in which the elided-lifetimes-in-paths lint undergoes a revolution

The existing elided-lifetimes-in-paths lint (introduced in Nov. 2017's
accd997b5 / #46254) lacked stuctured suggestions and—much more
alarmingly—produced false positives on associated functions (like
`Ref::clone`) and on anonymous '_ lifetimes (!!—yes, the very
anonymous lifetimes that we meant to suggest "instead"). That this
went apparently unnoticed for so long maybe tells you something about
how many people actually bother to flip on allow-by-default lints.

After many hours of good old-fashioned American elbow grease—and a
little help from expert reviewers—it turns out that getting the right
answer is a lot easier if we fire the lint while lowering the Higher
Intermediate Representation.

The lint is promoted to the idioms-2018 group.

Also, in the matter of test filenames, "elided" only has one 'l' (see,
e.g., https://en.wiktionary.org/wiki/elide).

Resolves #52041.

9 files changed, 324 insertions, 53 deletions

diff --git a/src/librustc/hir/lowering.rs b/src/librustc/hir/lowering.rs
index 99db718bdf8..c668dd433aa 100644
--- a/src/librustc/hir/lowering.rs
+++ b/src/librustc/hir/lowering.rs
@@ -47,7 +47,8 @@ use hir::map::{DefKey, DefPathData, Definitions};
 use hir::def_id::{DefId, DefIndex, DefIndexAddressSpace, CRATE_DEF_INDEX};
 use hir::def::{Def, PathResolution, PerNS};
 use hir::GenericArg;
-use lint::builtin::{self, PARENTHESIZED_PARAMS_IN_TYPES_AND_MODULES};
+use lint::builtin::{self, PARENTHESIZED_PARAMS_IN_TYPES_AND_MODULES,
+                    ELIDED_LIFETIMES_IN_PATHS};
 use middle::cstore::CrateStore;
 use rustc_data_structures::indexed_vec::IndexVec;
 use session::Session;
@@ -1754,13 +1755,40 @@ impl<'a> LoweringContext<'a> {
             GenericArg::Lifetime(_) => true,
             _ => false,
         });
+        let first_generic_span = generic_args.args.iter().map(|a| a.span())
+            .chain(generic_args.bindings.iter().map(|b| b.span)).next();
         if !generic_args.parenthesized && !has_lifetimes {
             generic_args.args =
                 self.elided_path_lifetimes(path_span, expected_lifetimes)
                     .into_iter()
                     .map(|lt| GenericArg::Lifetime(lt))
                     .chain(generic_args.args.into_iter())
-                    .collect();
+                .collect();
+            if expected_lifetimes > 0 && param_mode == ParamMode::Explicit {
+                let anon_lt_suggestion = vec!["'_"; expected_lifetimes].join(", ");
+                let no_ty_args = generic_args.args.len() == expected_lifetimes;
+                let no_bindings = generic_args.bindings.is_empty();
+                let (incl_angl_brckt, insertion_span, suggestion) = if no_ty_args && no_bindings {
+                    // If there are no (non-implicit) generic args or associated-type
+                    // bindings, our suggestion includes the angle brackets
+                    (true, path_span.shrink_to_hi(), format!("<{}>", anon_lt_suggestion))
+                } else {
+                    // Otherwise—sorry, this is kind of gross—we need to infer the
+                    // place to splice in the `'_, ` from the generics that do exist
+                    let first_generic_span = first_generic_span
+                        .expect("already checked that type args or bindings exist");
+                    (false, first_generic_span.shrink_to_lo(), format!("{}, ", anon_lt_suggestion))
+                };
+                self.sess.buffer_lint_with_diagnostic(
+                    ELIDED_LIFETIMES_IN_PATHS,
+                    CRATE_NODE_ID,
+                    path_span,
+                    "hidden lifetime parameters in types are deprecated",
+                    builtin::BuiltinLintDiagnostics::ElidedLifetimesInPaths(
+                        expected_lifetimes, path_span, incl_angl_brckt, insertion_span, suggestion
+                    )
+                );
+            }
         }
 
         hir::PathSegment::new(
diff --git a/src/librustc/lint/builtin.rs b/src/librustc/lint/builtin.rs
index 0ab10bf223f..47c5f464131 100644
--- a/src/librustc/lint/builtin.rs
+++ b/src/librustc/lint/builtin.rs
@@ -254,7 +254,7 @@ declare_lint! {
 declare_lint! {
     pub ELIDED_LIFETIMES_IN_PATHS,
     Allow,
-    "hidden lifetime parameters are deprecated, try `Foo<'_>`"
+    "hidden lifetime parameters in types are deprecated"
 }
 
