Implement builtin#format_args, using rustc's format_args parser

19 files changed, 1740 insertions, 14 deletions

diff --git a/Cargo.lock b/Cargo.lock
index f6900f883a1..c3d7b3f79b9 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -541,6 +541,7 @@ dependencies = [
  "mbe",
  "once_cell",
  "profile",
+ "ra-ap-rustc_lexer",
  "rustc-hash",
  "smallvec",
  "stdx",
diff --git a/crates/hir-def/Cargo.toml b/crates/hir-def/Cargo.toml
index 30307deb79b..4640ee5140f 100644
--- a/crates/hir-def/Cargo.toml
+++ b/crates/hir-def/Cargo.toml
@@ -33,6 +33,7 @@ triomphe.workspace = true
 
 rustc_abi = { version = "0.0.20221221", package = "hkalbasi-rustc-ap-rustc_abi", default-features = false }
 rustc_index = { version = "0.0.20221221", package = "hkalbasi-rustc-ap-rustc_index", default-features = false }
+rustc_lexer = { version = "0.1.0", package = "ra-ap-rustc_lexer" }
 
 # local deps
 stdx.workspace = true
diff --git a/crates/hir-def/src/body/lower.rs b/crates/hir-def/src/body/lower.rs
index 038032e05d8..bb0127c9ef6 100644
--- a/crates/hir-def/src/body/lower.rs
+++ b/crates/hir-def/src/body/lower.rs
@@ -29,9 +29,13 @@ use crate::{
     db::DefDatabase,
     expander::Expander,
     hir::{
-        dummy_expr_id, Array, Binding, BindingAnnotation, BindingId, BindingProblems, CaptureBy,
-        ClosureKind, Expr, ExprId, InlineAsm, Label, LabelId, Literal, LiteralOrConst, MatchArm,
-        Movability, OffsetOf, Pat, PatId, RecordFieldPat, RecordLitField, Statement,
+        dummy_expr_id,
+        format_args::{
+            self, FormatArgs, FormatArgument, FormatArgumentKind, FormatArgumentsCollector,
+        },
+        Array, Binding, BindingAnnotation, BindingId, BindingProblems, CaptureBy, ClosureKind,
+        Expr, ExprId, InlineAsm, Label, LabelId, Literal, LiteralOrConst, MatchArm, Movability,
+        OffsetOf, Pat, PatId, RecordFieldPat, RecordLitField, Statement,
     },
     item_scope::BuiltinShadowMode,
     lang_item::LangItem,
@@ -649,15 +653,58 @@ impl ExprCollector<'_> {
             }
             ast::Expr::UnderscoreExpr(_) => self.alloc_expr(Expr::Underscore, syntax_ptr),
             ast::Expr::AsmExpr(e) => {
-                let expr = Expr::InlineAsm(InlineAsm { e: self.collect_expr_opt(e.expr()) });
-                self.alloc_expr(expr, syntax_ptr)
+                let e = self.collect_expr_opt(e.expr());
+                self.alloc_expr(Expr::InlineAsm(InlineAsm { e }), syntax_ptr)
             }
             ast::Expr::OffsetOfExpr(e) => {
                 let container = Interned::new(TypeRef::from_ast_opt(&self.ctx(), e.ty()));
                 let fields = e.fields().map(|it| it.as_name()).collect();
                 self.alloc_expr(Expr::OffsetOf(OffsetOf { container, fields }), syntax_ptr)
             }
-            ast::Expr::FormatArgsExpr(_) => self.missing_expr(),
+            ast::Expr::FormatArgsExpr(f) => {
+                let mut args = FormatArgumentsCollector::new();
+                f.args().for_each(|arg| {
+                    args.add(FormatArgument {
+                        kind: match arg.name() {
+                            Some(name) => FormatArgumentKind::Named(name.as_name()),
+                            None => FormatArgumentKind::Normal,
+                        },
+                        expr: self.collect_expr_opt(arg.expr()),
+                    });
+                });
+                let template = f.template();
+                let fmt_snippet = template.as_ref().map(ToString::to_string);
+                let expr = self.collect_expr_opt(f.template());
+                if let Expr::Literal(Literal::String(_)) = self.body[expr] {
+                    let source = self.source_map.expr_map_back[expr].clone();
+                    let is_direct_literal = source.file_id == self.expander.current_file_id;
+                    if let ast::Expr::Literal(l) =
+                        source.value.to_node(&self.db.parse_or_expand(source.file_id))
+                    {
+                        if let ast::LiteralKind::String(s) = l.kind() {
+                            return Some(self.alloc_expr(
+                                Expr::FormatArgs(format_args::parse(
+                                    expr,
+                                    &s,
+                                    fmt_snippet,
+                                    args,
+                                    is_direct_literal,
+                                )),
+                                syntax_ptr,
+                            ));
+                        }
+                    }
+                }
+
+                self.alloc_expr(
+                    Expr::FormatArgs(FormatArgs {
+                        template_expr: expr,
+                        template: Default::default(),
+                        arguments: args.finish(),
+                    }),
+                    syntax_ptr,
+                )
+            }
         })
     }
 
diff --git a/crates/hir-def/src/body/pretty.rs b/crates/hir-def/src/body/pretty.rs
index 602a7983c7c..b67ed2fb385 100644
--- a/crates/hir-def/src/body/pretty.rs
+++ b/crates/hir-def/src/body/pretty.rs
@@ -156,6 +156,11 @@ impl Printer<'_> {
             Expr::Missing => w!(self, "�"),
             Expr::Underscore => w!(self, "_"),
             Expr::InlineAsm(_) => w!(self, "builtin#asm(_)"),
+            Expr::FormatArgs(_fmt_args) => {
+                w!(self, "builtin#format_args(");
+                // FIXME
+                w!(self, ")");
+            }
             Expr::OffsetOf(offset_of) => {
                 w!(self, "builtin#offset_of(");
                 self.print_type_ref(&offset_of.container);
diff --git a/crates/hir-def/src/hir.rs b/crates/hir-def/src/hir.rs
index 1c86af456d1..98220de388e 100644
--- a/crates/hir-def/src/hir.rs
+++ b/crates/hir-def/src/hir.rs
@@ -13,6 +13,7 @@
 //! See also a neighboring `body` module.
