Auto merge of #75016 - Manishearth:rollup-x553g7j, r=Manishearth

Rollup of 6 pull requests

Successful merges:

 - #74977 (Clean up E0741 error explanation)
 - #74981 (Some fixes for `plugin.md` in unstable-book)
 - #74983 (Replace a recursive algorithm with an iterative one and a stack.)
 - #74995 (Update the WASI libc build to LLVM 10.)
 - #74996 (submodules: update cargo from 974eb438d to 2d5c2381e)
 - #75007 (Clean up E0743 explanation)

Failed merges:

r? @ghost

src/ci/docker/host-x86_64/dist-various-2/build-wasi-toolchain.sh

@@ -4,10 +4,10 @@
 
 set -ex
 
-# Originally from https://releases.llvm.org/9.0.0/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-14.04.tar.xz
-curl https://ci-mirrors.rust-lang.org/rustc/clang%2Bllvm-9.0.0-x86_64-linux-gnu-ubuntu-14.04.tar.xz | \
+# Originally from https://github.com/llvm/llvm-project/releases/download/llvmorg-10.0.0/clang+llvm-10.0.0-x86_64-linux-gnu-ubuntu-18.04.tar.xz
+curl https://ci-mirrors.rust-lang.org/rustc/clang%2Bllvm-10.0.0-x86_64-linux-gnu-ubuntu-18.04.tar.xz | \
   tar xJf -
-export PATH=`pwd`/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-14.04/bin:$PATH
+export PATH=`pwd`/clang+llvm-10.0.0-x86_64-linux-gnu-ubuntu-18.04/bin:$PATH
 
 git clone https://github.com/WebAssembly/wasi-libc
 

src/doc/unstable-book/src/language-features/plugin.md

@@ -45,42 +45,40 @@ that warns about any item named `lintme`.
 extern crate rustc_ast;
 
 // Load rustc as a plugin to get macros
-#[macro_use]
-extern crate rustc;
 extern crate rustc_driver;
+#[macro_use]
+extern crate rustc_lint;
+#[macro_use]
+extern crate rustc_session;
 
-use rustc::lint::{EarlyContext, LintContext, LintPass, EarlyLintPass,
-                  EarlyLintPassObject, LintArray};
 use rustc_driver::plugin::Registry;
+use rustc_lint::{EarlyContext, EarlyLintPass, LintArray, LintContext, LintPass};
 use rustc_ast::ast;
-
 declare_lint!(TEST_LINT, Warn, "Warn about items named 'lintme'");
 
-struct Pass;
-
-impl LintPass for Pass {
-    fn get_lints(&self) -> LintArray {
-        lint_array!(TEST_LINT)
-    }
-}
+declare_lint_pass!(Pass => [TEST_LINT]);
 
 impl EarlyLintPass for Pass {
     fn check_item(&mut self, cx: &EarlyContext, it: &ast::Item) {
-        if it.ident.as_str() == "lintme" {
-            cx.span_lint(TEST_LINT, it.span, "item is named 'lintme'");
+        if it.ident.name.as_str() == "lintme" {
+            cx.lint(TEST_LINT, |lint| {
+                lint.build("item is named 'lintme'").set_span(it.span).emit()
+            });
         }
     }
 }
 
 #[plugin_registrar]
 pub fn plugin_registrar(reg: &mut Registry) {
-    reg.register_early_lint_pass(box Pass as EarlyLintPassObject);
+    reg.lint_store.register_lints(&[&TEST_LINT]);
+    reg.lint_store.register_early_pass(|| box Pass);
 }
 ```
 
 Then code like
 
 ```rust,ignore
+#![feature(plugin)]
 #![plugin(lint_plugin_test)]
 
 fn lintme() { }
@@ -107,7 +105,7 @@ The components of a lint plugin are:
 
 Lint passes are syntax traversals, but they run at a late stage of compilation
 where type information is available. `rustc`'s [built-in
-lints](https://github.com/rust-lang/rust/blob/master/src/librustc/lint/builtin.rs)
+lints](https://github.com/rust-lang/rust/blob/master/src/librustc_session/lint/builtin.rs)
 mostly use the same infrastructure as lint plugins, and provide examples of how
 to access type information.
 

src/librustc_error_codes/error_codes/E0741.md

@@ -1,5 +1,6 @@
-Only structural-match types (that is, types that derive `PartialEq` and `Eq`)
-may be used as the types of const generic parameters.
+A non-structural-match type was used as the type of a const generic parameter.
+
+Erroneous code example:
 
 ```compile_fail,E0741
 #![feature(const_generics)]
@@ -9,12 +10,15 @@ struct A;
 struct B<const X: A>; // error!
 ```
 