 declare_lint! {
@@ -402,6 +402,7 @@ pub enum BuiltinLintDiagnostics {
     AbsPathWithModule(Span),
     DuplicatedMacroExports(ast::Ident, Span, Span),
     ProcMacroDeriveResolutionFallback(Span),
+    ElidedLifetimesInPaths(usize, Span, bool, Span, String),
 }
 
 impl BuiltinLintDiagnostics {
@@ -442,6 +443,41 @@ impl BuiltinLintDiagnostics {
                 db.span_label(span, "names from parent modules are not \
                                      accessible without an explicit import");
             }
+            BuiltinLintDiagnostics::ElidedLifetimesInPaths(
+                n, path_span, incl_angl_brckt, insertion_span, anon_lts
+            ) => {
+                let (replace_span, suggestion) = if incl_angl_brckt {
+                    (insertion_span, anon_lts)
+                } else {
+                    // When possible, prefer a suggestion that replaces the whole
+                    // `Path<T>` expression with `Path<'_, T>`, rather than inserting `'_, `
+                    // at a point (which makes for an ugly/confusing label)
+                    if let Ok(snippet) = sess.codemap().span_to_snippet(path_span) {
+                        // But our spans can get out of whack due to macros; if the place we think
+                        // we want to insert `'_` isn't even within the path expression's span, we
+                        // should bail out of making any suggestion rather than panicking on a
+                        // subtract-with-overflow or string-slice-out-out-bounds (!)
+                        // FIXME: can we do better?
+                        if insertion_span.lo().0 < path_span.lo().0 {
+                            return;
+                        }
+                        let insertion_index = (insertion_span.lo().0 - path_span.lo().0) as usize;
+                        if insertion_index > snippet.len() {
+                            return;
+                        }
+                        let (before, after) = snippet.split_at(insertion_index);
+                        (path_span, format!("{}{}{}", before, anon_lts, after))
+                    } else {
+                        (insertion_span, anon_lts)
+                    }
+                };
+                db.span_suggestion_with_applicability(
+                    replace_span,
+                    &format!("indicate the anonymous lifetime{}", if n >= 2 { "s" } else { "" }),
+                    suggestion,
+                    Applicability::MachineApplicable
+                );
+            }
         }
     }
 }
diff --git a/src/librustc/middle/resolve_lifetime.rs b/src/librustc/middle/resolve_lifetime.rs
index 39a57e985e8..1309540717c 100644
--- a/src/librustc/middle/resolve_lifetime.rs
+++ b/src/librustc/middle/resolve_lifetime.rs
@@ -609,7 +609,7 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
                         // resolved the same as the `'_` in `&'_ Foo`.
                         //
                         // cc #48468
-                        self.resolve_elided_lifetimes(vec![lifetime], false)
+                        self.resolve_elided_lifetimes(vec![lifetime])
                     }
                     LifetimeName::Param(_) | LifetimeName::Static => {
                         // If the user wrote an explicit name, use that.
@@ -893,7 +893,7 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
 
     fn visit_lifetime(&mut self, lifetime_ref: &'tcx hir::Lifetime) {
         if lifetime_ref.is_elided() {
-            self.resolve_elided_lifetimes(vec![lifetime_ref], false);
+            self.resolve_elided_lifetimes(vec![lifetime_ref]);
             return;
         }
         if lifetime_ref.is_static() {
@@ -1728,7 +1728,7 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
             _ => None,
         }).collect();
         if elide_lifetimes {
-            self.resolve_elided_lifetimes(lifetimes, true);
+            self.resolve_elided_lifetimes(lifetimes);
         } else {
             lifetimes.iter().for_each(|lt| self.visit_lifetime(lt));
         }
@@ -2106,26 +2106,14 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
     }
 
     fn resolve_elided_lifetimes(&mut self,
-                                lifetime_refs: Vec<&'tcx hir::Lifetime>,
-                                deprecated: bool) {
+                                lifetime_refs: Vec<&'tcx hir::Lifetime>) {
         if lifetime_refs.is_empty() {
             return;
         }
 