 
 pub mod type_ref;
+pub mod format_args;
 
 use std::fmt;
 
@@ -24,6 +25,7 @@ use syntax::ast;
 
 use crate::{
     builtin_type::{BuiltinFloat, BuiltinInt, BuiltinUint},
+    hir::format_args::{FormatArgs, FormatArgumentKind},
     path::{GenericArgs, Path},
     type_ref::{Mutability, Rawness, TypeRef},
     BlockId, ConstBlockId,
@@ -117,7 +119,6 @@ impl From<ast::LiteralKind> for Literal {
     fn from(ast_lit_kind: ast::LiteralKind) -> Self {
         use ast::LiteralKind;
         match ast_lit_kind {
-            // FIXME: these should have actual values filled in, but unsure on perf impact
             LiteralKind::IntNumber(lit) => {
                 if let builtin @ Some(_) = lit.suffix().and_then(BuiltinFloat::from_suffix) {
                     Literal::Float(
@@ -283,6 +284,7 @@ pub enum Expr {
     Underscore,
     OffsetOf(OffsetOf),
     InlineAsm(InlineAsm),
+    FormatArgs(FormatArgs),
 }
 
 #[derive(Debug, Clone, PartialEq, Eq)]
@@ -355,7 +357,15 @@ impl Expr {
         match self {
             Expr::Missing => {}
             Expr::Path(_) | Expr::OffsetOf(_) => {}
-            Expr::InlineAsm(e) => f(e.e),
+            Expr::InlineAsm(it) => f(it.e),
+            Expr::FormatArgs(it) => {
+                f(it.template_expr);
+                it.arguments
+                    .arguments
+                    .iter()
+                    .filter(|it| !matches!(it.kind, FormatArgumentKind::Captured(_)))
+                    .for_each(|it| f(it.expr));
+            }
             Expr::If { condition, then_branch, else_branch } => {
                 f(*condition);
                 f(*then_branch);
diff --git a/crates/hir-def/src/hir/format_args.rs b/crates/hir-def/src/hir/format_args.rs
new file mode 100644
index 00000000000..8fa8b7246bc
--- /dev/null
+++ b/crates/hir-def/src/hir/format_args.rs
@@ -0,0 +1,511 @@
+use std::mem;
+
+use hir_expand::name::Name;
+use syntax::{
+    ast::{self, IsString},
+    AstToken, SmolStr, TextRange,
+};
+
+use crate::hir::{dummy_expr_id, ExprId};
+
+mod parse;
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct FormatArgs {
+    pub template_expr: ExprId,
+    pub template: Box<[FormatArgsPiece]>,
+    pub arguments: FormatArguments,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct FormatArguments {
+    pub arguments: Box<[FormatArgument]>,
+    pub num_unnamed_args: usize,
+    pub num_explicit_args: usize,
+    pub names: Box<[(Name, usize)]>,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum FormatArgsPiece {
+    Literal(Box<str>),
+    Placeholder(FormatPlaceholder),
+}
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct FormatPlaceholder {
+    /// Index into [`FormatArgs::arguments`].
+    pub argument: FormatArgPosition,
+    /// The span inside the format string for the full `{…}` placeholder.
+    pub span: Option<TextRange>,
+    /// `{}`, `{:?}`, or `{:x}`, etc.
+    pub format_trait: FormatTrait,
+    /// `{}` or `{:.5}` or `{:-^20}`, etc.
+    pub format_options: FormatOptions,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct FormatArgPosition {
+    /// Which argument this position refers to (Ok),
+    /// or would've referred to if it existed (Err).
+    pub index: Result<usize, usize>,
+    /// What kind of position this is. See [`FormatArgPositionKind`].
+    pub kind: FormatArgPositionKind,
+    /// The span of the name or number.
+    pub span: Option<TextRange>,
+}
+
+#[derive(Copy, Debug, Clone, PartialEq, Eq)]
+pub enum FormatArgPositionKind {
+    /// `{}` or `{:.*}`
+    Implicit,
+    /// `{1}` or `{:1$}` or `{:.1$}`
+    Number,
+    /// `{a}` or `{:a$}` or `{:.a$}`
+    Named,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum FormatTrait {
+    /// `{}`
+    Display,
+    /// `{:?}`
+    Debug,
+    /// `{:e}`
+    LowerExp,
+    /// `{:E}`
+    UpperExp,
+    /// `{:o}`
+    Octal,
+    /// `{:p}`
+    Pointer,
+    /// `{:b}`
+    Binary,
+    /// `{:x}`
+    LowerHex,
+    /// `{:X}`
+    UpperHex,
+}
+
+#[derive(Clone, Default, Debug, PartialEq, Eq)]
+pub struct FormatOptions {
+    /// The width. E.g. `{:5}` or `{:width$}`.
+    pub width: Option<FormatCount>,
+    /// The precision. E.g. `{:.5}` or `{:.precision$}`.
+    pub precision: Option<FormatCount>,
+    /// The alignment. E.g. `{:>}` or `{:<}` or `{:^}`.
+    pub alignment: Option<FormatAlignment>,
+    /// The fill character. E.g. the `.` in `{:.>10}`.
+    pub fill: Option<char>,
+    /// The `+` or `-` flag.
+    pub sign: Option<FormatSign>,
+    /// The `#` flag.
+    pub alternate: bool,
+    /// The `0` flag. E.g. the `0` in `{:02x}`.
+    pub zero_pad: bool,
+    /// The `x` or `X` flag (for `Debug` only). E.g. the `x` in `{:x?}`.
