Rollup merge of #108230 - LittleFall:enhance/warning, r=estebank

Convert a hard-warning about named static lifetimes into lint "unused_lifetimes"

Fixes #96956.

Some changes are ported from #98079, thanks to jeremydavis519.

r? `@estebank` `@petrochenkov`

Any feedback is appreciated!

## Actions
- [x] resolve conflicts
- [x] fix build
- [x] address review comments in last pr
- [x] update tests

compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs

@@ -18,6 +18,7 @@ use rustc_middle::bug;
 use rustc_middle::hir::nested_filter;
 use rustc_middle::middle::resolve_bound_vars::*;
 use rustc_middle::ty::{self, ir::TypeVisitor, DefIdTree, TyCtxt, TypeSuperVisitable};
+use rustc_session::lint;
 use rustc_span::def_id::DefId;
 use rustc_span::symbol::{sym, Ident};
 use rustc_span::Span;
@@ -923,17 +924,16 @@ impl<'a, 'tcx> Visitor<'tcx> for BoundVarContext<'a, 'tcx> {
                         origin,
                         ..
                     }) => {
-
                         let (bound_vars, binders): (FxIndexMap<LocalDefId, ResolvedArg>, Vec<_>) =
                             bound_generic_params
-                            .iter()
-                            .enumerate()
-                            .map(|(late_bound_idx, param)| {
-                                let pair = ResolvedArg::late(late_bound_idx as u32, param);
-                                let r = late_arg_as_bound_arg(this.tcx, &pair.1, param);
-                                (pair, r)
-                            })
-                            .unzip();
+                                .iter()
+                                .enumerate()
+                                .map(|(late_bound_idx, param)| {
+                                    let pair = ResolvedArg::late(late_bound_idx as u32, param);
+                                    let r = late_arg_as_bound_arg(this.tcx, &pair.1, param);
+                                    (pair, r)
+                                })
+                                .unzip();
                         this.record_late_bound_vars(hir_id, binders.clone());
                         // Even if there are no lifetimes defined here, we still wrap it in a binder
                         // scope. If there happens to be a nested poly trait ref (an error), that
@@ -968,20 +968,22 @@ impl<'a, 'tcx> Visitor<'tcx> for BoundVarContext<'a, 'tcx> {
                                     continue;
                                 }
                                 this.insert_lifetime(lt, ResolvedArg::StaticLifetime);
-                                this.tcx
-                                    .sess
-                                    .struct_span_warn(
-                                        lifetime.ident.span,
-                                        &format!(
-                                            "unnecessary lifetime parameter `{}`",
+                                this.tcx.struct_span_lint_hir(
+                                    lint::builtin::UNUSED_LIFETIMES,
+                                    lifetime.hir_id,
+                                    lifetime.ident.span,
+                                    format!(
+                                        "unnecessary lifetime parameter `{}`",
+                                        lifetime.ident
+                                    ),
+                                    |lint| {
+                                        let help = &format!(
+                                            "you can use the `'static` lifetime directly, in place of `{}`",
                                             lifetime.ident,
-                                        ),
-                                    )
-                                    .help(&format!(
-                                        "you can use the `'static` lifetime directly, in place of `{}`",
-                                        lifetime.ident,
-                                    ))
-                                    .emit();
+                                        );
+                                        lint.help(help)
+                                    },
+                                );
                             }
                         }
                     }

tests/ui/generic-associated-types/method-unsatified-assoc-type-predicate.stderr → tests/ui/generic-associated-types/method-unsatisfied-assoc-type-predicate.stderr

@@ -1,5 +1,5 @@
 error[E0599]: the method `f` exists for struct `S`, but its trait bounds were not satisfied
-  --> $DIR/method-unsatified-assoc-type-predicate.rs:28:7
+  --> $DIR/method-unsatisfied-assoc-type-predicate.rs:28:7
    |
 LL | struct S;
    | --------
@@ -12,7 +12,7 @@ LL |     a.f();
    |       ^ method cannot be called on `S` due to unsatisfied trait bounds
    |
 note: trait bound `<S as X>::Y<i32> = i32` was not satisfied
-  --> $DIR/method-unsatified-assoc-type-predicate.rs:12:11
+  --> $DIR/method-unsatisfied-assoc-type-predicate.rs:12:11
    |
 LL | impl<T: X<Y<i32> = i32>> M for T {}
    |           ^^^^^^^^^^^^   -     -

tests/ui/generic-associated-types/unsatified-item-lifetime-bound.rs → tests/ui/generic-associated-types/unsatisfied-item-lifetime-bound.rs