         let span = lifetime_refs[0].span;
-        let id = lifetime_refs[0].id;
         let mut late_depth = 0;
         let mut scope = self.scope;
-        if deprecated {
-            self.tcx
-                .struct_span_lint_node(
-                    lint::builtin::ELIDED_LIFETIMES_IN_PATHS,
-                    id,
-                    span,
-                    &format!("hidden lifetime parameters are deprecated, try `Foo<'_>`"),
-                )
-                .emit();
-        }
         let error = loop {
             match *scope {
                 // Do not assign any resolution, it will be inferred.
diff --git a/src/librustc_lint/lib.rs b/src/librustc_lint/lib.rs
index e875df0d22b..cab51fbd987 100644
--- a/src/librustc_lint/lib.rs
+++ b/src/librustc_lint/lib.rs
@@ -44,7 +44,8 @@ extern crate syntax_pos;
 
 use rustc::lint;
 use rustc::lint::{LateContext, LateLintPass, LintPass, LintArray};
-use rustc::lint::builtin::{BARE_TRAIT_OBJECTS, ABSOLUTE_PATHS_NOT_STARTING_WITH_CRATE};
+use rustc::lint::builtin::{BARE_TRAIT_OBJECTS, ABSOLUTE_PATHS_NOT_STARTING_WITH_CRATE,
+                           ELIDED_LIFETIMES_IN_PATHS};
 use rustc::lint::builtin::MACRO_USE_EXTERN_CRATE;
 use rustc::session;
 use rustc::util;
@@ -195,6 +196,7 @@ pub fn register_builtins(store: &mut lint::LintStore, sess: Option<&Session>) {
                     UNREACHABLE_PUB,
                     UNUSED_EXTERN_CRATES,
                     MACRO_USE_EXTERN_CRATE,
+                    ELIDED_LIFETIMES_IN_PATHS,
                     ELLIPSIS_INCLUSIVE_RANGE_PATTERNS);
 