+    pub debug_hex: Option<FormatDebugHex>,
+}
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub enum FormatSign {
+    /// The `+` flag.
+    Plus,
+    /// The `-` flag.
+    Minus,
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub enum FormatDebugHex {
+    /// The `x` flag in `{:x?}`.
+    Lower,
+    /// The `X` flag in `{:X?}`.
+    Upper,
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub enum FormatAlignment {
+    /// `{:<}`
+    Left,
+    /// `{:>}`
+    Right,
+    /// `{:^}`
+    Center,
+}
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub enum FormatCount {
+    /// `{:5}` or `{:.5}`
+    Literal(usize),
+    /// `{:.*}`, `{:.5$}`, or `{:a$}`, etc.
+    Argument(FormatArgPosition),
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct FormatArgument {
+    pub kind: FormatArgumentKind,
+    pub expr: ExprId,
+}
+
+#[derive(Clone, PartialEq, Eq, Debug)]
+pub enum FormatArgumentKind {
+    /// `format_args(…, arg)`
+    Normal,
+    /// `format_args(…, arg = 1)`
+    Named(Name),
+    /// `format_args("… {arg} …")`
+    Captured(Name),
+}
+
+// Only used in parse_args and report_invalid_references,
+// to indicate how a referred argument was used.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+enum PositionUsedAs {
+    Placeholder(Option<TextRange>),
+    Precision,
+    Width,
+}
+use PositionUsedAs::*;
+
+pub(crate) fn parse(
+    expr: ExprId,
+    s: &ast::String,
+    fmt_snippet: Option<String>,
+    mut args: FormatArgumentsCollector,
+    is_direct_literal: bool,
+) -> FormatArgs {
+    let text = s.text();
+    let str_style = match s.quote_offsets() {
+        Some(offsets) => {
+            let raw = u32::from(offsets.quotes.0.len()) - 1;
+            (raw != 0).then_some(raw as usize)
+        }
+        None => None,
+    };
+    let mut parser =
+        parse::Parser::new(text, str_style, fmt_snippet, false, parse::ParseMode::Format);
+
+    let mut pieces = Vec::new();
+    while let Some(piece) = parser.next() {
+        if !parser.errors.is_empty() {
+            break;
+        } else {
+            pieces.push(piece);
+        }
+    }
+    let is_source_literal = parser.is_source_literal;
+    if !parser.errors.is_empty() {
+        // FIXME: Diagnose
+        return FormatArgs {
+            template_expr: expr,
+            template: Default::default(),
+            arguments: args.finish(),
+        };
+    }
+
+    let to_span = |inner_span: parse::InnerSpan| {
+        is_source_literal.then(|| {
+            TextRange::new(inner_span.start.try_into().unwrap(), inner_span.end.try_into().unwrap())
+        })
+    };
+
+    let mut used = vec![false; args.explicit_args().len()];
+    let mut invalid_refs = Vec::new();
+    let mut numeric_refences_to_named_arg = Vec::new();
+
+    enum ArgRef<'a> {
+        Index(usize),
+        Name(&'a str, Option<TextRange>),
+    }
+    let mut lookup_arg = |arg: ArgRef<'_>,
+                          span: Option<TextRange>,
+                          used_as: PositionUsedAs,
+                          kind: FormatArgPositionKind|
+     -> FormatArgPosition {
+        let index = match arg {
+            ArgRef::Index(index) => {
+                if let Some(arg) = args.by_index(index) {
+                    used[index] = true;
+                    if arg.kind.ident().is_some() {
+                        // This was a named argument, but it was used as a positional argument.
+                        numeric_refences_to_named_arg.push((index, span, used_as));
+                    }
+                    Ok(index)
+                } else {
+                    // Doesn't exist as an explicit argument.
+                    invalid_refs.push((index, span, used_as, kind));
+                    Err(index)
+                }
+            }
+            ArgRef::Name(name, _span) => {
+                let name = Name::new_text_dont_use(SmolStr::new(name));
+                if let Some((index, _)) = args.by_name(&name) {
+                    // Name found in `args`, so we resolve it to its index.
+                    if index < args.explicit_args().len() {
+                        // Mark it as used, if it was an explicit argument.
+                        used[index] = true;
+                    }
+                    Ok(index)
+                } else {
+                    // Name not found in `args`, so we add it as an implicitly captured argument.
+                    if !is_direct_literal {
+                        // For the moment capturing variables from format strings expanded from macros is
+                        // disabled (see RFC #2795)
+                        // FIXME: Diagnose
+                    }
+                    Ok(args.add(FormatArgument {
+                        kind: FormatArgumentKind::Captured(name),
+                        // FIXME: This is problematic, we might want to synthesize a dummy
+                        // expression proper and/or desugar these.