@@ -1,3 +1,5 @@
+#![warn(unused_lifetimes)]
+
 pub trait X {
     type Y<'a: 'static>;
     //~^ WARNING unnecessary lifetime parameter

tests/ui/generic-associated-types/unsatified-item-lifetime-bound.stderr → tests/ui/generic-associated-types/unsatisfied-item-lifetime-bound.stderr

@@ -1,45 +1,50 @@
 warning: unnecessary lifetime parameter `'a`
-  --> $DIR/unsatified-item-lifetime-bound.rs:2:12
+  --> $DIR/unsatisfied-item-lifetime-bound.rs:4:12
    |
 LL |     type Y<'a: 'static>;
    |            ^^
    |
    = help: you can use the `'static` lifetime directly, in place of `'a`
+note: the lint level is defined here
+  --> $DIR/unsatisfied-item-lifetime-bound.rs:1:9
+   |
+LL | #![warn(unused_lifetimes)]
+   |         ^^^^^^^^^^^^^^^^
 
 error[E0478]: lifetime bound not satisfied
-  --> $DIR/unsatified-item-lifetime-bound.rs:11:8
+  --> $DIR/unsatisfied-item-lifetime-bound.rs:13:8
    |
 LL |     f: <T as X>::Y<'a>,
    |        ^^^^^^^^^^^^^^^
    |
 note: lifetime parameter instantiated with the lifetime `'a` as defined here
-  --> $DIR/unsatified-item-lifetime-bound.rs:10:10
+  --> $DIR/unsatisfied-item-lifetime-bound.rs:12:10
    |
 LL | struct B<'a, T: for<'r> X<Y<'r> = &'r ()>> {
    |          ^^
    = note: but lifetime parameter must outlive the static lifetime
 
 error[E0478]: lifetime bound not satisfied
-  --> $DIR/unsatified-item-lifetime-bound.rs:16:8
+  --> $DIR/unsatisfied-item-lifetime-bound.rs:18:8
    |
 LL |     f: <T as X>::Y<'a>,
    |        ^^^^^^^^^^^^^^^
    |
 note: lifetime parameter instantiated with the lifetime `'a` as defined here
-  --> $DIR/unsatified-item-lifetime-bound.rs:15:10
+  --> $DIR/unsatisfied-item-lifetime-bound.rs:17:10
    |
 LL | struct C<'a, T: X> {
    |          ^^
    = note: but lifetime parameter must outlive the static lifetime
 
 error[E0478]: lifetime bound not satisfied
-  --> $DIR/unsatified-item-lifetime-bound.rs:21:8
+  --> $DIR/unsatisfied-item-lifetime-bound.rs:23:8
    |
 LL |     f: <() as X>::Y<'a>,
    |        ^^^^^^^^^^^^^^^^
    |
 note: lifetime parameter instantiated with the lifetime `'a` as defined here
-  --> $DIR/unsatified-item-lifetime-bound.rs:20:10
+  --> $DIR/unsatisfied-item-lifetime-bound.rs:22:10
    |
 LL | struct D<'a> {
    |          ^^

tests/ui/impl-trait/equal-hidden-lifetimes.rs

@@ -5,7 +5,6 @@
 
 // `'a == 'static` so `&'a i32` is fine as the return type
 fn equal_regions_static<'a: 'static>(x: &'a i32) -> impl Sized {
-    //~^ WARNING unnecessary lifetime parameter `'a`
     x
 }
 

tests/ui/issues/issue-30438-c.rs

@@ -5,7 +5,6 @@ trait Trait { type Out; }
 struct Test<'a> { s: &'a str }
 
 fn silly<'y, 'z>(_s: &'y Test<'z>) -> &'y <Test<'z> as Trait>::Out where 'z: 'static {
-    //~^ WARN unnecessary lifetime parameter `'z`
     let x = Test { s: "this cannot last" };
     &x
     //~^ ERROR: cannot return reference to local variable `x`

tests/ui/issues/issue-30438-c.stderr

@@ -1,17 +1,9 @@
-warning: unnecessary lifetime parameter `'z`
-  --> $DIR/issue-30438-c.rs:7:74
-   |
-LL | fn silly<'y, 'z>(_s: &'y Test<'z>) -> &'y <Test<'z> as Trait>::Out where 'z: 'static {
-   |                                                                          ^^
-   |
-   = help: you can use the `'static` lifetime directly, in place of `'z`
-
 error[E0515]: cannot return reference to local variable `x`
-  --> $DIR/issue-30438-c.rs:10:5
+  --> $DIR/issue-30438-c.rs:9:5
    |
 LL |     &x
    |     ^^ returns a reference to data owned by the current function
 