     // Guidelines for creating a future incompatibility lint:
diff --git a/src/test/ui/in-band-lifetimes/elided-lifetimes.fixed b/src/test/ui/in-band-lifetimes/elided-lifetimes.fixed
new file mode 100644
index 00000000000..c9381e6350f
--- /dev/null
+++ b/src/test/ui/in-band-lifetimes/elided-lifetimes.fixed
@@ -0,0 +1,97 @@
+// Copyright 2018 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
+// compile-flags: --edition 2018
+
+#![allow(unused)]
+#![deny(elided_lifetimes_in_paths)]
+//~^ NOTE lint level defined here
+
+use std::cell::{RefCell, Ref};
+
+
+struct Foo<'a> { x: &'a u32 }
+
+fn foo(x: &Foo<'_>) {
+    //~^ ERROR hidden lifetime parameters in types are deprecated
+    //~| HELP indicate the anonymous lifetime
+}
+
+fn bar(x: &Foo<'_>) {}
+
+
+struct Wrapped<'a>(&'a str);
+
+struct WrappedWithBow<'a> {
+    gift: &'a str
+}
+
+struct MatchedSet<'a, 'b> {
+    one: &'a str,
+    another: &'b str,
+}
+
+fn wrap_gift(gift: &str) -> Wrapped<'_> {
+    //~^ ERROR hidden lifetime parameters in types are deprecated
+    //~| HELP indicate the anonymous lifetime
+    Wrapped(gift)
+}
+
+fn wrap_gift_with_bow(gift: &str) -> WrappedWithBow<'_> {
+    //~^ ERROR hidden lifetime parameters in types are deprecated
+    //~| HELP indicate the anonymous lifetime
+    WrappedWithBow { gift }
+}
+
+fn inspect_matched_set(set: MatchedSet<'_, '_>) {
+    //~^ ERROR hidden lifetime parameters in types are deprecated
+    //~| HELP indicate the anonymous lifetime
+    println!("{} {}", set.one, set.another);
+}
+
+macro_rules! autowrapper {
+    ($type_name:ident, $fn_name:ident, $lt:lifetime) => {
+        struct $type_name<$lt> {
+            gift: &$lt str
+        }
+
+        fn $fn_name(gift: &str) -> $type_name<'_> {
+            //~^ ERROR hidden lifetime parameters in types are deprecated
+            //~| HELP indicate the anonymous lifetime
+            $type_name { gift }
+        }
+    }
+}
+
+autowrapper!(Autowrapped, autowrap_gift, 'a);
+//~^ NOTE in this expansion of autowrapper!
+//~| NOTE in this expansion of autowrapper!
+
+macro_rules! anytuple_ref_ty {
+    ($($types:ty),*) => {
+        Ref<'_, ($($types),*)>
+        //~^ ERROR hidden lifetime parameters in types are deprecated
+        //~| HELP indicate the anonymous lifetime
+    }
+}
+
+fn main() {
+    let honesty = RefCell::new((4, 'e'));
+    let loyalty: Ref<'_, (u32, char)> = honesty.borrow();
+    //~^ ERROR hidden lifetime parameters in types are deprecated
+    //~| HELP indicate the anonymous lifetime
+    let generosity = Ref::map(loyalty, |t| &t.0);
+
+    let laughter = RefCell::new((true, "magic"));
+    let yellow: anytuple_ref_ty!(bool, &str) = laughter.borrow();
+    //~^ NOTE in this expansion of anytuple_ref_ty!
+    //~| NOTE in this expansion of anytuple_ref_ty!
+}
diff --git a/src/test/ui/in-band-lifetimes/elided-lifetimes.rs b/src/test/ui/in-band-lifetimes/elided-lifetimes.rs
new file mode 100644
index 00000000000..8151dd01a98
--- /dev/null
+++ b/src/test/ui/in-band-lifetimes/elided-lifetimes.rs
@@ -0,0 +1,97 @@
+// Copyright 2018 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
+// compile-flags: --edition 2018
+
+#![allow(unused)]
+#![deny(elided_lifetimes_in_paths)]
+//~^ NOTE lint level defined here
+
+use std::cell::{RefCell, Ref};
+
+
+struct Foo<'a> { x: &'a u32 }
+
+fn foo(x: &Foo) {
+    //~^ ERROR hidden lifetime parameters in types are deprecated
+    //~| HELP indicate the anonymous lifetime
+}
+
+fn bar(x: &Foo<'_>) {}
+
+
+struct Wrapped<'a>(&'a str);
+
+struct WrappedWithBow<'a> {
+    gift: &'a str
+}
+
+struct MatchedSet<'a, 'b> {
+    one: &'a str,
+    another: &'b str,
+}
+
+fn wrap_gift(gift: &str) -> Wrapped {
+    //~^ ERROR hidden lifetime parameters in types are deprecated
+    //~| HELP indicate the anonymous lifetime
+    Wrapped(gift)
+}
+
+fn wrap_gift_with_bow(gift: &str) -> WrappedWithBow {
+    //~^ ERROR hidden lifetime parameters in types are deprecated
+    //~| HELP indicate the anonymous lifetime
+    WrappedWithBow { gift }
+}
+
+fn inspect_matched_set(set: MatchedSet) {
+    //~^ ERROR hidden lifetime parameters in types are deprecated
+    //~| HELP indicate the anonymous lifetime
+    println!("{} {}", set.one, set.another);
+}
+
+macro_rules! autowrapper {
+    ($type_name:ident, $fn_name:ident, $lt:lifetime) => {
+        struct $type_name<$lt> {
+            gift: &$lt str
+        }
+
+        fn $fn_name(gift: &str) -> $type_name {
+            //~^ ERROR hidden lifetime parameters in types are deprecated
+            //~| HELP indicate the anonymous lifetime
+            $type_name { gift }
