Auto merge of #91359 - dtolnay:args, r=Mark-Simulacrum

Emit simpler code from format_args

I made this PR so that `cargo expand` dumps a less overwhelming amount of formatting-related code.

<br>

`println!("rust")` **Before:**

```rust
{
    ::std::io::_print(::core::fmt::Arguments::new_v1(&["rust\n"],
                                                     &match () {
                                                          _args => [],
                                                      }));
};
```

**After:**

```rust
{ ::std::io::_print(::core::fmt::Arguments::new_v1(&["rust\n"], &[])); };
```

`println!("{}", x)` **Before:**

```rust
{
    ::std::io::_print(::core::fmt::Arguments::new_v1(
        &["", "\n"],
        &match (&x,) {
            _args => [::core::fmt::ArgumentV1::new(
                _args.0,
                ::core::fmt::Display::fmt,
            )],
        },
    ));
};
```

**After:**

```rust
{
    ::std::io::_print(::core::fmt::Arguments::new_v1(
        &["", "\n"],
        &[::core::fmt::ArgumentV1::new(&x, ::core::fmt::Display::fmt)],
    ));
};
```

11 files changed, 123 insertions, 91 deletions

diff --git a/compiler/rustc_ast/src/ast.rs b/compiler/rustc_ast/src/ast.rs
index 565488ab6a5..438168f4fcc 100644
--- a/compiler/rustc_ast/src/ast.rs
+++ b/compiler/rustc_ast/src/ast.rs
@@ -39,6 +39,7 @@ use rustc_span::{Span, DUMMY_SP};
 use std::cmp::Ordering;
 use std::convert::TryFrom;
 use std::fmt;
+use std::mem;
 
 #[cfg(test)]
 mod tests;
@@ -1276,6 +1277,19 @@ impl Expr {
             ExprKind::Err => ExprPrecedence::Err,
         }
     }
+
+    pub fn take(&mut self) -> Self {
+        mem::replace(
+            self,
+            Expr {
+                id: DUMMY_NODE_ID,
+                kind: ExprKind::Err,
+                span: DUMMY_SP,
+                attrs: ThinVec::new(),
+                tokens: None,
+            },
+        )
+    }
 }
 
 /// Limit types of a range (inclusive or exclusive)
diff --git a/compiler/rustc_builtin_macros/src/format.rs b/compiler/rustc_builtin_macros/src/format.rs
index 407aaacb889..d1393528d1c 100644
--- a/compiler/rustc_builtin_macros/src/format.rs
+++ b/compiler/rustc_builtin_macros/src/format.rs
@@ -11,6 +11,7 @@ use rustc_expand::base::{self, *};
 use rustc_parse_format as parse;
 use rustc_span::symbol::{sym, Ident, Symbol};
 use rustc_span::{MultiSpan, Span};
+use smallvec::SmallVec;
 
 use std::borrow::Cow;
 use std::collections::hash_map::Entry;
@@ -744,78 +745,95 @@ impl<'a, 'b> Context<'a, 'b> {
     /// Actually builds the expression which the format_args! block will be
     /// expanded to.
     fn into_expr(self) -> P<ast::Expr> {
-        let mut args = Vec::with_capacity(
+        let mut original_args = self.args;
+        let mut fmt_args = Vec::with_capacity(
             self.arg_unique_types.iter().map(|v| v.len()).sum::<usize>() + self.count_args.len(),
         );
-        let mut heads = Vec::with_capacity(self.args.len());
 
         // First, build up the static array which will become our precompiled
         // format "string"
         let pieces = self.ecx.expr_vec_slice(self.fmtsp, self.str_pieces);
 
