Rollup merge of rust-lang#55173 - estebank:suggest-static, r=oli-obk

Suggest appropriate syntax on missing lifetime specifier in return type

Suggest using `'static` when a lifetime is missing in the return type
with a structured suggestion instead of a note.

Fix rust-lang#55170.

Author: pietroalbini

src/librustc/hir/lowering.rs

@@ -1147,7 +1147,7 @@ impl<'a> LoweringContext<'a> {
             TyKind::Slice(ref ty) => hir::TyKind::Slice(self.lower_ty(ty, itctx)),
             TyKind::Ptr(ref mt) => hir::TyKind::Ptr(self.lower_mt(mt, itctx)),
             TyKind::Rptr(ref region, ref mt) => {
-                let span = t.span.shrink_to_lo();
+                let span = self.sess.source_map().next_point(t.span.shrink_to_lo());
                 let lifetime = match *region {
                     Some(ref lt) => self.lower_lifetime(lt),
                     None => self.elided_ref_lifetime(span),

src/librustc/middle/resolve_lifetime.rs

@@ -2235,21 +2235,46 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
         };
 
         let mut err = report_missing_lifetime_specifiers(self.tcx.sess, span, lifetime_refs.len());
+        let mut add_label = true;
 
         if let Some(params) = error {
             if lifetime_refs.len() == 1 {
-                self.report_elision_failure(&mut err, params);
+                add_label = add_label && self.report_elision_failure(&mut err, params, span);
             }
         }
+        if add_label {
+            add_missing_lifetime_specifiers_label(&mut err, span, lifetime_refs.len());
+        }
 
         err.emit();
     }
 
+    fn suggest_lifetime(&self, db: &mut DiagnosticBuilder<'_>, span: Span, msg: &str) -> bool {
+        match self.tcx.sess.source_map().span_to_snippet(span) {
+            Ok(ref snippet) => {
+                let (sugg, applicability) = if snippet == "&" {
+                    ("&'static ".to_owned(), Applicability::MachineApplicable)
+                } else if snippet == "'_" {
+                    ("'static".to_owned(), Applicability::MachineApplicable)
+                } else {
+                    (format!("{} + 'static", snippet), Applicability::MaybeIncorrect)
+                };
+                db.span_suggestion_with_applicability(span, msg, sugg, applicability);
+                false
+            }
+            Err(_) => {
+                db.help(msg);
+                true
+            }
+        }
+    }
+
     fn report_elision_failure(
         &mut self,
         db: &mut DiagnosticBuilder<'_>,
         params: &[ElisionFailureInfo],
-    ) {
+        span: Span,
+    ) -> bool {
         let mut m = String::new();
         let len = params.len();
 
@@ -2304,33 +2329,32 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
                 "this function's return type contains a borrowed value, but \
                  there is no value for it to be borrowed from"
             );
-            help!(db, "consider giving it a 'static lifetime");
+            self.suggest_lifetime(db, span, "consider giving it a 'static lifetime")
         } else if elided_len == 0 {
             help!(
                 db,
                 "this function's return type contains a borrowed value with \
                  an elided lifetime, but the lifetime cannot be derived from \
                  the arguments"
             );
-            help!(
-                db,
-                "consider giving it an explicit bounded or 'static \
-                 lifetime"
-            );
+            let msg = "consider giving it an explicit bounded or 'static lifetime";
+            self.suggest_lifetime(db, span, msg)
         } else if elided_len == 1 {
             help!(
                 db,
                 "this function's return type contains a borrowed value, but \
                  the signature does not say which {} it is borrowed from",
                 m
             );
+            true
         } else {
             help!(
                 db,
                 "this function's return type contains a borrowed value, but \
                  the signature does not say whether it is borrowed from {}",
                 m
             );
+            true
         }
     }
 
@@ -2744,26 +2768,28 @@ fn insert_late_bound_lifetimes(
     }
 }
 
-pub fn report_missing_lifetime_specifiers(
+fn report_missing_lifetime_specifiers(
     sess: &Session,
     span: Span,
     count: usize,
 ) -> DiagnosticBuilder<'_> {
-    let mut err = struct_span_err!(
+    struct_span_err!(
         sess,
         span,
         E0106,
         "missing lifetime specifier{}",
         if count > 1 { "s" } else { "" }
-    );
+    )
+}
 
