Improve diagnostics when a static lifetime is expected

compiler/rustc_infer/src/infer/error_reporting/nice_region_error/named_anon_conflict.rs

@@ -3,8 +3,6 @@
 use crate::infer::error_reporting::nice_region_error::find_anon_type::find_anon_type;
 use crate::infer::error_reporting::nice_region_error::NiceRegionError;
 use rustc_errors::{struct_span_err, Applicability, DiagnosticBuilder};
-use rustc_hir::intravisit::Visitor;
-use rustc_hir::FnRetTy;
 use rustc_middle::ty;
 
 impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
@@ -48,19 +46,24 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
             return None; // inapplicable
         };
 
+        // Suggesting to add a `'static` lifetime to a parameter is nearly always incorrect,
+        // and can steer users down the wrong path.
+        if *named == ty::ReStatic {
+            return None;
+        }
+
         debug!("try_report_named_anon_conflict: named = {:?}", named);
         debug!("try_report_named_anon_conflict: anon_param_info = {:?}", anon_param_info);
         debug!("try_report_named_anon_conflict: region_info = {:?}", region_info);
 
-        let (param, new_ty, new_ty_span, br, is_first, scope_def_id, is_impl_item) = (
-            anon_param_info.param,
-            anon_param_info.param_ty,
-            anon_param_info.param_ty_span,
-            anon_param_info.bound_region,
-            anon_param_info.is_first,
-            region_info.def_id,
-            region_info.is_impl_item,
-        );
+        let param = anon_param_info.param;
+        let new_ty = anon_param_info.param_ty;
+        let new_ty_span = anon_param_info.param_ty_span;
+        let br = anon_param_info.bound_region;
+        let is_first = anon_param_info.is_first;
+        let scope_def_id = region_info.def_id;
+        let is_impl_item = region_info.is_impl_item;
+
         match br {
             ty::BrAnon(_) => {}
             _ => {
@@ -75,26 +78,10 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
             return None;
         }
 
-        if let Some((_, fndecl)) = find_anon_type(self.tcx(), anon, &br) {
-            if self.is_self_anon(is_first, scope_def_id) {
-                return None;
-            }
-
-            if let FnRetTy::Return(ty) = &fndecl.output {
-                let mut v = ty::TraitObjectVisitor(vec![], self.tcx().hir());
-                v.visit_ty(ty);
-
-                debug!("try_report_named_anon_conflict: ret ty {:?}", ty);
-                if sub == &ty::ReStatic
-                    && v.0.into_iter().any(|t| t.span.desugaring_kind().is_none())
-                {
-                    // If the failure is due to a `'static` requirement coming from a `dyn` or
-                    // `impl` Trait that *isn't* caused by `async fn` desugaring, handle this case
-                    // better in `static_impl_trait`.
-                    debug!("try_report_named_anon_conflict: impl Trait + 'static");
-                    return None;
-                }
-            }
+        if find_anon_type(self.tcx(), anon, &br).is_some()
+            && self.is_self_anon(is_first, scope_def_id)
+        {
+            return None;
         }
 
         let (error_var, span_label_var) = match param.pat.simple_ident() {
@@ -114,16 +101,12 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
         );
 
         diag.span_label(span, format!("lifetime `{}` required", named));
-        // Suggesting `'static` is nearly always incorrect, and can steer users
-        // down the wrong path.
-        if *named != ty::ReStatic {
-            diag.span_suggestion(
-                new_ty_span,
-                &format!("add explicit lifetime `{}` to {}", named, span_label_var),
-                new_ty.to_string(),
-                Applicability::Unspecified,
-            );
-        }
+        diag.span_suggestion(
+            new_ty_span,
+            &format!("add explicit lifetime `{}` to {}", named, span_label_var),
+            new_ty.to_string(),
+            Applicability::Unspecified,
+        );
 
