ -77,11 +77,11 @@ fn main() {
 
     struct NoIntoIter();
     impl NoIntoIter {
-        fn iter(&self) -> slice::Iter<u8> {
+        fn iter(&self) -> slice::Iter<'_, u8> {
             unimplemented!()
         }
 
-        fn iter_mut(&mut self) -> slice::IterMut<u8> {
+        fn iter_mut(&mut self) -> slice::IterMut<'_, u8> {
             unimplemented!()
         }
     }

tests/ui/explicit_iter_loop.rs

@@ -77,11 +77,11 @@ fn main() {
 
     struct NoIntoIter();
     impl NoIntoIter {
-        fn iter(&self) -> slice::Iter<u8> {
+        fn iter(&self) -> slice::Iter<'_, u8> {
             unimplemented!()
         }
 
-        fn iter_mut(&mut self) -> slice::IterMut<u8> {
+        fn iter_mut(&mut self) -> slice::IterMut<'_, u8> {
             unimplemented!()
         }
     }

tests/ui/iter_next_loop.rs

@@ -8,7 +8,7 @@ fn main() {
 
     struct Unrelated(&'static [u8]);
     impl Unrelated {
-        fn next(&self) -> std::slice::Iter<u8> {
+        fn next(&self) -> std::slice::Iter<'_, u8> {
             self.0.iter()
         }
     }

tests/ui/iter_not_returning_iterator.rs

@@ -71,7 +71,7 @@ impl S {
 
 struct S2([u8]);
 impl S2 {
-    fn iter(&self) -> core::slice::Iter<u8> {
+    fn iter(&self) -> core::slice::Iter<'_, u8> {
         self.0.iter()
     }
 }

tests/ui/methods.rs

@@ -49,7 +49,7 @@ struct Lt2<'a> {
 
 impl<'a> Lt2<'a> {
     // The lifetime is different, but that’s irrelevant; see issue #734.
-    pub fn new(s: &str) -> Lt2 {
+    pub fn new(s: &str) -> Lt2<'_> {
         unimplemented!()
     }
 }

tests/ui/needless_lifetimes.fixed

@@ -10,7 +10,7 @@
     clippy::unnecessary_wraps,
     dyn_drop,
     clippy::get_first,
-    elided_named_lifetimes
+    mismatched_lifetime_syntaxes,
 )]
 
 extern crate proc_macros;

tests/ui/needless_lifetimes.rs

@@ -10,7 +10,7 @@
     clippy::unnecessary_wraps,
     dyn_drop,
     clippy::get_first,
-    elided_named_lifetimes
+    mismatched_lifetime_syntaxes,
 )]
 
 extern crate proc_macros;

tests/ui/ptr_arg.rs

@@ -312,7 +312,7 @@ mod issue_9218 {
 
     // Inferred to be `&'a str`, afaik.
     fn cow_good_ret_ty<'a>(input: &'a Cow<'a, str>) -> &str {
-        //~^ ERROR: elided lifetime has a name
+        //~^ ERROR: lifetime flowing from input to output with different syntax
         todo!()
     }
 }

tests/ui/ptr_arg.stderr

@@ -1,12 +1,3 @@
-error: elided lifetime has a name
-  --> tests/ui/ptr_arg.rs:314:56
-   |
-LL |     fn cow_good_ret_ty<'a>(input: &'a Cow<'a, str>) -> &str {
-   |                        -- lifetime `'a` declared here  ^ this elided lifetime gets resolved as `'a`
-   |
-   = note: `-D elided-named-lifetimes` implied by `-D warnings`
-   = help: to override `-D warnings` add `#[allow(elided_named_lifetimes)]`
-
 error: writing `&Vec` instead of `&[_]` involves a new object where a slice will do
   --> tests/ui/ptr_arg.rs:13:14
    |
@@ -240,5 +231,21 @@ error: writing `&String` instead of `&str` involves a new object where a slice w
 LL |     fn good(v1: &String, v2: &String) {
    |                              ^^^^^^^ help: change this to: `&str`
 
+error: lifetime flowing from input to output with different syntax can be confusing
+  --> tests/ui/ptr_arg.rs:314:36
+   |
+LL |     fn cow_good_ret_ty<'a>(input: &'a Cow<'a, str>) -> &str {
+   |                                    ^^     ^^           ---- the lifetime gets resolved as `'a`
+   |                                    |      |
+   |                                    |      these lifetimes flow to the output
+   |                                    these lifetimes flow to the output
+   |
+   = note: `-D mismatched-lifetime-syntaxes` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(mismatched_lifetime_syntaxes)]`
+help: one option is to consistently use `'a`
+   |
+LL |     fn cow_good_ret_ty<'a>(input: &'a Cow<'a, str>) -> &'a str {
+   |                                                         ++
+
 error: aborting due to 27 previous errors
 

tests/ui/significant_drop_in_scrutinee.rs

@@ -191,7 +191,7 @@ struct CounterWrapper<'a> {
 }
 
 impl<'a> CounterWrapper<'a> {
-    fn new(counter: &Counter) -> CounterWrapper {
+    fn new(counter: &Counter) -> CounterWrapper<'_> {
         counter.i.fetch_add(1, Ordering::Relaxed);
         CounterWrapper { counter }
     }
@@ -204,7 +204,7 @@ impl<'a> Drop for CounterWrapper<'a> {
 }
 
 impl Counter {
-    fn temp_increment(&self) -> Vec<CounterWrapper> {
+    fn temp_increment(&self) -> Vec<CounterWrapper<'_>> {
         vec![CounterWrapper::new(self), CounterWrapper::new(self)]
     }
 }
@@ -480,7 +480,7 @@ impl StateWithBoxedMutexGuard {
     fn new() -> StateWithBoxedMutexGuard {
         StateWithBoxedMutexGuard { u: Mutex::new(42) }
     }
-    fn lock(&self) -> Box<MutexGuard<u64>> {
+    fn lock(&self) -> Box<MutexGuard<'_, u64>> {
         Box::new(self.u.lock().unwrap())
     }
 }
@@ -507,7 +507,7 @@ impl StateStringWithBoxedMutexGuard {
             s: Mutex::new("A String".to_owned()),
         }
     }
-    fn lock(&self) -> Box<MutexGuard<String>> {
+    fn lock(&self) -> Box<MutexGuard<'_, String>> {
         Box::new(self.s.lock().unwrap())
     }
 }
@@ -686,11 +686,11 @@ struct Guard<'a, T>(MutexGuard<'a, T>);
 struct Ref<'a, T>(&'a T);
 
 impl<'a, T> Guard<'a, T> {
-    fn guard(&self) -> &MutexGuard<T> {
+    fn guard(&self) -> &MutexGuard<'_, T> {
         &self.0
     }
 
-    fn guard_ref(&self) -> Ref<MutexGuard<T>> {
+    fn guard_ref(&self) -> Ref<'_, MutexGuard<'_, T>> {
         Ref(&self.0)
     }