-        // Before consuming the expressions, we have to remember spans for
-        // count arguments as they are now generated separate from other
-        // arguments, hence have no access to the `P<ast::Expr>`'s.
-        let spans_pos: Vec<_> = self.args.iter().map(|e| e.span).collect();
-
-        // Right now there is a bug such that for the expression:
-        //      foo(bar(&1))
-        // the lifetime of `1` doesn't outlast the call to `bar`, so it's not
-        // valid for the call to `foo`. To work around this all arguments to the
-        // format! string are shoved into locals. Furthermore, we shove the address
-        // of each variable because we don't want to move out of the arguments
-        // passed to this function.
-        for (i, e) in self.args.into_iter().enumerate() {
-            for arg_ty in self.arg_unique_types[i].iter() {
-                args.push(Context::format_arg(self.ecx, self.macsp, e.span, arg_ty, i));
-            }
-            // use the arg span for `&arg` so that borrowck errors
-            // point to the specific expression passed to the macro
-            // (the span is otherwise unavailable in MIR)
-            heads.push(self.ecx.expr_addr_of(e.span.with_ctxt(self.macsp.ctxt()), e));
-        }
-        for index in self.count_args {
-            let span = spans_pos[index];
-            args.push(Context::format_arg(self.ecx, self.macsp, span, &Count, index));
+        // We need to construct a &[ArgumentV1] to pass into the fmt::Arguments
+        // constructor. In general the expressions in this slice might be
+        // permuted from their order in original_args (such as in the case of
+        // "{1} {0}"), or may have multiple entries referring to the same
+        // element of original_args ("{0} {0}").
+        //
+        // The following vector has one item per element of our output slice,
+        // identifying the index of which element of original_args it's passing,
+        // and that argument's type.
+        let mut fmt_arg_index_and_ty = SmallVec::<[(usize, &ArgumentType); 8]>::new();
+        for (i, unique_types) in self.arg_unique_types.iter().enumerate() {
+            fmt_arg_index_and_ty.extend(unique_types.iter().map(|ty| (i, ty)));
         }
+        fmt_arg_index_and_ty.extend(self.count_args.iter().map(|&i| (i, &Count)));
 
-        let args_array = self.ecx.expr_vec(self.macsp, args);
-
-        // Constructs an AST equivalent to:
-        //
-        //      match (&arg0, &arg1) {
-        //          (tmp0, tmp1) => args_array
-        //      }
+        // Figure out whether there are permuted or repeated elements. If not,
+        // we can generate simpler code.
         //
-        // It was:
+        // The sequence has no indices out of order or repeated if: for every
+        // adjacent pair of elements, the first one's index is less than the
+        // second one's index.
+        let nicely_ordered =
+            fmt_arg_index_and_ty.array_windows().all(|[(i, _i_ty), (j, _j_ty)]| i < j);
+
+        // We want to emit:
         //
-        //      let tmp0 = &arg0;
-        //      let tmp1 = &arg1;
-        //      args_array
+        //     [ArgumentV1::new(&$arg0, …), ArgumentV1::new(&$arg1, …), …]
         //
-        // Because of #11585 the new temporary lifetime rule, the enclosing
-        // statements for these temporaries become the let's themselves.
-        // If one or more of them are RefCell's, RefCell borrow() will also
-        // end there; they don't last long enough for args_array to use them.
-        // The match expression solves the scope problem.
+        // However, it's only legal to do so if $arg0, $arg1, … were written in
+        // exactly that order by the programmer. When arguments are permuted, we
+        // want them evaluated in the order written by the programmer, not in
+        // the order provided to fmt::Arguments. When arguments are repeated, we
+        // want the expression evaluated only once.
         //
-        // Note, it may also very well be transformed to:
+        // Thus in the not nicely ordered case we emit the following instead:
         //
-        //      match arg0 {
-        //          ref tmp0 => {
-        //              match arg1 => {
-        //                  ref tmp1 => args_array } } }
+        //     match (&$arg0, &$arg1, …) {
+        //         _args => [ArgumentV1::new(_args.$i, …), ArgumentV1::new(_args.$j, …), …]
+        //     }
         //
-        // But the nested match expression is proved to perform not as well
-        // as series of let's; the first approach does.
-        let args_match = {
-            let pat = self.ecx.pat_ident(self.macsp, Ident::new(sym::_args, self.macsp));
-            let arm = self.ecx.arm(self.macsp, pat, args_array);
-            let head = self.ecx.expr(self.macsp, ast::ExprKind::Tup(heads));
-            self.ecx.expr_match(self.macsp, head, vec![arm])
-        };
+        // for the sequence of indices $i, $j, … governed by fmt_arg_index_and_ty.
+        for (arg_index, arg_ty) in fmt_arg_index_and_ty {
+            let e = &mut original_args[arg_index];
+            let span = e.span;
+            let arg = if nicely_ordered {
+                let expansion_span = e.span.with_ctxt(self.macsp.ctxt());
+                // The indices are strictly ordered so e has not been taken yet.
+                self.ecx.expr_addr_of(expansion_span, P(e.take()))
+            } else {
+                let def_site = self.ecx.with_def_site_ctxt(span);
+                let args_tuple = self.ecx.expr_ident(def_site, Ident::new(sym::_args, def_site));
+                let member = Ident::new(sym::integer(arg_index), def_site);
+                self.ecx.expr(def_site, ast::ExprKind::Field(args_tuple, member))
+            };
+            fmt_args.push(Context::format_arg(self.ecx, self.macsp, span, arg_ty, arg));
+        }
 