+                        expr: dummy_expr_id(),
+                    }))
+                }
+            }
+        };
+        FormatArgPosition { index, kind, span }
+    };
+
+    let mut template = Vec::new();
+    let mut unfinished_literal = String::new();
+    let mut placeholder_index = 0;
+
+    for piece in pieces {
+        match piece {
+            parse::Piece::String(s) => {
+                unfinished_literal.push_str(s);
+            }
+            parse::Piece::NextArgument(arg) => {
+                let parse::Argument { position, position_span, format } = *arg;
+                if !unfinished_literal.is_empty() {
+                    template.push(FormatArgsPiece::Literal(
+                        mem::take(&mut unfinished_literal).into_boxed_str(),
+                    ));
+                }
+
+                let span = parser.arg_places.get(placeholder_index).and_then(|&s| to_span(s));
+                placeholder_index += 1;
+
+                let position_span = to_span(position_span);
+                let argument = match position {
+                    parse::ArgumentImplicitlyIs(i) => lookup_arg(
+                        ArgRef::Index(i),
+                        position_span,
+                        Placeholder(span),
+                        FormatArgPositionKind::Implicit,
+                    ),
+                    parse::ArgumentIs(i) => lookup_arg(
+                        ArgRef::Index(i),
+                        position_span,
+                        Placeholder(span),
+                        FormatArgPositionKind::Number,
+                    ),
+                    parse::ArgumentNamed(name) => lookup_arg(
+                        ArgRef::Name(name, position_span),
+                        position_span,
+                        Placeholder(span),
+                        FormatArgPositionKind::Named,
+                    ),
+                };
+
+                let alignment = match format.align {
+                    parse::AlignUnknown => None,
+                    parse::AlignLeft => Some(FormatAlignment::Left),
+                    parse::AlignRight => Some(FormatAlignment::Right),
+                    parse::AlignCenter => Some(FormatAlignment::Center),
+                };
+
+                let format_trait = match format.ty {
+                    "" => FormatTrait::Display,
+                    "?" => FormatTrait::Debug,
+                    "e" => FormatTrait::LowerExp,
+                    "E" => FormatTrait::UpperExp,
+                    "o" => FormatTrait::Octal,
+                    "p" => FormatTrait::Pointer,
+                    "b" => FormatTrait::Binary,
+                    "x" => FormatTrait::LowerHex,
+                    "X" => FormatTrait::UpperHex,
+                    _ => {
+                        // FIXME: Diagnose
+                        FormatTrait::Display
+                    }
+                };
+
+                let precision_span = format.precision_span.and_then(to_span);
+                let precision = match format.precision {
+                    parse::CountIs(n) => Some(FormatCount::Literal(n)),
+                    parse::CountIsName(name, name_span) => Some(FormatCount::Argument(lookup_arg(
+                        ArgRef::Name(name, to_span(name_span)),
+                        precision_span,
+                        Precision,
+                        FormatArgPositionKind::Named,
+                    ))),
+                    parse::CountIsParam(i) => Some(FormatCount::Argument(lookup_arg(
+                        ArgRef::Index(i),
+                        precision_span,
+                        Precision,
+                        FormatArgPositionKind::Number,
+                    ))),
+                    parse::CountIsStar(i) => Some(FormatCount::Argument(lookup_arg(
+                        ArgRef::Index(i),
+                        precision_span,
+                        Precision,
+                        FormatArgPositionKind::Implicit,
+                    ))),
+                    parse::CountImplied => None,
+                };
+
+                let width_span = format.width_span.and_then(to_span);
+                let width = match format.width {
+                    parse::CountIs(n) => Some(FormatCount::Literal(n)),
+                    parse::CountIsName(name, name_span) => Some(FormatCount::Argument(lookup_arg(
+                        ArgRef::Name(name, to_span(name_span)),
+                        width_span,
+                        Width,
+                        FormatArgPositionKind::Named,
+                    ))),
+                    parse::CountIsParam(i) => Some(FormatCount::Argument(lookup_arg(
+                        ArgRef::Index(i),
+                        width_span,
+                        Width,
+                        FormatArgPositionKind::Number,
+                    ))),
+                    parse::CountIsStar(_) => unreachable!(),
+                    parse::CountImplied => None,
+                };
+
+                template.push(FormatArgsPiece::Placeholder(FormatPlaceholder {
+                    argument,
+                    span,
+                    format_trait,
+                    format_options: FormatOptions {
+                        fill: format.fill,
+                        alignment,
+                        sign: format.sign.map(|s| match s {
+                            parse::Sign::Plus => FormatSign::Plus,
+                            parse::Sign::Minus => FormatSign::Minus,
+                        }),
+                        alternate: format.alternate,
+                        zero_pad: format.zero_pad,
+                        debug_hex: format.debug_hex.map(|s| match s {
+                            parse::DebugHex::Lower => FormatDebugHex::Lower,
+                            parse::DebugHex::Upper => FormatDebugHex::Upper,
+                        }),
+                        precision,
+                        width,
+                    },
+                }));
+            }
+        }
+    }
+
+    if !unfinished_literal.is_empty() {
+        template.push(FormatArgsPiece::Literal(unfinished_literal.into_boxed_str()));
+    }
+
+    if !invalid_refs.is_empty() {
+        // FIXME: Diagnose
+    }
+
+    let unused = used
+        .iter()
+        .enumerate()
+        .filter(|&(_, used)| !used)
+        .map(|(i, _)| {
+            let named = matches!(args.explicit_args()[i].kind, FormatArgumentKind::Named(_));
+            (args.explicit_args()[i].expr, named)
+        })
+        .collect::<Vec<_>>();
+
+    if !unused.is_empty() {
+        // FIXME: Diagnose
+    }
+
+    FormatArgs {
+        template_expr: expr,
+        template: template.into_boxed_slice(),
+        arguments: args.finish(),
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct FormatArgumentsCollector {
+    arguments: Vec<FormatArgument>,
+    num_unnamed_args: usize,
+    num_explicit_args: usize,
+    names: Vec<(Name, usize)>,
+}
+
+impl FormatArgumentsCollector {
+    pub(crate) fn finish(self) -> FormatArguments {
+        FormatArguments {
+            arguments: self.arguments.into_boxed_slice(),
+            num_unnamed_args: self.num_unnamed_args,
+            num_explicit_args: self.num_explicit_args,
+            names: self.names.into_boxed_slice(),
+        }
+    }
+
+    pub fn new() -> Self {
+        Self { arguments: vec![], names: vec![], num_unnamed_args: 0, num_explicit_args: 0 }
+    }
+
+    pub fn add(&mut self, arg: FormatArgument) -> usize {
+        let index = self.arguments.len();
+        if let Some(name) = arg.kind.ident() {
+            self.names.push((name.clone(), index));
+        } else if self.names.is_empty() {
+            // Only count the unnamed args before the first named arg.
+            // (Any later ones are errors.)