+        }
+    }
+}
+
+autowrapper!(Autowrapped, autowrap_gift, 'a);
+//~^ NOTE in this expansion of autowrapper!
+//~| NOTE in this expansion of autowrapper!
+
+macro_rules! anytuple_ref_ty {
+    ($($types:ty),*) => {
+        Ref<($($types),*)>
+        //~^ ERROR hidden lifetime parameters in types are deprecated
+        //~| HELP indicate the anonymous lifetime
+    }
+}
+
+fn main() {
+    let honesty = RefCell::new((4, 'e'));
+    let loyalty: Ref<(u32, char)> = honesty.borrow();
+    //~^ ERROR hidden lifetime parameters in types are deprecated
+    //~| HELP indicate the anonymous lifetime
+    let generosity = Ref::map(loyalty, |t| &t.0);
+
+    let laughter = RefCell::new((true, "magic"));
+    let yellow: anytuple_ref_ty!(bool, &str) = laughter.borrow();
+    //~^ NOTE in this expansion of anytuple_ref_ty!
+    //~| NOTE in this expansion of anytuple_ref_ty!
+}
diff --git a/src/test/ui/in-band-lifetimes/elided-lifetimes.stderr b/src/test/ui/in-band-lifetimes/elided-lifetimes.stderr
new file mode 100644
index 00000000000..e7a1b42b8c9
--- /dev/null
+++ b/src/test/ui/in-band-lifetimes/elided-lifetimes.stderr
@@ -0,0 +1,56 @@
+error: hidden lifetime parameters in types are deprecated
+  --> $DIR/elided-lifetimes.rs:23:12
+   |
+LL | fn foo(x: &Foo) {
+   |            ^^^- help: indicate the anonymous lifetime: `<'_>`
+   |
+note: lint level defined here
+  --> $DIR/elided-lifetimes.rs:15:9
+   |
+LL | #![deny(elided_lifetimes_in_paths)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: hidden lifetime parameters in types are deprecated
+  --> $DIR/elided-lifetimes.rs:42:29
+   |
+LL | fn wrap_gift(gift: &str) -> Wrapped {
+   |                             ^^^^^^^- help: indicate the anonymous lifetime: `<'_>`
+
+error: hidden lifetime parameters in types are deprecated
+  --> $DIR/elided-lifetimes.rs:48:38
+   |
+LL | fn wrap_gift_with_bow(gift: &str) -> WrappedWithBow {
+   |                                      ^^^^^^^^^^^^^^- help: indicate the anonymous lifetime: `<'_>`
+
+error: hidden lifetime parameters in types are deprecated
+  --> $DIR/elided-lifetimes.rs:54:29
+   |
+LL | fn inspect_matched_set(set: MatchedSet) {
+   |                             ^^^^^^^^^^- help: indicate the anonymous lifetimes: `<'_, '_>`
+
+error: hidden lifetime parameters in types are deprecated
+  --> $DIR/elided-lifetimes.rs:66:36
+   |
+LL |         fn $fn_name(gift: &str) -> $type_name {
+   |                                    ^^^^^^^^^^- help: indicate the anonymous lifetime: `<'_>`
+...
+LL | autowrapper!(Autowrapped, autowrap_gift, 'a);
+   | --------------------------------------------- in this macro invocation
+
+error: hidden lifetime parameters in types are deprecated
+  --> $DIR/elided-lifetimes.rs:88:18
+   |
+LL |     let loyalty: Ref<(u32, char)> = honesty.borrow();
+   |                  ^^^^^^^^^^^^^^^^ help: indicate the anonymous lifetime: `Ref<'_, (u32, char)>`
+
+error: hidden lifetime parameters in types are deprecated
+  --> $DIR/elided-lifetimes.rs:80:9
+   |
+LL |         Ref<($($types),*)>
+   |         ^^^^^^^^^^^^^^^^^^ help: indicate the anonymous lifetime: `Ref<'_, ($($types),*)>`
+...
+LL |     let yellow: anytuple_ref_ty!(bool, &str) = laughter.borrow();
+   |                 ---------------------------- in this macro invocation
+
+error: aborting due to 7 previous errors
+
diff --git a/src/test/ui/in-band-lifetimes/ellided-lifetimes.rs b/src/test/ui/in-band-lifetimes/ellided-lifetimes.rs
deleted file mode 100644
index 3739ffe6a26..00000000000
--- a/src/test/ui/in-band-lifetimes/ellided-lifetimes.rs
+++ /dev/null
@@ -1,19 +0,0 @@
-// Copyright 2018 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.
-#![allow(warnings)]
-#![allow(unused_variables, dead_code, unused, bad_style)]
-#![deny(elided_lifetimes_in_paths)]
-
-struct Foo<'a> { x: &'a u32 }
-fn foo(x: &Foo) {
-    //~^ ERROR: hidden lifetime parameters are deprecated, try `Foo<'_>`
-}
-
-fn main() {}
diff --git a/src/test/ui/in-band-lifetimes/ellided-lifetimes.stderr b/src/test/ui/in-band-lifetimes/ellided-lifetimes.stderr
deleted file mode 100644
index c2bd2c261ac..00000000000
--- a/src/test/ui/in-band-lifetimes/ellided-lifetimes.stderr
+++ /dev/null
@@ -1,14 +0,0 @@
-error: hidden lifetime parameters are deprecated, try `Foo<'_>`
-  --> $DIR/ellided-lifetimes.rs:15:12
-   |
-LL | fn foo(x: &Foo) {
-   |            ^^^
-   |
-note: lint level defined here
-  --> $DIR/ellided-lifetimes.rs:12:9
-   |
-LL | #![deny(elided_lifetimes_in_paths)]
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^
-
-error: aborting due to previous error
-