Do not print single ::<'_> for fn items.

Author: cjgillot

compiler/rustc_const_eval/src/interpret/intrinsics/type_name.rs

@@ -135,6 +135,7 @@ impl<'tcx> Printer<'tcx> for AbsolutePathPrinter<'tcx> {
 
     fn path_generic_args(
         mut self,
+        _: bool,
         print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
         args: &[GenericArg<'tcx>],
     ) -> Result<Self::Path, Self::Error> {

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -574,6 +574,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
             }
             fn path_generic_args(
                 self,
+                _: bool,
                 print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
                 _args: &[GenericArg<'tcx>],
             ) -> Result<Self::Path, Self::Error> {

compiler/rustc_lint/src/context.rs

@@ -1232,6 +1232,7 @@ impl<'tcx> LateContext<'tcx> {
 
             fn path_generic_args(
                 self,
+                _: bool,
                 print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
                 _args: &[GenericArg<'tcx>],
             ) -> Result<Self::Path, Self::Error> {

compiler/rustc_middle/src/ty/error.rs

@@ -985,7 +985,7 @@ fn foo(&self) -> Self::T { String::new() }
 
     fn format_generic_args(self, args: &[ty::GenericArg<'tcx>]) -> String {
         FmtPrinter::new(self, hir::def::Namespace::TypeNS)
-            .path_generic_args(Ok, args)
+            .path_generic_args(true, Ok, args)
             .expect("could not write to `String`.")
             .into_buffer()
     }

compiler/rustc_middle/src/ty/print/mod.rs

@@ -3,6 +3,7 @@ use crate::ty::{self, DefIdTree, Ty, TyCtxt};
 
 use rustc_data_structures::fx::FxHashSet;
 use rustc_data_structures::sso::SsoHashSet;
+use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{CrateNum, DefId, LocalDefId};
 use rustc_hir::definitions::{DefPathData, DisambiguatedDefPathData};
 
@@ -92,6 +93,7 @@ pub trait Printer<'tcx>: Sized {
 
     fn path_generic_args(
         self,
+        forbid_lifetime_elision: bool,
         print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
         args: &[GenericArg<'tcx>],
     ) -> Result<Self::Path, Self::Error>;
@@ -150,7 +152,12 @@ pub trait Printer<'tcx>: Sized {
                         _ => {
                             if !generics.params.is_empty() && substs.len() >= generics.count() {
                                 let args = generics.own_substs_no_defaults(self.tcx(), substs);
+                                let forbid_elision = !matches!(
+                                    self.tcx().def_kind(def_id),
+                                    DefKind::Fn | DefKind::AssocFn
+                                );
                                 return self.path_generic_args(
+                                    forbid_elision,
                                     |cx| cx.print_def_path(def_id, parent_substs),
                                     args,
                                 );

compiler/rustc_middle/src/ty/print/pretty.rs

@@ -1845,12 +1845,18 @@ impl<'tcx> Printer<'tcx> for FmtPrinter<'_, 'tcx> {
 
     fn path_generic_args(
         mut self,
+        forbid_lifetime_elision: bool,
         print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
         args: &[GenericArg<'tcx>],
     ) -> Result<Self::Path, Self::Error> {
         self = print_prefix(self)?;
 
-        if args.first().is_some() {
+        let all_elided = args.iter().all(|arg| match arg.unpack() {
+            ty::GenericArgKind::Lifetime(lt) => !lt.has_name(),
+            _ => false,
+        });
+
+        if !args.is_empty() && (forbid_lifetime_elision || self.tcx.sess.verbose() || !all_elided) {
             if self.in_value {
                 write!(self, "::")?;
             }

compiler/rustc_symbol_mangling/src/legacy.rs

@@ -350,6 +350,7 @@ impl<'tcx> Printer<'tcx> for &mut SymbolPrinter<'tcx> {
     }
     fn path_generic_args(
         mut self,
+        _: bool,
         print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
         args: &[GenericArg<'tcx>],
     ) -> Result<Self::Path, Self::Error> {

compiler/rustc_symbol_mangling/src/v0.rs

@@ -303,6 +303,7 @@ impl<'tcx> Printer<'tcx> for &mut SymbolMangler<'tcx> {
         // polymorphization is enabled) and this isn't an inherent impl.
         if impl_trait_ref.is_some() && substs.iter().any(|a| a.has_non_region_param()) {
             self = self.path_generic_args(
+                false,
                 |this| {
                     this.path_append_ns(
                         |cx| cx.print_def_path(parent_def_id, &[]),
@@ -808,6 +809,7 @@ impl<'tcx> Printer<'tcx> for &mut SymbolMangler<'tcx> {
 
     fn path_generic_args(
         mut self,
+        _: bool,
         print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
         args: &[GenericArg<'tcx>],
     ) -> Result<Self::Path, Self::Error> {

src/test/ui/regions/region-lifetime-bounds-on-fns-where-clause.stderr

@@ -5,7 +5,7 @@ LL |     let _: fn(&mut &isize, &mut &isize) = a;
    |                                           ^ one type is more general than the other
    |
    = note: expected fn pointer `for<'a, 'b, 'c, 'd> fn(&'a mut &'b isize, &'c mut &'d isize)`
-                 found fn item `for<'a, 'b> fn(&'a mut &isize, &'b mut &isize) {a::<'_, '_>}`
+                 found fn item `for<'a, 'b> fn(&'a mut &isize, &'b mut &isize) {a}`
 
 error: aborting due to previous error
 

src/test/ui/regions/region-multiple-lifetime-bounds-on-fns-where-clause.stderr

@@ -5,7 +5,7 @@ LL |     let _: fn(&mut &isize, &mut &isize, &mut &isize) = a;
    |                                                        ^ one type is more general than the other
    |
    = note: expected fn pointer `for<'a, 'b, 'c, 'd, 'e, 'f> fn(&'a mut &'b isize, &'c mut &'d isize, &'e mut &'f isize)`
-                 found fn item `for<'a, 'b, 'c> fn(&'a mut &isize, &'b mut &isize, &'c mut &isize) {a::<'_, '_, '_>}`
+                 found fn item `for<'a, 'b, 'c> fn(&'a mut &isize, &'b mut &isize, &'c mut &isize) {a}`
 
 error: aborting due to previous error