+            self.num_unnamed_args += 1;
+        }
+        if !matches!(arg.kind, FormatArgumentKind::Captured(..)) {
+            // This is an explicit argument.
+            // Make sure that all arguments so far are explicit.
+            assert_eq!(
+                self.num_explicit_args,
+                self.arguments.len(),
+                "captured arguments must be added last"
+            );
+            self.num_explicit_args += 1;
+        }
+        self.arguments.push(arg);
+        index
+    }
+
+    pub fn by_name(&self, name: &Name) -> Option<(usize, &FormatArgument)> {
+        let &(_, i) = self.names.iter().find(|(n, _)| n == name)?;
+        Some((i, &self.arguments[i]))
+    }
+
+    pub fn by_index(&self, i: usize) -> Option<&FormatArgument> {
+        (i < self.num_explicit_args).then(|| &self.arguments[i])
+    }
+
+    pub fn unnamed_args(&self) -> &[FormatArgument] {
+        &self.arguments[..self.num_unnamed_args]
+    }
+
+    pub fn named_args(&self) -> &[FormatArgument] {
+        &self.arguments[self.num_unnamed_args..self.num_explicit_args]
+    }
+
+    pub fn explicit_args(&self) -> &[FormatArgument] {
+        &self.arguments[..self.num_explicit_args]
+    }
+
+    pub fn all_args(&self) -> &[FormatArgument] {
+        &self.arguments[..]
+    }
+
+    pub fn all_args_mut(&mut self) -> &mut Vec<FormatArgument> {
+        &mut self.arguments
+    }
+}
+
+impl FormatArgumentKind {
+    pub fn ident(&self) -> Option<&Name> {
+        match self {
+            Self::Normal => None,
+            Self::Named(id) => Some(id),
+            Self::Captured(id) => Some(id),
+        }
+    }
+}
diff --git a/crates/hir-def/src/hir/format_args/parse.rs b/crates/hir-def/src/hir/format_args/parse.rs
new file mode 100644
index 00000000000..22efa3883d4
--- /dev/null
+++ b/crates/hir-def/src/hir/format_args/parse.rs
@@ -0,0 +1,1023 @@
+//! Macro support for format strings
+//!
+//! These structures are used when parsing format strings for the compiler.
+//! Parsing does not happen at runtime: structures of `std::fmt::rt` are
+//! generated instead.
+
+// This is a copy of
+// https://github.com/Veykril/rust/blob/b89d7d6882532686fd90a89cec1a0fd386f0ade3/compiler/rustc_parse_format/src/lib.rs#L999-L1000
+// with the dependency of rustc-data-structures stripped out.
+
+// #![doc(
+//     html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/",
+//     html_playground_url = "https://play.rust-lang.org/",
+//     test(attr(deny(warnings)))
+// )]
+// #![deny(rustc::untranslatable_diagnostic)]
+// #![deny(rustc::diagnostic_outside_of_impl)]
+// We want to be able to build this crate with a stable compiler, so no
+// `#![feature]` attributes should be added.
+#![allow(dead_code, unreachable_pub)]
+
+use rustc_lexer::unescape;
+pub use Alignment::*;
+pub use Count::*;
+pub use Piece::*;
+pub use Position::*;
+
+use std::iter;
+use std::str;
+use std::string;
+
+// Note: copied from rustc_span
+/// Range inside of a `Span` used for diagnostics when we only have access to relative positions.
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+pub struct InnerSpan {
+    pub start: usize,
+    pub end: usize,
+}
+
+impl InnerSpan {
+    pub fn new(start: usize, end: usize) -> InnerSpan {
+        InnerSpan { start, end }
+    }
+}
+
+/// The location and before/after width of a character whose width has changed from its source code
+/// representation
+#[derive(Copy, Clone, PartialEq, Eq)]
+pub struct InnerWidthMapping {
+    /// Index of the character in the source
+    pub position: usize,
+    /// The inner width in characters
+    pub before: usize,
+    /// The transformed width in characters
+    pub after: usize,
+}
+
+impl InnerWidthMapping {
+    pub fn new(position: usize, before: usize, after: usize) -> InnerWidthMapping {
+        InnerWidthMapping { position, before, after }
+    }
+}
+
+/// Whether the input string is a literal. If yes, it contains the inner width mappings.
+#[derive(Clone, PartialEq, Eq)]
+enum InputStringKind {
+    NotALiteral,
+    Literal { width_mappings: Vec<InnerWidthMapping> },
+}
+
+/// The type of format string that we are parsing.
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+pub enum ParseMode {
+    /// A normal format string as per `format_args!`.
+    Format,
+    /// An inline assembly template string for `asm!`.
+    InlineAsm,
+}
+
+#[derive(Copy, Clone)]
+struct InnerOffset(usize);
+
+impl InnerOffset {
+    fn to(self, end: InnerOffset) -> InnerSpan {
+        InnerSpan::new(self.0, end.0)
+    }
+}
+
+/// A piece is a portion of the format string which represents the next part
+/// to emit. These are emitted as a stream by the `Parser` class.
+#[derive(Clone, Debug, PartialEq)]
+pub enum Piece<'a> {
+    /// A literal string which should directly be emitted
+    String(&'a str),
+    /// This describes that formatting should process the next argument (as
+    /// specified inside) for emission.
+    NextArgument(Box<Argument<'a>>),
+}
+
+/// Representation of an argument specification.
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct Argument<'a> {
+    /// Where to find this argument
+    pub position: Position<'a>,
+    /// The span of the position indicator. Includes any whitespace in implicit
+    /// positions (`{  }`).
+    pub position_span: InnerSpan,
+    /// How to format the argument
+    pub format: FormatSpec<'a>,
+}
+
+/// Specification for the formatting of an argument in the format string.
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct FormatSpec<'a> {
+    /// Optionally specified character to fill alignment with.
+    pub fill: Option<char>,
+    /// Span of the optionally specified fill character.