-        let args_slice = self.ecx.expr_addr_of(self.macsp, args_match);
+        let args_array = self.ecx.expr_vec(self.macsp, fmt_args);
+        let args_slice = self.ecx.expr_addr_of(
+            self.macsp,
+            if nicely_ordered {
+                args_array
+            } else {
+                // In the !nicely_ordered case, none of the exprs were moved
+                // away in the previous loop.
+                //
+                // This uses the arg span for `&arg` so that borrowck errors
+                // point to the specific expression passed to the macro (the
+                // span is otherwise unavailable in the MIR used by borrowck).
+                let heads = original_args
+                    .into_iter()
+                    .map(|e| self.ecx.expr_addr_of(e.span.with_ctxt(self.macsp.ctxt()), e))
+                    .collect();
+
+                let pat = self.ecx.pat_ident(self.macsp, Ident::new(sym::_args, self.macsp));
+                let arm = self.ecx.arm(self.macsp, pat, args_array);
+                let head = self.ecx.expr(self.macsp, ast::ExprKind::Tup(heads));
+                self.ecx.expr_match(self.macsp, head, vec![arm])
+            },
+        );
 
         // Now create the fmt::Arguments struct with all our locals we created.
         let (fn_name, fn_args) = if self.all_pieces_simple {
@@ -848,11 +866,9 @@ impl<'a, 'b> Context<'a, 'b> {
         macsp: Span,
         mut sp: Span,
         ty: &ArgumentType,
-        arg_index: usize,
+        arg: P<ast::Expr>,
     ) -> P<ast::Expr> {
         sp = ecx.with_def_site_ctxt(sp);
-        let arg = ecx.expr_ident(sp, Ident::new(sym::_args, sp));
-        let arg = ecx.expr(sp, ast::ExprKind::Field(arg, Ident::new(sym::integer(arg_index), sp)));
         let trait_ = match *ty {
             Placeholder(trait_) if trait_ == "<invalid>" => return DummyResult::raw_expr(sp, true),
             Placeholder(trait_) => trait_,
diff --git a/compiler/rustc_builtin_macros/src/lib.rs b/compiler/rustc_builtin_macros/src/lib.rs
index 65f785b9097..6c16c285492 100644
--- a/compiler/rustc_builtin_macros/src/lib.rs
+++ b/compiler/rustc_builtin_macros/src/lib.rs
@@ -2,10 +2,12 @@
 //! injecting code into the crate before it is lowered to HIR.
 
