Auto merge of rust-lang#71664 - Dylan-DPC:rollup-eng60x9, r=Dylan-DPC

Rollup of 5 pull requests

Successful merges:

 - rust-lang#71217 (Suggest `;` or assignment to drop borrows in tail exprs)
 - rust-lang#71286 (Add regression test for rust-lang#69654)
 - rust-lang#71296 (Change wording on read_vectored docs)
 - rust-lang#71654 (Update link to unstable book for llvm_asm macro)
 - rust-lang#71657 (Add rust-lang#24949 assoc constant static recursion test)

Failed merges:

r? @ghost

Author: bors

src/libcore/macros/mod.rs

@@ -1316,7 +1316,7 @@ pub(crate) mod builtin {
     ///
     /// Read the [unstable book] for the usage.
     ///
-    /// [unstable book]: ../unstable-book/library-features/asm.html
+    /// [unstable book]: ../unstable-book/library-features/llvm-asm.html
     #[unstable(
         feature = "llvm_asm",
         issue = "70173",

src/librustc_middle/mir/mod.rs

@@ -672,6 +672,9 @@ pub struct BlockTailInfo {
     /// Examples include `{ ...; tail };` and `let _ = { ...; tail };`
     /// but not e.g., `let _x = { ...; tail };`
     pub tail_result_is_ignored: bool,
+
+    /// `Span` of the tail expression.
+    pub span: Span,
 }
 
 /// A MIR local.

src/librustc_mir/borrow_check/diagnostics/explain_borrow.rs

@@ -156,23 +156,32 @@ impl BorrowExplanation {
                         err.span_label(body.source_info(drop_loc).span, message);
 
                         if let Some(info) = &local_decl.is_block_tail {
-                            // FIXME: use span_suggestion instead, highlighting the
-                            // whole block tail expression.
-                            let msg = if info.tail_result_is_ignored {
-                                "The temporary is part of an expression at the end of a block. \
-                                 Consider adding semicolon after the expression so its temporaries \
-                                 are dropped sooner, before the local variables declared by the \
-                                 block are dropped."
+                            if info.tail_result_is_ignored {
+                                err.span_suggestion_verbose(
+                                    info.span.shrink_to_hi(),
+                                    "consider adding semicolon after the expression so its \
+                                     temporaries are dropped sooner, before the local variables \
+                                     declared by the block are dropped",
+                                    ";".to_string(),
+                                    Applicability::MaybeIncorrect,
+                                );
                             } else {
-                                "The temporary is part of an expression at the end of a block. \
-                                 Consider forcing this temporary to be dropped sooner, before \
-                                 the block's local variables are dropped. \
-                                 For example, you could save the expression's value in a new \
-                                 local variable `x` and then make `x` be the expression \
-                                 at the end of the block."
+                                err.note(
+                                    "the temporary is part of an expression at the end of a \
+                                     block;\nconsider forcing this temporary to be dropped sooner, \
+                                     before the block's local variables are dropped",
+                                );
+                                err.multipart_suggestion(
+                                    "for example, you could save the expression's value in a new \
+                                     local variable `x` and then make `x` be the expression at the \
+                                     end of the block",
+                                    vec![
+                                        (info.span.shrink_to_lo(), "let x = ".to_string()),
+                                        (info.span.shrink_to_hi(), "; x".to_string()),
+                                    ],
+                                    Applicability::MaybeIncorrect,
+                                );
                             };
-
-                            err.note(msg);
                         }
                     }
                 }

src/librustc_mir_build/build/block.rs

@@ -173,7 +173,11 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         if let Some(expr) = expr {
             let tail_result_is_ignored =
                 destination_ty.is_unit() || this.block_context.currently_ignores_tail_results();
-            this.block_context.push(BlockFrame::TailExpr { tail_result_is_ignored });
+            let span = match expr {
+                ExprRef::Hair(expr) => expr.span,
+                ExprRef::Mirror(ref expr) => expr.span,
+            };
+            this.block_context.push(BlockFrame::TailExpr { tail_result_is_ignored, span });
 
             unpack!(block = this.into(destination, block, expr));
             let popped = this.block_context.pop();

src/librustc_mir_build/build/expr/stmt.rs

@@ -151,7 +151,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                                 }
                             }
                             this.block_context
-                                .push(BlockFrame::TailExpr { tail_result_is_ignored: true });
+                                .push(BlockFrame::TailExpr { tail_result_is_ignored: true, span: expr.span });
                             return Some(expr.span);
                         }
                     }

src/librustc_mir_build/build/mod.rs

@@ -242,6 +242,9 @@ enum BlockFrame {
         ///
         /// Example: `let _ = { STMT_1; EXPR };`
         tail_result_is_ignored: bool,
+
+        /// `Span` of the tail expression.
+        span: Span,
     },
 