+    pub fill_span: Option<InnerSpan>,
+    /// Optionally specified alignment.
+    pub align: Alignment,
+    /// The `+` or `-` flag.
+    pub sign: Option<Sign>,
+    /// The `#` flag.
+    pub alternate: bool,
+    /// The `0` flag.
+    pub zero_pad: bool,
+    /// The `x` or `X` flag. (Only for `Debug`.)
+    pub debug_hex: Option<DebugHex>,
+    /// The integer precision to use.
+    pub precision: Count<'a>,
+    /// The span of the precision formatting flag (for diagnostics).
+    pub precision_span: Option<InnerSpan>,
+    /// The string width requested for the resulting format.
+    pub width: Count<'a>,
+    /// The span of the width formatting flag (for diagnostics).
+    pub width_span: Option<InnerSpan>,
+    /// The descriptor string representing the name of the format desired for
+    /// this argument, this can be empty or any number of characters, although
+    /// it is required to be one word.
+    pub ty: &'a str,
+    /// The span of the descriptor string (for diagnostics).
+    pub ty_span: Option<InnerSpan>,
+}
+
+/// Enum describing where an argument for a format can be located.
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub enum Position<'a> {
+    /// The argument is implied to be located at an index
+    ArgumentImplicitlyIs(usize),
+    /// The argument is located at a specific index given in the format,
+    ArgumentIs(usize),
+    /// The argument has a name.
+    ArgumentNamed(&'a str),
+}
+
+impl Position<'_> {
+    pub fn index(&self) -> Option<usize> {
+        match self {
+            ArgumentIs(i, ..) | ArgumentImplicitlyIs(i) => Some(*i),
+            _ => None,
+        }
+    }
+}
+
+/// Enum of alignments which are supported.
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub enum Alignment {
+    /// The value will be aligned to the left.
+    AlignLeft,
+    /// The value will be aligned to the right.
+    AlignRight,
+    /// The value will be aligned in the center.
+    AlignCenter,
+    /// The value will take on a default alignment.
+    AlignUnknown,
+}
+
+/// Enum for the sign flags.
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub enum Sign {
+    /// The `+` flag.
+    Plus,
+    /// The `-` flag.
+    Minus,
+}
+
+/// Enum for the debug hex flags.
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub enum DebugHex {
+    /// The `x` flag in `{:x?}`.
+    Lower,
+    /// The `X` flag in `{:X?}`.
+    Upper,
+}
+
+/// A count is used for the precision and width parameters of an integer, and
+/// can reference either an argument or a literal integer.
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub enum Count<'a> {
+    /// The count is specified explicitly.
+    CountIs(usize),
+    /// The count is specified by the argument with the given name.
+    CountIsName(&'a str, InnerSpan),
+    /// The count is specified by the argument at the given index.
+    CountIsParam(usize),
+    /// The count is specified by a star (like in `{:.*}`) that refers to the argument at the given index.
+    CountIsStar(usize),
+    /// The count is implied and cannot be explicitly specified.
+    CountImplied,
+}
+
+pub struct ParseError {
+    pub description: string::String,
+    pub note: Option<string::String>,
+    pub label: string::String,
+    pub span: InnerSpan,
+    pub secondary_label: Option<(string::String, InnerSpan)>,
+    pub should_be_replaced_with_positional_argument: bool,
+}
+
+/// The parser structure for interpreting the input format string. This is
+/// modeled as an iterator over `Piece` structures to form a stream of tokens
+/// being output.
+///
+/// This is a recursive-descent parser for the sake of simplicity, and if
+/// necessary there's probably lots of room for improvement performance-wise.
+pub struct Parser<'a> {
+    mode: ParseMode,
+    input: &'a str,
+    cur: iter::Peekable<str::CharIndices<'a>>,
+    /// Error messages accumulated during parsing
+    pub errors: Vec<ParseError>,
+    /// Current position of implicit positional argument pointer
+    pub curarg: usize,
+    /// `Some(raw count)` when the string is "raw", used to position spans correctly
+    style: Option<usize>,
+    /// Start and end byte offset of every successfully parsed argument
+    pub arg_places: Vec<InnerSpan>,
+    /// Characters whose length has been changed from their in-code representation
+    width_map: Vec<InnerWidthMapping>,
+    /// Span of the last opening brace seen, used for error reporting
+    last_opening_brace: Option<InnerSpan>,
+    /// Whether the source string is comes from `println!` as opposed to `format!` or `print!`
+    append_newline: bool,
+    /// Whether this formatting string was written directly in the source. This controls whether we
+    /// can use spans to refer into it and give better error messages.
+    /// N.B: This does _not_ control whether implicit argument captures can be used.
+    pub is_source_literal: bool,
+    /// Start position of the current line.
+    cur_line_start: usize,
+    /// Start and end byte offset of every line of the format string. Excludes
+    /// newline characters and leading whitespace.
