Account for trailing comma when suggesting where clauses

Fix rust-lang#72693.

Author: estebank

src/librustc_middle/ty/diagnostics.rs

@@ -7,7 +7,6 @@ use rustc_errors::{Applicability, DiagnosticBuilder};
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_hir::{QPath, TyKind, WhereBoundPredicate, WherePredicate};
-use rustc_span::{BytePos, Span};
 
 impl<'tcx> TyS<'tcx> {
     /// Similar to `TyS::is_primitive`, but also considers inferred numeric values to be primitive.
@@ -221,18 +220,16 @@ pub fn suggest_constraining_type_param(
             }
         }
 
-        let where_clause_span = generics.where_clause.span_for_predicates_or_empty_place();
         // Account for `fn foo<T>(t: T) where T: Foo,` so we don't suggest two trailing commas.
-        let mut trailing_comma = false;
-        if let Ok(snippet) = tcx.sess.source_map().span_to_snippet(where_clause_span) {
-            trailing_comma = snippet.ends_with(',');
-        }
-        let where_clause_span = if trailing_comma {
-            let hi = where_clause_span.hi();
-            Span::new(hi - BytePos(1), hi, where_clause_span.ctxt())
-        } else {
-            where_clause_span.shrink_to_hi()
-        };
+        let end = generics.where_clause.span_for_predicates_or_empty_place().shrink_to_hi();
+        let where_clause_span = generics
+            .where_clause
+            .predicates
+            .last()
+            .map(|p| p.span())
+            .unwrap_or(end)
+            .shrink_to_hi()
+            .to(end);
 
         match &param_spans[..] {
             &[&param_span] => suggest_restrict(param_span.shrink_to_hi()),

src/librustc_trait_selection/traits/error_reporting/suggestions.rs

@@ -169,8 +169,17 @@ pub trait InferCtxtExt<'tcx> {
 }
 
 fn predicate_constraint(generics: &hir::Generics<'_>, pred: String) -> (Span, String) {
+    let end = generics.where_clause.span_for_predicates_or_empty_place().shrink_to_hi();
     (
-        generics.where_clause.span_for_predicates_or_empty_place().shrink_to_hi(),
+        // Account for `where T: Foo,` so we don't suggest two trailing commas.
+        generics
+            .where_clause
+            .predicates
+            .last()
+            .map(|p| p.span())
+            .unwrap_or(end)
+            .shrink_to_hi()
+            .to(end),
         format!(
             "{} {}",
             if !generics.where_clause.predicates.is_empty() { "," } else { " where" },

src/test/ui/bound-suggestions.rs

@@ -19,7 +19,7 @@ fn test_one_bound<T: Sized>(t: T) {
 }
 
 #[allow(dead_code)]
-fn test_no_bounds_where<X, Y>(x: X, y: Y) where X: std::fmt::Debug {
+fn test_no_bounds_where<X, Y>(x: X, y: Y) where X: std::fmt::Debug, {
     println!("{:?} {:?}", x, y);
     //~^ ERROR doesn't implement
 }