         Some(diag)
     }

src/test/ui/closures/closure-bounds-static-cant-capture-borrowed.nll.stderr

@@ -1,11 +1,18 @@
-error[E0621]: explicit lifetime required in the type of `x`
+error[E0521]: borrowed data escapes outside of function
   --> $DIR/closure-bounds-static-cant-capture-borrowed.rs:5:5
    |
+LL |   fn foo(x: &()) {
+   |          -  - let's call the lifetime of this reference `'1`
+   |          |
+   |          `x` is a reference that is only valid in the function body
 LL | /     bar(|| {
 LL | |
 LL | |         let _ = x;
 LL | |     })
-   | |______^ lifetime `'static` required
+   | |      ^
+   | |      |
+   | |______`x` escapes the function body here
+   |        argument requires that `'1` must outlive `'static`
 
 error[E0373]: closure may outlive the current function, but it borrows `x`, which is owned by the current function
   --> $DIR/closure-bounds-static-cant-capture-borrowed.rs:5:9
@@ -31,5 +38,5 @@ LL |     bar(move || {
 
 error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0373, E0621.
+Some errors have detailed explanations: E0373, E0521.
 For more information about an error, try `rustc --explain E0373`.

src/test/ui/closures/closure-bounds-static-cant-capture-borrowed.rs

@@ -3,7 +3,7 @@ fn bar<F>(blk: F) where F: FnOnce() + 'static {
 
 fn foo(x: &()) {
     bar(|| {
-        //~^ ERROR explicit lifetime required in the type of `x` [E0621]
+        //~^ ERROR `x` has an anonymous lifetime `'_` but it needs to satisfy a `'static` lifetime requirement [E0759]
         let _ = x;
     })
 }

src/test/ui/closures/closure-bounds-static-cant-capture-borrowed.stderr

@@ -1,9 +1,21 @@
-error[E0621]: explicit lifetime required in the type of `x`
+error[E0759]: `x` has an anonymous lifetime `'_` but it needs to satisfy a `'static` lifetime requirement
+  --> $DIR/closure-bounds-static-cant-capture-borrowed.rs:5:9
+   |
+LL |   fn foo(x: &()) {
+   |             --- this data with an anonymous lifetime `'_`...
+LL |       bar(|| {
+   |  _________^
+LL | |
+LL | |         let _ = x;
+LL | |     })
+   | |_____^ ...is captured here...
+   |
+note: ...and is required to live as long as `'static` here
   --> $DIR/closure-bounds-static-cant-capture-borrowed.rs:5:5
    |
 LL |     bar(|| {
-   |     ^^^ lifetime `'static` required
+   |     ^^^
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0621`.
+For more information about this error, try `rustc --explain E0759`.

src/test/ui/generator/generator-region-requirements.nll.stderr

@@ -0,0 +1,11 @@
+error: lifetime may not live long enough
+  --> $DIR/generator-region-requirements.rs:13:51
+   |
+LL | fn dangle(x: &mut i32) -> &'static mut i32 {
+   |              - let's call the lifetime of this reference `'1`
+...
+LL |             GeneratorState::Complete(c) => return c,
+   |                                                   ^ returning this value requires that `'1` must outlive `'static`
+
+error: aborting due to previous error
+

src/test/ui/generator/generator-region-requirements.rs

@@ -6,11 +6,11 @@ fn dangle(x: &mut i32) -> &'static mut i32 {
     let mut g = || {
         yield;
         x
+        //~^ ERROR `x` has an anonymous lifetime `'_` but it needs to satisfy a `'static` lifetime requirement [E0759]
     };
     loop {
         match Pin::new(&mut g).resume(()) {
             GeneratorState::Complete(c) => return c,
-            //~^ ERROR explicit lifetime required
             GeneratorState::Yielded(_) => (),
         }
     }

src/test/ui/generator/generator-region-requirements.stderr

@@ -1,9 +1,15 @@
-error[E0621]: explicit lifetime required in the type of `x`
-  --> $DIR/generator-region-requirements.rs:12:51
+error[E0759]: `x` has an anonymous lifetime `'_` but it needs to satisfy a `'static` lifetime requirement
+  --> $DIR/generator-region-requirements.rs:8:9
    |
+LL | fn dangle(x: &mut i32) -> &'static mut i32 {
+   |              -------- this data with an anonymous lifetime `'_`...
+...
+LL |         x
+   |         ^ ...is captured here...
+...
 LL |             GeneratorState::Complete(c) => return c,
-   |                                                   ^ lifetime `'static` required
+   |                                                   - ...and is required to live as long as `'static` here
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0621`.
+For more information about this error, try `rustc --explain E0759`.

src/test/ui/generic-associated-types/projection-type-lifetime-mismatch.nll.stderr