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
+#![feature(array_windows)]
 #![feature(box_patterns)]
 #![feature(bool_to_option)]
 #![feature(crate_visibility_modifier)]
 #![feature(decl_macro)]
+#![feature(is_sorted)]
 #![feature(nll)]
 #![feature(proc_macro_internals)]
 #![feature(proc_macro_quote)]
diff --git a/src/test/pretty/dollar-crate.pp b/src/test/pretty/dollar-crate.pp
index 84eda08d203..3af37955f23 100644
--- a/src/test/pretty/dollar-crate.pp
+++ b/src/test/pretty/dollar-crate.pp
@@ -9,10 +9,5 @@ extern crate std;
 // pp-exact:dollar-crate.pp
 
 fn main() {
-    {
-        ::std::io::_print(::core::fmt::Arguments::new_v1(&["rust\n"],
-                                                         &match () {
-                                                              _args => [],
-                                                          }));
-    };
+    { ::std::io::_print(::core::fmt::Arguments::new_v1(&["rust\n"], &[])); };
 }
diff --git a/src/test/pretty/issue-4264.pp b/src/test/pretty/issue-4264.pp
index 529daab9038..93967e720c1 100644
--- a/src/test/pretty/issue-4264.pp
+++ b/src/test/pretty/issue-4264.pp
@@ -41,16 +41,7 @@ pub fn bar() ({
                                                                                                                                                               [&str; 1])
                                                                                                                                                             as
                                                                                                                                                             &[&str; 1]),
-                                                                                                                                                        (&(match (()
-                                                                                                                                                                     as
-                                                                                                                                                                     ())
-                                                                                                                                                               {
-                                                                                                                                                               _args
-                                                                                                                                                               =>
-                                                                                                                                                               ([]
-                                                                                                                                                                   as
-                                                                                                                                                                   [ArgumentV1; 0]),
-                                                                                                                                                           }
+                                                                                                                                                        (&([]
                                                                                                                                                               as
                                                                                                                                                               [ArgumentV1; 0])
                                                                                                                                                             as
diff --git a/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.closure.txt b/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.closure.txt
index 5715e0cc269..7eb393bb448 100644
--- a/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.closure.txt
+++ b/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.closure.txt
@@ -29,8 +29,8 @@
    29|      1|    some_string = Some(String::from("the string content"));
    30|      1|    let
    31|      1|        a
-   32|      1|    =
-   33|      1|        ||
+   32|       |    =
+   33|       |        ||
    34|      0|    {
    35|      0|        let mut countdown = 0;
    36|      0|        if is_false {
@@ -173,7 +173,7 @@
   169|       |    ;
   170|       |
   171|      1|    let short_used_not_covered_closure_line_break_no_block_embedded_branch =
-  172|      1|        | _unused_arg: u8 |
+  172|       |        | _unused_arg: u8 |
   173|      0|            println!(
   174|      0|                "not called: {}",
   175|      0|                if is_true { "check" } else { "me" }
diff --git a/src/test/ui/attributes/key-value-expansion.stderr b/src/test/ui/attributes/key-value-expansion.stderr