-    let msg: Cow<'static, str> = if count > 1 {
-        format!("expected {} lifetime parameters", count).into()
+fn add_missing_lifetime_specifiers_label(
+    err: &mut DiagnosticBuilder<'_>,
+    span: Span,
+    count: usize,
+) {
+    if count > 1 {
+        err.span_label(span, format!("expected {} lifetime parameters", count));
     } else {
-        "expected lifetime parameter".into()
+        err.span_label(span, "expected lifetime parameter");
     };
-
-    err.span_label(span, msg);
-
-    err
 }

src/test/ui/associated-types/bound-lifetime-in-binding-only.elision.stderr

@@ -2,10 +2,9 @@ error[E0106]: missing lifetime specifier
   --> $DIR/bound-lifetime-in-binding-only.rs:62:23
    |
 LL | fn elision<T: Fn() -> &i32>() {
-   |                       ^ expected lifetime parameter
+   |                       ^ help: consider giving it a 'static lifetime: `&'static`
    |
    = help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
-   = help: consider giving it a 'static lifetime
 
 error: aborting due to previous error
 

src/test/ui/associated-types/bound-lifetime-in-return-only.elision.stderr

@@ -2,10 +2,9 @@ error[E0106]: missing lifetime specifier
   --> $DIR/bound-lifetime-in-return-only.rs:44:23
    |
 LL | fn elision(_: fn() -> &i32) {
-   |                       ^ expected lifetime parameter
+   |                       ^ help: consider giving it a 'static lifetime: `&'static`
    |
    = help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
-   = help: consider giving it a 'static lifetime
 
 error: aborting due to previous error
 

src/test/ui/foreign-fn-return-lifetime.fixed

@@ -0,0 +1,18 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// run-rustfix
+
+extern "C" {
+    pub fn g(_: &u8) -> &u8; // OK
+    pub fn f() -> &'static u8; //~ ERROR missing lifetime specifier
+}
+
+fn main() {}

src/test/ui/foreign-fn-return-lifetime.rs

@@ -8,9 +8,11 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// run-rustfix
+
 extern "C" {
-    fn g(_: &u8) -> &u8; // OK
-    fn f() -> &u8; //~ ERROR missing lifetime specifier
+    pub fn g(_: &u8) -> &u8; // OK
+    pub fn f() -> &u8; //~ ERROR missing lifetime specifier
 }
 
 fn main() {}

src/test/ui/foreign-fn-return-lifetime.stderr

@@ -1,11 +1,10 @@
 error[E0106]: missing lifetime specifier
-  --> $DIR/foreign-fn-return-lifetime.rs:13:15
+  --> $DIR/foreign-fn-return-lifetime.rs:15:19
    |
-LL |     fn f() -> &u8; //~ ERROR missing lifetime specifier
-   |               ^ expected lifetime parameter
+LL |     pub fn f() -> &u8; //~ ERROR missing lifetime specifier
+   |                   ^ help: consider giving it a 'static lifetime: `&'static`
    |
    = help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
-   = help: consider giving it a 'static lifetime
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-13497.stderr

@@ -2,10 +2,9 @@ error[E0106]: missing lifetime specifier
   --> $DIR/issue-13497.rs:12:5
    |
 LL |     &str //~ ERROR missing lifetime specifier
-   |     ^ expected lifetime parameter
+   |     ^ help: consider giving it a 'static lifetime: `&'static`
    |
    = help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
-   = help: consider giving it a 'static lifetime
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-26638.stderr

@@ -10,19 +10,17 @@ error[E0106]: missing lifetime specifier
   --> $DIR/issue-26638.rs:14:40
    |
 LL | fn parse_type_2(iter: fn(&u8)->&u8) -> &str { iter() }
-   |                                        ^ expected lifetime parameter
+   |                                        ^ help: consider giving it an explicit bounded or 'static lifetime: `&'static`
    |
    = help: this function's return type contains a borrowed value with an elided lifetime, but the lifetime cannot be derived from the arguments
-   = help: consider giving it an explicit bounded or 'static lifetime
 
 error[E0106]: missing lifetime specifier
   --> $DIR/issue-26638.rs:17:22
    |
 LL | fn parse_type_3() -> &str { unimplemented!() }
-   |                      ^ expected lifetime parameter
+   |                      ^ help: consider giving it a 'static lifetime: `&'static`
    |
    = help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
-   = help: consider giving it a 'static lifetime
 
 error: aborting due to 3 previous errors
 

src/test/ui/lifetimes/lifetime-elision-return-type-requires-explicit-lifetime.stderr