+    pub line_spans: Vec<InnerSpan>,
+}
+
+impl<'a> Iterator for Parser<'a> {
+    type Item = Piece<'a>;
+
+    fn next(&mut self) -> Option<Piece<'a>> {
+        if let Some(&(pos, c)) = self.cur.peek() {
+            match c {
+                '{' => {
+                    let curr_last_brace = self.last_opening_brace;
+                    let byte_pos = self.to_span_index(pos);
+                    let lbrace_end = InnerOffset(byte_pos.0 + self.to_span_width(pos));
+                    self.last_opening_brace = Some(byte_pos.to(lbrace_end));
+                    self.cur.next();
+                    if self.consume('{') {
+                        self.last_opening_brace = curr_last_brace;
+
+                        Some(String(self.string(pos + 1)))
+                    } else {
+                        let arg = self.argument(lbrace_end);
+                        if let Some(rbrace_pos) = self.consume_closing_brace(&arg) {
+                            if self.is_source_literal {
+                                let lbrace_byte_pos = self.to_span_index(pos);
+                                let rbrace_byte_pos = self.to_span_index(rbrace_pos);
+
+                                let width = self.to_span_width(rbrace_pos);
+
+                                self.arg_places.push(
+                                    lbrace_byte_pos.to(InnerOffset(rbrace_byte_pos.0 + width)),
+                                );
+                            }
+                        } else {
+                            if let Some(&(_, maybe)) = self.cur.peek() {
+                                if maybe == '?' {
+                                    self.suggest_format();
+                                } else {
+                                    self.suggest_positional_arg_instead_of_captured_arg(arg);
+                                }
+                            }
+                        }
+                        Some(NextArgument(Box::new(arg)))
+                    }
+                }
+                '}' => {
+                    self.cur.next();
+                    if self.consume('}') {
+                        Some(String(self.string(pos + 1)))
+                    } else {
+                        let err_pos = self.to_span_index(pos);
+                        self.err_with_note(
+                            "unmatched `}` found",
+                            "unmatched `}`",
+                            "if you intended to print `}`, you can escape it using `}}`",
+                            err_pos.to(err_pos),
+                        );
+                        None
+                    }
+                }
+                _ => Some(String(self.string(pos))),
+            }
+        } else {
+            if self.is_source_literal {
+                let span = self.span(self.cur_line_start, self.input.len());
+                if self.line_spans.last() != Some(&span) {
+                    self.line_spans.push(span);
+                }
+            }
+            None
+        }
+    }
+}
+
+impl<'a> Parser<'a> {
+    /// Creates a new parser for the given format string
+    pub fn new(
+        s: &'a str,
+        style: Option<usize>,
+        snippet: Option<string::String>,
+        append_newline: bool,
+        mode: ParseMode,
+    ) -> Parser<'a> {
+        let input_string_kind = find_width_map_from_snippet(s, snippet, style);
+        let (width_map, is_source_literal) = match input_string_kind {
+            InputStringKind::Literal { width_mappings } => (width_mappings, true),
+            InputStringKind::NotALiteral => (Vec::new(), false),
+        };
+
+        Parser {
+            mode,
+            input: s,
+            cur: s.char_indices().peekable(),
+            errors: vec![],
+            curarg: 0,
+            style,
+            arg_places: vec![],
+            width_map,
+            last_opening_brace: None,
+            append_newline,
+            is_source_literal,
+            cur_line_start: 0,
+            line_spans: vec![],
+        }
+    }
+
+    /// Notifies of an error. The message doesn't actually need to be of type
+    /// String, but I think it does when this eventually uses conditions so it
+    /// might as well start using it now.
+    fn err<S1: Into<string::String>, S2: Into<string::String>>(
+        &mut self,
+        description: S1,
+        label: S2,
+        span: InnerSpan,
+    ) {
+        self.errors.push(ParseError {
+            description: description.into(),
+            note: None,
+            label: label.into(),
+            span,
+            secondary_label: None,
+            should_be_replaced_with_positional_argument: false,
+        });
+    }
+
+    /// Notifies of an error. The message doesn't actually need to be of type
+    /// String, but I think it does when this eventually uses conditions so it
+    /// might as well start using it now.
+    fn err_with_note<
+        S1: Into<string::String>,
+        S2: Into<string::String>,
+        S3: Into<string::String>,
+    >(
+        &mut self,
+        description: S1,
+        label: S2,
+        note: S3,
+        span: InnerSpan,
+    ) {
+        self.errors.push(ParseError {
+            description: description.into(),
+            note: Some(note.into()),
+            label: label.into(),
+            span,
+            secondary_label: None,
+            should_be_replaced_with_positional_argument: false,
+        });
+    }
+
+    /// Optionally consumes the specified character. If the character is not at
+    /// the current position, then the current iterator isn't moved and `false` is
+    /// returned, otherwise the character is consumed and `true` is returned.
+    fn consume(&mut self, c: char) -> bool {
+        self.consume_pos(c).is_some()
+    }
+
+    /// Optionally consumes the specified character. If the character is not at
+    /// the current position, then the current iterator isn't moved and `None` is
+    /// returned, otherwise the character is consumed and the current position is
+    /// returned.
+    fn consume_pos(&mut self, c: char) -> Option<usize> {
+        if let Some(&(pos, maybe)) = self.cur.peek() {
+            if c == maybe {
+                self.cur.next();
+                return Some(pos);
+            }
+        }
+        None
+    }
+
+    fn remap_pos(&self, mut pos: usize) -> InnerOffset {
+        for width in &self.width_map {
+            if pos > width.position {
+                pos += width.before - width.after;
+            } else if pos == width.position && width.after == 0 {
+                pos += width.before;
+            } else {
+                break;
+            }
+        }
+
+        InnerOffset(pos)
+    }
+
+    fn to_span_index(&self, pos: usize) -> InnerOffset {
+        // This handles the raw string case, the raw argument is the number of #
+        // in r###"..."### (we need to add one because of the `r`).
+        let raw = self.style.map_or(0, |raw| raw + 1);
+        let pos = self.remap_pos(pos);
+        InnerOffset(raw + pos.0 + 1)
+    }
+
+    fn to_span_width(&self, pos: usize) -> usize {
+        let pos = self.remap_pos(pos);
+        match self.width_map.iter().find(|w| w.position == pos.0) {
+            Some(w) => w.before,
+            None => 1,
+        }
+    }
+
+    fn span(&self, start_pos: usize, end_pos: usize) -> InnerSpan {
+        let start = self.to_span_index(start_pos);
+        let end = self.to_span_index(end_pos);
+        start.to(end)
+    }
+
+    /// Forces consumption of the specified character. If the character is not
+    /// found, an error is emitted.