@@ -0,0 +1,26 @@
+error: lifetime may not live long enough
+  --> $DIR/projection-type-lifetime-mismatch.rs:17:5
+   |
+LL | fn f(x: &impl for<'a> X<Y<'a> = &'a ()>) -> &'static () {
+   |         - let's call the lifetime of this reference `'1`
+LL |     x.m()
+   |     ^^^^^ returning this value requires that `'1` must outlive `'static`
+
+error: lifetime may not live long enough
+  --> $DIR/projection-type-lifetime-mismatch.rs:22:5
+   |
+LL | fn g<T: for<'a> X<Y<'a> = &'a ()>>(x: &T) -> &'static () {
+   |                                       - let's call the lifetime of this reference `'1`
+LL |     x.m()
+   |     ^^^^^ returning this value requires that `'1` must outlive `'static`
+
+error: lifetime may not live long enough
+  --> $DIR/projection-type-lifetime-mismatch.rs:27:5
+   |
+LL | fn h(x: &()) -> &'static () {
+   |         - let's call the lifetime of this reference `'1`
+LL |     x.m()
+   |     ^^^^^ returning this value requires that `'1` must outlive `'static`
+
+error: aborting due to 3 previous errors
+

src/test/ui/generic-associated-types/projection-type-lifetime-mismatch.rs

@@ -15,17 +15,17 @@ impl X for () {
 
 fn f(x: &impl for<'a> X<Y<'a> = &'a ()>) -> &'static () {
     x.m()
-    //~^ ERROR explicit lifetime required
+    //~^ ERROR `x` has an anonymous lifetime `'_` but it needs to satisfy a `'static` lifetime requirement [E0759]
 }
 
 fn g<T: for<'a> X<Y<'a> = &'a ()>>(x: &T) -> &'static () {
     x.m()
-    //~^ ERROR explicit lifetime required
+    //~^ ERROR `x` has an anonymous lifetime `'_` but it needs to satisfy a `'static` lifetime requirement [E0759]
 }
 
 fn h(x: &()) -> &'static () {
     x.m()
-    //~^ ERROR explicit lifetime required
+    //~^ ERROR `x` has an anonymous lifetime `'_` but it needs to satisfy a `'static` lifetime requirement [E0759]
 }
 
 fn main() {

src/test/ui/generic-associated-types/projection-type-lifetime-mismatch.stderr

@@ -1,21 +1,27 @@
-error[E0621]: explicit lifetime required in the type of `x`
-  --> $DIR/projection-type-lifetime-mismatch.rs:17:5
+error[E0759]: `x` has an anonymous lifetime `'_` but it needs to satisfy a `'static` lifetime requirement
+  --> $DIR/projection-type-lifetime-mismatch.rs:17:7
    |
+LL | fn f(x: &impl for<'a> X<Y<'a> = &'a ()>) -> &'static () {
+   |         ------------------------------- this data with an anonymous lifetime `'_`...
 LL |     x.m()
-   |     ^^^^^ lifetime `'static` required
+   |     --^-- ...is captured and required to live as long as `'static` here
 
-error[E0621]: explicit lifetime required in the type of `x`
-  --> $DIR/projection-type-lifetime-mismatch.rs:22:5
+error[E0759]: `x` has an anonymous lifetime `'_` but it needs to satisfy a `'static` lifetime requirement
+  --> $DIR/projection-type-lifetime-mismatch.rs:22:7
    |
+LL | fn g<T: for<'a> X<Y<'a> = &'a ()>>(x: &T) -> &'static () {
+   |                                       -- this data with an anonymous lifetime `'_`...
 LL |     x.m()
-   |     ^^^^^ lifetime `'static` required
+   |     --^-- ...is captured and required to live as long as `'static` here
 
-error[E0621]: explicit lifetime required in the type of `x`
-  --> $DIR/projection-type-lifetime-mismatch.rs:27:5
+error[E0759]: `x` has an anonymous lifetime `'_` but it needs to satisfy a `'static` lifetime requirement
+  --> $DIR/projection-type-lifetime-mismatch.rs:27:7
    |
+LL | fn h(x: &()) -> &'static () {
+   |         --- this data with an anonymous lifetime `'_`...
 LL |     x.m()
-   |     ^^^^^ lifetime `'static` required
+   |     --^-- ...is captured and required to live as long as `'static` here
 
 error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0621`.
+For more information about this error, try `rustc --explain E0759`.