-error: aborting due to previous error; 1 warning emitted
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0515`.

tests/ui/regions/regions-free-region-outlives-static-outlives-free-region.rs

@@ -8,6 +8,8 @@
 //
 //     'a : 'b
 
+#![warn(unused_lifetimes)]
+
 fn test<'a,'b>(x: &'a i32) -> &'b i32
     where 'a: 'static //~ WARN unnecessary lifetime parameter `'a`
 {

tests/ui/regions/regions-free-region-outlives-static-outlives-free-region.stderr

@@ -1,10 +1,15 @@
 warning: unnecessary lifetime parameter `'a`
-  --> $DIR/regions-free-region-outlives-static-outlives-free-region.rs:12:11
+  --> $DIR/regions-free-region-outlives-static-outlives-free-region.rs:14:11
    |
 LL |     where 'a: 'static
    |           ^^
    |
    = help: you can use the `'static` lifetime directly, in place of `'a`
+note: the lint level is defined here
+  --> $DIR/regions-free-region-outlives-static-outlives-free-region.rs:11:9
+   |
+LL | #![warn(unused_lifetimes)]
+   |         ^^^^^^^^^^^^^^^^
 
 warning: 1 warning emitted
 

tests/ui/regions/regions-static-bound-rpass.rs

@@ -1,5 +1,7 @@
 // run-pass
 
+#![warn(unused_lifetimes)]
+
 fn invariant_id<'a,'b>(t: &'b mut &'static ()) -> &'b mut &'a ()
     where 'a: 'static { t }
 //~^ WARN unnecessary lifetime parameter `'a`

tests/ui/regions/regions-static-bound-rpass.stderr

@@ -1,21 +1,26 @@
 warning: unnecessary lifetime parameter `'a`
-  --> $DIR/regions-static-bound-rpass.rs:4:11
+  --> $DIR/regions-static-bound-rpass.rs:6:11
    |
 LL |     where 'a: 'static { t }
    |           ^^
    |
    = help: you can use the `'static` lifetime directly, in place of `'a`
+note: the lint level is defined here
+  --> $DIR/regions-static-bound-rpass.rs:3:9
+   |
+LL | #![warn(unused_lifetimes)]
+   |         ^^^^^^^^^^^^^^^^
 
 warning: unnecessary lifetime parameter `'a`
-  --> $DIR/regions-static-bound-rpass.rs:8:11
+  --> $DIR/regions-static-bound-rpass.rs:10:11
    |
 LL |     where 'a: 'static { t }
    |           ^^
    |
    = help: you can use the `'static` lifetime directly, in place of `'a`
 
 warning: unnecessary lifetime parameter `'b`
-  --> $DIR/regions-static-bound-rpass.rs:12:19
+  --> $DIR/regions-static-bound-rpass.rs:14:19
    |
 LL |     where 'a: 'b, 'b: 'static { t }
    |                   ^^

tests/ui/regions/regions-static-bound.rs

@@ -1,6 +1,8 @@
-fn static_id<'a,'b>(t: &'a ()) -> &'static ()
-    where 'a: 'static { t }
-//~^ WARN unnecessary lifetime parameter `'a`
+#![warn(unused_lifetimes)]
+
+fn static_id<'a,'b>(t: &'a ()) -> &'static () where 'a: 'static { t }
+//~^ WARN lifetime parameter `'b` never used
+//~| WARN unnecessary lifetime parameter `'a`
 
 fn static_id_indirect<'a,'b>(t: &'a ()) -> &'static ()
     where 'a: 'b, 'b: 'static { t }

tests/ui/regions/regions-static-bound.stderr

@@ -1,29 +1,43 @@
+warning: lifetime parameter `'b` never used
+  --> $DIR/regions-static-bound.rs:3:17
+   |
+LL | fn static_id<'a,'b>(t: &'a ()) -> &'static () where 'a: 'static { t }
+   |                -^^
+   |                |
+   |                help: elide the unused lifetime
+   |
+note: the lint level is defined here
+  --> $DIR/regions-static-bound.rs:1:9
+   |
+LL | #![warn(unused_lifetimes)]
+   |         ^^^^^^^^^^^^^^^^
+
 warning: unnecessary lifetime parameter `'a`
-  --> $DIR/regions-static-bound.rs:2:11
+  --> $DIR/regions-static-bound.rs:3:53
    |
-LL |     where 'a: 'static { t }
-   |           ^^
+LL | fn static_id<'a,'b>(t: &'a ()) -> &'static () where 'a: 'static { t }
+   |                                                     ^^
    |
    = help: you can use the `'static` lifetime directly, in place of `'a`
 