@@ -2,10 +2,9 @@ error[E0106]: missing lifetime specifier
   --> $DIR/lifetime-elision-return-type-requires-explicit-lifetime.rs:12:11
    |
 LL | fn f() -> &isize {    //~ ERROR missing lifetime specifier
-   |           ^ expected lifetime parameter
+   |           ^ help: consider giving it a 'static lifetime: `&'static`
    |
    = help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
-   = help: consider giving it a 'static lifetime
 
 error[E0106]: missing lifetime specifier
   --> $DIR/lifetime-elision-return-type-requires-explicit-lifetime.rs:17:33
@@ -27,28 +26,25 @@ error[E0106]: missing lifetime specifier
   --> $DIR/lifetime-elision-return-type-requires-explicit-lifetime.rs:31:20
    |
 LL | fn i(_x: isize) -> &isize { //~ ERROR missing lifetime specifier
-   |                    ^ expected lifetime parameter
+   |                    ^ help: consider giving it an explicit bounded or 'static lifetime: `&'static`
    |
    = help: this function's return type contains a borrowed value with an elided lifetime, but the lifetime cannot be derived from the arguments
-   = help: consider giving it an explicit bounded or 'static lifetime
 
 error[E0106]: missing lifetime specifier
   --> $DIR/lifetime-elision-return-type-requires-explicit-lifetime.rs:44:24
    |
 LL | fn j(_x: StaticStr) -> &isize { //~ ERROR missing lifetime specifier
-   |                        ^ expected lifetime parameter
+   |                        ^ help: consider giving it an explicit bounded or 'static lifetime: `&'static`
    |
    = help: this function's return type contains a borrowed value with an elided lifetime, but the lifetime cannot be derived from the arguments
-   = help: consider giving it an explicit bounded or 'static lifetime
 
 error[E0106]: missing lifetime specifier
   --> $DIR/lifetime-elision-return-type-requires-explicit-lifetime.rs:50:49
    |
 LL | fn k<'a, T: WithLifetime<'a>>(_x: T::Output) -> &isize {
-   |                                                 ^ expected lifetime parameter
+   |                                                 ^ help: consider giving it an explicit bounded or 'static lifetime: `&'static`
    |
    = help: this function's return type contains a borrowed value with an elided lifetime, but the lifetime cannot be derived from the arguments
-   = help: consider giving it an explicit bounded or 'static lifetime
 
 error: aborting due to 6 previous errors
 

src/test/ui/lifetimes/lifetime-elision-return-type-trait.rs

@@ -0,0 +1,12 @@
+trait Future {
+    type Item;
+    type Error;
+}
+
+use std::error::Error;
+
+fn foo() -> impl Future<Item=(), Error=Box<Error>> {
+    Ok(())
+}
+
+fn main() {}

src/test/ui/lifetimes/lifetime-elision-return-type-trait.stderr

@@ -0,0 +1,11 @@
+error[E0106]: missing lifetime specifier
+  --> $DIR/lifetime-elision-return-type-trait.rs:8:44
+   |
+LL | fn foo() -> impl Future<Item=(), Error=Box<Error>> {
+   |                                            ^^^^^ help: consider giving it a 'static lifetime: `Error + 'static`
+   |
+   = help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0106`.

src/test/ui/underscore-lifetime/underscore-lifetime-binders.stderr

@@ -20,10 +20,9 @@ error[E0106]: missing lifetime specifier
   --> $DIR/underscore-lifetime-binders.rs:20:29
    |
 LL | fn meh() -> Box<for<'_> Meh<'_>> //~ ERROR cannot be used here
-   |                             ^^ expected lifetime parameter
+   |                             ^^ help: consider giving it a 'static lifetime: `'static`
    |
    = help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
-   = help: consider giving it a 'static lifetime
 
 error[E0106]: missing lifetime specifier
   --> $DIR/underscore-lifetime-binders.rs:26:35