index 878afb39214..268e382327e 100644
--- a/src/test/ui/attributes/key-value-expansion.stderr
+++ b/src/test/ui/attributes/key-value-expansion.stderr
@@ -18,11 +18,8 @@ LL | bug!();
 error: unexpected token: `{
            let res =
                ::alloc::fmt::format(::core::fmt::Arguments::new_v1(&[""],
-                                                                   &match (&"u8",) {
-                                                                        _args =>
-                                                                        [::core::fmt::ArgumentV1::new(_args.0,
-                                                                                                      ::core::fmt::Display::fmt)],
-                                                                    }));
+                                                                   &[::core::fmt::ArgumentV1::new(&"u8",
+                                                                                                  ::core::fmt::Display::fmt)]));
            res
        }.as_str()`
   --> $DIR/key-value-expansion.rs:48:23
diff --git a/src/test/ui/closures/print/closure-print-generic-trim-off-verbose-2.stderr b/src/test/ui/closures/print/closure-print-generic-trim-off-verbose-2.stderr
index ee394d64a1d..bdc71b8dcaa 100644
--- a/src/test/ui/closures/print/closure-print-generic-trim-off-verbose-2.stderr
+++ b/src/test/ui/closures/print/closure-print-generic-trim-off-verbose-2.stderr
@@ -9,7 +9,7 @@ LL |         let c1 : () = c;
    |                  expected due to this
    |
    = note: expected unit type `()`
-                found closure `[mod1::f<T>::{closure#0} closure_substs=(unavailable) substs=[T, _#22t, extern "rust-call" fn(()), _#23t]]`
+                found closure `[mod1::f<T>::{closure#0} closure_substs=(unavailable) substs=[T, _#19t, extern "rust-call" fn(()), _#20t]]`
 help: use parentheses to call this closure
    |
 LL |         let c1 : () = c();
diff --git a/src/test/ui/closures/print/closure-print-generic-verbose-2.stderr b/src/test/ui/closures/print/closure-print-generic-verbose-2.stderr
index 11b9fa7e40c..453f18891d3 100644
--- a/src/test/ui/closures/print/closure-print-generic-verbose-2.stderr
+++ b/src/test/ui/closures/print/closure-print-generic-verbose-2.stderr
@@ -9,7 +9,7 @@ LL |         let c1 : () = c;
    |                  expected due to this
    |
    = note: expected unit type `()`
-                found closure `[f<T>::{closure#0} closure_substs=(unavailable) substs=[T, _#22t, extern "rust-call" fn(()), _#23t]]`
+                found closure `[f<T>::{closure#0} closure_substs=(unavailable) substs=[T, _#19t, extern "rust-call" fn(()), _#20t]]`
 help: use parentheses to call this closure
    |
 LL |         let c1 : () = c();
diff --git a/src/test/ui/issues/issue-69455.stderr b/src/test/ui/issues/issue-69455.stderr
index da84a6b52da..95617e4ecc8 100644
--- a/src/test/ui/issues/issue-69455.stderr
+++ b/src/test/ui/issues/issue-69455.stderr
@@ -1,8 +1,16 @@
-error[E0284]: type annotations needed: cannot satisfy `<u64 as Test<_>>::Output == _`
-  --> $DIR/issue-69455.rs:29:26
+error[E0282]: type annotations needed
+  --> $DIR/issue-69455.rs:29:5
    |
+LL |     type Output;
+   |     ------------ `<Self as Test<Rhs>>::Output` defined here
+...
 LL |     println!("{}", 23u64.test(xs.iter().sum()));
-   |                          ^^^^ cannot satisfy `<u64 as Test<_>>::Output == _`
+   |     ^^^^^^^^^^^^^^^---------------------------^
+   |     |              |
+   |     |              this method call resolves to `<Self as Test<Rhs>>::Output`
+   |     cannot infer type for type parameter `T` declared on the associated function `new`
+   |
+   = note: this error originates in the macro `$crate::format_args_nl` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0283]: type annotations needed
   --> $DIR/issue-69455.rs:29:26
@@ -25,5 +33,5 @@ LL |     println!("{}", 23u64.test(xs.iter().sum::<S>()));
 
 error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0283, E0284.
-For more information about an error, try `rustc --explain E0283`.
+Some errors have detailed explanations: E0282, E0283.
+For more information about an error, try `rustc --explain E0282`.
diff --git a/src/tools/clippy/tests/ui/to_string_in_display.stderr b/src/tools/clippy/tests/ui/to_string_in_display.stderr
index 5f26ef413e2..80189ca1f0a 100644
--- a/src/tools/clippy/tests/ui/to_string_in_display.stderr
+++ b/src/tools/clippy/tests/ui/to_string_in_display.stderr
@@ -6,5 +6,14 @@ LL |         write!(f, "{}", self.to_string())
    |
    = note: `-D clippy::to-string-in-display` implied by `-D warnings`
 
-error: aborting due to previous error
+error: unnecessary use of `to_string`
+  --> $DIR/to_string_in_display.rs:55:50
+   |
+LL |             Self::E(string) => write!(f, "E {}", string.to_string()),
+   |                                                  ^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::unnecessary-to-owned` implied by `-D warnings`
+   = note: this error originates in the macro `$crate::format_args` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 2 previous errors