-To fix this example, we derive `PartialEq` and `Eq`.
+Only structural-match types (that is, types that derive `PartialEq` and `Eq`)
+may be used as the types of const generic parameters.
+
+To fix the previous code example, we derive `PartialEq` and `Eq`:
 
 ```
 #![feature(const_generics)]
 
-#[derive(PartialEq, Eq)]
+#[derive(PartialEq, Eq)] // We derive both traits here.
 struct A;
 
 struct B<const X: A>; // ok!

src/librustc_error_codes/error_codes/E0743.md

@@ -8,10 +8,9 @@ Erroneous code example:
 fn foo2(x: u8, y: &...) {} // error!
 ```
 
-Only foreign functions can use the C-variadic type (`...`).
-In such functions, `...` may only occur non-nested.
-That is, `y: &'a ...` is not allowed.
+Only foreign functions can use the C-variadic type (`...`). In such functions,
+`...` may only occur non-nested. That is, `y: &'a ...` is not allowed.
 
-A C-variadic type is used to give an undefined number
-of parameters to a given function (like `printf` in C).
-The equivalent in Rust would be to use macros directly.
+A C-variadic type is used to give an undefined number of parameters to a given
+function (like `printf` in C). The equivalent in Rust would be to use macros
+directly (like `println!` for example).

src/librustc_mir/transform/simplify.rs

@@ -33,6 +33,7 @@ use rustc_index::vec::{Idx, IndexVec};
 use rustc_middle::mir::visit::{MutVisitor, MutatingUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::*;
 use rustc_middle::ty::TyCtxt;
+use smallvec::SmallVec;
 use std::borrow::Cow;
 
 pub struct SimplifyCfg {
@@ -172,9 +173,12 @@ impl<'a, 'tcx> CfgSimplifier<'a, 'tcx> {
         }
     }
 
-    // Collapse a goto chain starting from `start`
-    fn collapse_goto_chain(&mut self, start: &mut BasicBlock, changed: &mut bool) {
-        let mut terminator = match self.basic_blocks[*start] {
+    /// This function will return `None` if
+    /// * the block has statements
+    /// * the block has a terminator other than `goto`
+    /// * the block has no terminator (meaning some other part of the current optimization stole it)
+    fn take_terminator_if_simple_goto(&mut self, bb: BasicBlock) -> Option<Terminator<'tcx>> {
+        match self.basic_blocks[bb] {
             BasicBlockData {
                 ref statements,
                 terminator:
@@ -183,32 +187,45 @@ impl<'a, 'tcx> CfgSimplifier<'a, 'tcx> {
             } if statements.is_empty() => terminator.take(),
             // if `terminator` is None, this means we are in a loop. In that
             // case, let all the loop collapse to its entry.
-            _ => return,
-        };
-
-        let target = match terminator {
-            Some(Terminator { kind: TerminatorKind::Goto { ref mut target }, .. }) => {
-                self.collapse_goto_chain(target, changed);
-                *target
-            }
-            _ => unreachable!(),
-        };
-        self.basic_blocks[*start].terminator = terminator;
-
-        debug!("collapsing goto chain from {:?} to {:?}", *start, target);
-
-        *changed |= *start != target;
+            _ => None,
+        }
+    }
 
-        if self.pred_count[*start] == 1 {
-            // This is the last reference to *start, so the pred-count to
-            // to target is moved into the current block.
-            self.pred_count[*start] = 0;
-        } else {
-            self.pred_count[target] += 1;
-            self.pred_count[*start] -= 1;
+    /// Collapse a goto chain starting from `start`
+    fn collapse_goto_chain(&mut self, start: &mut BasicBlock, changed: &mut bool) {
+        // Using `SmallVec` here, because in some logs on libcore oli-obk saw many single-element
+        // goto chains. We should probably benchmark different sizes.
+        let mut terminators: SmallVec<[_; 1]> = Default::default();
+        let mut current = *start;
+        while let Some(terminator) = self.take_terminator_if_simple_goto(current) {
+            let target = match terminator {
+                Terminator { kind: TerminatorKind::Goto { target }, .. } => target,
+                _ => unreachable!(),
+            };
+            terminators.push((current, terminator));
+            current = target;
         }
+        let last = current;
+        *start = last;
+        while let Some((current, mut terminator)) = terminators.pop() {
+            let target = match terminator {
+                Terminator { kind: TerminatorKind::Goto { ref mut target }, .. } => target,
+                _ => unreachable!(),
+            };
+            *target = last;
+            debug!("collapsing goto chain from {:?} to {:?}", current, target);
 
-        *start = target;
+            if self.pred_count[current] == 1 {
+                // This is the last reference to current, so the pred-count to
+                // to target is moved into the current block.
+                self.pred_count[current] = 0;
+            } else {
+                self.pred_count[*target] += 1;
+                self.pred_count[current] -= 1;
+            }
+            *changed = true;
+            self.basic_blocks[current].terminator = Some(terminator);
+        }
     }
 
     // merge a block with 1 `goto` predecessor to its parent