 warning: unnecessary lifetime parameter `'b`
-  --> $DIR/regions-static-bound.rs:6:19
+  --> $DIR/regions-static-bound.rs:8:19
    |
 LL |     where 'a: 'b, 'b: 'static { t }
    |                   ^^
    |
    = help: you can use the `'static` lifetime directly, in place of `'b`
 
 error: lifetime may not live long enough
-  --> $DIR/regions-static-bound.rs:10:5
+  --> $DIR/regions-static-bound.rs:12:5
    |
 LL | fn static_id_wrong_way<'a>(t: &'a ()) -> &'static () where 'static: 'a {
    |                        -- lifetime `'a` defined here
 LL |     t
    |     ^ returning this value requires that `'a` must outlive `'static`
 
 error[E0521]: borrowed data escapes outside of function
-  --> $DIR/regions-static-bound.rs:15:5
+  --> $DIR/regions-static-bound.rs:17:5
    |
 LL | fn error(u: &(), v: &()) {
    |          -  - let's call the lifetime of this reference `'1`
@@ -36,7 +50,7 @@ LL |     static_id(&u);
    |     argument requires that `'1` must outlive `'static`
 
 error[E0521]: borrowed data escapes outside of function
-  --> $DIR/regions-static-bound.rs:17:5
+  --> $DIR/regions-static-bound.rs:19:5
    |
 LL | fn error(u: &(), v: &()) {
    |                  -  - let's call the lifetime of this reference `'2`
@@ -49,6 +63,6 @@ LL |     static_id_indirect(&v);
    |     `v` escapes the function body here
    |     argument requires that `'2` must outlive `'static`
 
-error: aborting due to 3 previous errors; 2 warnings emitted
+error: aborting due to 3 previous errors; 3 warnings emitted
 
 For more information about this error, try `rustc --explain E0521`.

tests/ui/static/static-lifetime-bound.rs

@@ -1,4 +1,4 @@
-fn f<'a: 'static>(_: &'a i32) {} //~WARN unnecessary lifetime parameter `'a`
+fn f<'a: 'static>(_: &'a i32) {}
 
 fn main() {
     let x = 0;

tests/ui/static/static-lifetime-bound.stderr

@@ -1,11 +1,3 @@
-warning: unnecessary lifetime parameter `'a`
-  --> $DIR/static-lifetime-bound.rs:1:6
-   |
-LL | fn f<'a: 'static>(_: &'a i32) {}
-   |      ^^
-   |
-   = help: you can use the `'static` lifetime directly, in place of `'a`
-
 error[E0597]: `x` does not live long enough
   --> $DIR/static-lifetime-bound.rs:5:7
    |
@@ -19,6 +11,6 @@ LL |     f(&x);
 LL | }
    | - `x` dropped here while still borrowed
 
-error: aborting due to previous error; 1 warning emitted
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0597`.

tests/ui/type-alias-impl-trait/bounds-are-checked.rs

@@ -6,7 +6,6 @@
 type X<'a> = impl Into<&'static str> + From<&'a str>;
 
 fn f<'a: 'static>(t: &'a str) -> X<'a> {
-    //~^ WARNING unnecessary lifetime parameter
     t
     //~^ ERROR expected generic lifetime parameter, found `'static`
 }

tests/ui/type-alias-impl-trait/bounds-are-checked.stderr

@@ -1,20 +1,12 @@
-warning: unnecessary lifetime parameter `'a`
-  --> $DIR/bounds-are-checked.rs:8:6
-   |
-LL | fn f<'a: 'static>(t: &'a str) -> X<'a> {
-   |      ^^
-   |
-   = help: you can use the `'static` lifetime directly, in place of `'a`
-
 error[E0792]: expected generic lifetime parameter, found `'static`
-  --> $DIR/bounds-are-checked.rs:10:5
+  --> $DIR/bounds-are-checked.rs:9:5
    |
 LL | type X<'a> = impl Into<&'static str> + From<&'a str>;
    |        -- cannot use static lifetime; use a bound lifetime instead or remove the lifetime parameter from the opaque type
 ...
 LL |     t
    |     ^
 
-error: aborting due to previous error; 1 warning emitted
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0792`.