src/test/ui/issues/issue-46983.nll.stderr

@@ -0,0 +1,10 @@
+error: lifetime may not live long enough
+  --> $DIR/issue-46983.rs:2:5
+   |
+LL | fn foo(x: &u32) -> &'static u32 {
+   |           - let's call the lifetime of this reference `'1`
+LL |     &*x
+   |     ^^^ returning this value requires that `'1` must outlive `'static`
+
+error: aborting due to previous error
+

src/test/ui/issues/issue-46983.rs

@@ -1,6 +1,6 @@
 fn foo(x: &u32) -> &'static u32 {
     &*x
-    //~^ ERROR explicit lifetime required in the type of `x` [E0621]
+    //~^ ERROR `x` has an anonymous lifetime `'_` but it needs to satisfy a `'static` lifetime requirement [E0759]
 }
 
 fn main() {}

src/test/ui/issues/issue-46983.stderr

@@ -1,9 +1,11 @@
-error[E0621]: explicit lifetime required in the type of `x`
+error[E0759]: `x` has an anonymous lifetime `'_` but it needs to satisfy a `'static` lifetime requirement
   --> $DIR/issue-46983.rs:2:5
    |
+LL | fn foo(x: &u32) -> &'static u32 {
+   |           ---- this data with an anonymous lifetime `'_`...
 LL |     &*x
-   |     ^^^ lifetime `'static` required
+   |     ^^^ ...is captured and required to live as long as `'static` here
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0621`.
+For more information about this error, try `rustc --explain E0759`.

src/test/ui/lifetimes/issue-90600-expected-return-static-indirect.nll.stderr

@@ -0,0 +1,24 @@
+error[E0597]: `foo` does not live long enough
+  --> $DIR/issue-90600-expected-return-static-indirect.rs:7:32
+   |
+LL |     let refcell = RefCell::new(&mut foo);
+   |                                ^^^^^^^^ borrowed value does not live long enough
+LL |
+LL |     let read = &refcell as &RefCell<dyn Read>;
+   |                -------- cast requires that `foo` is borrowed for `'static`
+...
+LL | }
+   | - `foo` dropped here while still borrowed
+
+error: lifetime may not live long enough
+  --> $DIR/issue-90600-expected-return-static-indirect.rs:9:16
+   |
+LL | fn inner(mut foo: &[u8]) {
+   |                   - let's call the lifetime of this reference `'1`
+...
+LL |     let read = &refcell as &RefCell<dyn Read>;
+   |                ^^^^^^^^ cast requires that `'1` must outlive `'static`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0597`.

src/test/ui/lifetimes/issue-90600-expected-return-static-indirect.rs

@@ -0,0 +1,14 @@
+use std::cell::RefCell;
+use std::io::Read;
+
+fn main() {}
+
+fn inner(mut foo: &[u8]) {
+    let refcell = RefCell::new(&mut foo);
+    //~^ ERROR `foo` has an anonymous lifetime `'_` but it needs to satisfy a `'static` lifetime requirement [E0759]
+    let read = &refcell as &RefCell<dyn Read>;
+
+    read_thing(read);
+}
+
+fn read_thing(refcell: &RefCell<dyn Read>) {}

src/test/ui/lifetimes/issue-90600-expected-return-static-indirect.stderr

@@ -0,0 +1,14 @@
+error[E0759]: `foo` has an anonymous lifetime `'_` but it needs to satisfy a `'static` lifetime requirement
+  --> $DIR/issue-90600-expected-return-static-indirect.rs:7:32
+   |
+LL | fn inner(mut foo: &[u8]) {
+   |                   ----- this data with an anonymous lifetime `'_`...
+LL |     let refcell = RefCell::new(&mut foo);
+   |                                ^^^^^^^^ ...is captured here...
+...
+LL |     read_thing(read);
+   |                ---- ...and is required to live as long as `'static` here
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0759`.

src/test/ui/nll/closure-requirements/region-lbr-anon-does-not-outlive-static.rs

@@ -7,7 +7,7 @@
 
 fn foo(x: &u32) -> &'static u32 {
     &*x
-        //~^ ERROR explicit lifetime required in the type of `x`
+    //~^ ERROR lifetime may not live long enough
 }
 
 fn main() { }