     /// Generic mark meaning that the block occurred as a subexpression
@@ -369,8 +372,8 @@ impl BlockContext {
             match bf {
                 BlockFrame::SubExpr => continue,
                 BlockFrame::Statement { .. } => break,
-                &BlockFrame::TailExpr { tail_result_is_ignored } => {
-                    return Some(BlockTailInfo { tail_result_is_ignored });
+                &BlockFrame::TailExpr { tail_result_is_ignored, span } => {
+                    return Some(BlockTailInfo { tail_result_is_ignored, span });
                 }
             }
         }
@@ -394,7 +397,7 @@ impl BlockContext {
 
             // otherwise: use accumulated is_ignored state.
             Some(
-                BlockFrame::TailExpr { tail_result_is_ignored: ignored }
+                BlockFrame::TailExpr { tail_result_is_ignored: ignored, .. }
                 | BlockFrame::Statement { ignores_expr_result: ignored },
             ) => *ignored,
         }

src/libstd/io/mod.rs

@@ -571,8 +571,9 @@ pub trait Read {
     /// Like `read`, except that it reads into a slice of buffers.
     ///
     /// Data is copied to fill each buffer in order, with the final buffer
-    /// written to possibly being only partially filled. This method must behave
-    /// as a single call to `read` with the buffers concatenated would.
+    /// written to possibly being only partially filled. This method must
+    /// behave equivalently to a single call to `read` with concatenated
+    /// buffers.
     ///
     /// The default implementation calls `read` with either the first nonempty
     /// buffer provided, or an empty one if none exists.

src/test/ui/associated-consts/issue-24949-assoc-const-static-recursion-impl.rs

@@ -0,0 +1,15 @@
+// Check for recursion involving references to impl-associated const.
+
+trait Foo {
+    const BAR: u32;
+}
+
+const IMPL_REF_BAR: u32 = GlobalImplRef::BAR; //~ ERROR E0391
+
+struct GlobalImplRef;
+
+impl GlobalImplRef {
+    const BAR: u32 = IMPL_REF_BAR;
+}
+
+fn main() {}

src/test/ui/associated-consts/issue-24949-assoc-const-static-recursion-impl.stderr

@@ -0,0 +1,44 @@
+error[E0391]: cycle detected when const-evaluating + checking `IMPL_REF_BAR`
+  --> $DIR/issue-24949-assoc-const-static-recursion-impl.rs:7:1
+   |
+LL | const IMPL_REF_BAR: u32 = GlobalImplRef::BAR;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: ...which requires const-evaluating + checking `IMPL_REF_BAR`...
+  --> $DIR/issue-24949-assoc-const-static-recursion-impl.rs:7:1
+   |
+LL | const IMPL_REF_BAR: u32 = GlobalImplRef::BAR;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: ...which requires const-evaluating `IMPL_REF_BAR`...
+  --> $DIR/issue-24949-assoc-const-static-recursion-impl.rs:7:1
+   |
+LL | const IMPL_REF_BAR: u32 = GlobalImplRef::BAR;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: ...which requires normalizing `<impl at $DIR/issue-24949-assoc-const-static-recursion-impl.rs:11:1: 13:2>::BAR`...
+note: ...which requires const-evaluating + checking `<impl at $DIR/issue-24949-assoc-const-static-recursion-impl.rs:11:1: 13:2>::BAR`...
+  --> $DIR/issue-24949-assoc-const-static-recursion-impl.rs:12:5
+   |
+LL |     const BAR: u32 = IMPL_REF_BAR;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: ...which requires const-evaluating + checking `<impl at $DIR/issue-24949-assoc-const-static-recursion-impl.rs:11:1: 13:2>::BAR`...
+  --> $DIR/issue-24949-assoc-const-static-recursion-impl.rs:12:5
+   |
+LL |     const BAR: u32 = IMPL_REF_BAR;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: ...which requires const-evaluating `<impl at $DIR/issue-24949-assoc-const-static-recursion-impl.rs:11:1: 13:2>::BAR`...
+  --> $DIR/issue-24949-assoc-const-static-recursion-impl.rs:12:5
+   |
+LL |     const BAR: u32 = IMPL_REF_BAR;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: ...which requires processing `<impl at $DIR/issue-24949-assoc-const-static-recursion-impl.rs:11:1: 13:2>::BAR`...
+  --> $DIR/issue-24949-assoc-const-static-recursion-impl.rs:12:5
+   |
+LL |     const BAR: u32 = IMPL_REF_BAR;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: ...which requires normalizing `IMPL_REF_BAR`...
+   = note: ...which again requires const-evaluating + checking `IMPL_REF_BAR`, completing the cycle
+   = note: cycle used when running analysis passes on this crate
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0391`.

src/test/ui/associated-consts/issue-24949-assoc-const-static-recursion-trait-default.rs

@@ -0,0 +1,17 @@
+// Check for recursion involving references to trait-associated const default.
+
+trait Foo {
+    const BAR: u32;
+}
+
+trait FooDefault {
+    const BAR: u32 = DEFAULT_REF_BAR;
+}
+
+const DEFAULT_REF_BAR: u32 = <GlobalDefaultRef>::BAR; //~ ERROR E0391
+
+struct GlobalDefaultRef;
+
+impl FooDefault for GlobalDefaultRef {}
+
+fn main() {}