+    fn consume_closing_brace(&mut self, arg: &Argument<'_>) -> Option<usize> {
+        self.ws();
+
+        let pos;
+        let description;
+
+        if let Some(&(peek_pos, maybe)) = self.cur.peek() {
+            if maybe == '}' {
+                self.cur.next();
+                return Some(peek_pos);
+            }
+
+            pos = peek_pos;
+            description = format!("expected `'}}'`, found `{maybe:?}`");
+        } else {
+            description = "expected `'}'` but string was terminated".to_owned();
+            // point at closing `"`
+            pos = self.input.len() - if self.append_newline { 1 } else { 0 };
+        }
+
+        let pos = self.to_span_index(pos);
+
+        let label = "expected `'}'`".to_owned();
+        let (note, secondary_label) = if arg.format.fill == Some('}') {
+            (
+                Some("the character `'}'` is interpreted as a fill character because of the `:` that precedes it".to_owned()),
+                arg.format.fill_span.map(|sp| ("this is not interpreted as a formatting closing brace".to_owned(), sp)),
+            )
+        } else {
+            (
+                Some("if you intended to print `{`, you can escape it using `{{`".to_owned()),
+                self.last_opening_brace.map(|sp| ("because of this opening brace".to_owned(), sp)),
+            )
+        };
+
+        self.errors.push(ParseError {
+            description,
+            note,
+            label,
+            span: pos.to(pos),
+            secondary_label,
+            should_be_replaced_with_positional_argument: false,
+        });
+
+        None
+    }
+
+    /// Consumes all whitespace characters until the first non-whitespace character
+    fn ws(&mut self) {
+        while let Some(&(_, c)) = self.cur.peek() {
+            if c.is_whitespace() {
+                self.cur.next();
+            } else {
+                break;
+            }
+        }
+    }
+
+    /// Parses all of a string which is to be considered a "raw literal" in a
+    /// format string. This is everything outside of the braces.
+    fn string(&mut self, start: usize) -> &'a str {
+        // we may not consume the character, peek the iterator
+        while let Some(&(pos, c)) = self.cur.peek() {
+            match c {
+                '{' | '}' => {
+                    return &self.input[start..pos];
+                }
+                '\n' if self.is_source_literal => {
+                    self.line_spans.push(self.span(self.cur_line_start, pos));
+                    self.cur_line_start = pos + 1;
+                    self.cur.next();
+                }
+                _ => {
+                    if self.is_source_literal && pos == self.cur_line_start && c.is_whitespace() {
+                        self.cur_line_start = pos + c.len_utf8();
+                    }
+                    self.cur.next();
+                }
+            }
+        }
+        &self.input[start..self.input.len()]
+    }
+
+    /// Parses an `Argument` structure, or what's contained within braces inside the format string.
+    fn argument(&mut self, start: InnerOffset) -> Argument<'a> {
+        let pos = self.position();
+
+        let end = self
+            .cur
+            .clone()
+            .find(|(_, ch)| !ch.is_whitespace())
+            .map_or(start, |(end, _)| self.to_span_index(end));
+        let position_span = start.to(end);
+
+        let format = match self.mode {
+            ParseMode::Format => self.format(),
+            ParseMode::InlineAsm => self.inline_asm(),
+        };
+
+        // Resolve position after parsing format spec.
+        let pos = match pos {
+            Some(position) => position,
+            None => {
+                let i = self.curarg;
+                self.curarg += 1;
+                ArgumentImplicitlyIs(i)
+            }
+        };
+
+        Argument { position: pos, position_span, format }
+    }
+
+    /// Parses a positional argument for a format. This could either be an
+    /// integer index of an argument, a named argument, or a blank string.
+    /// Returns `Some(parsed_position)` if the position is not implicitly
+    /// consuming a macro argument, `None` if it's the case.
+    fn position(&mut self) -> Option<Position<'a>> {
+        if let Some(i) = self.integer() {
+            Some(ArgumentIs(i))
+        } else {
+            match self.cur.peek() {
+                Some(&(_, c)) if rustc_lexer::is_id_start(c) => Some(ArgumentNamed(self.word())),
+
+                // This is an `ArgumentNext`.
+                // Record the fact and do the resolution after parsing the
+                // format spec, to make things like `{:.*}` work.
+                _ => None,
+            }
+        }
+    }
+
+    fn current_pos(&mut self) -> usize {
+        if let Some(&(pos, _)) = self.cur.peek() {
+            pos
+        } else {
+            self.input.len()
+        }
+    }
+
+    /// Parses a format specifier at the current position, returning all of the
+    /// relevant information in the `FormatSpec` struct.
+    fn format(&mut self) -> FormatSpec<'a> {
+        let mut spec = FormatSpec {
+            fill: None,
+            fill_span: None,
+            align: AlignUnknown,
+            sign: None,
+            alternate: false,
+            zero_pad: false,
+            debug_hex: None,
+            precision: CountImplied,
+            precision_span: None,
+            width: CountImplied,
+            width_span: None,
+            ty: &self.input[..0],
+            ty_span: None,
+        };
+        if !self.consume(':') {
+            return spec;
+        }
+
+        // fill character
+        if let Some(&(idx, c)) = self.cur.peek() {
+            if let Some((_, '>' | '<' | '^')) = self.cur.clone().nth(1) {
+                spec.fill = Some(c);
+                spec.fill_span = Some(self.span(idx, idx + 1));
+                self.cur.next();
+            }
+        }
+        // Alignment
+        if self.consume('<') {
+            spec.align = AlignLeft;
+        } else if self.consume('>') {
+            spec.align = AlignRight;
+        } else if self.consume('^') {
+            spec.align = AlignCenter;
+        }
+        // Sign flags
+        if self.consume('+') {
+            spec.sign = Some(Sign::Plus);
+        } else if self.consume('-') {
+            spec.sign = Some(Sign::Minus);
+        }
+        // Alternate marker
+        if self.consume('#') {
+            spec.alternate = true;
+        }
+        // Width and precision
+        let mut havewidth = false;
+
+        if self.consume('0') {
+            // small ambiguity with '0$' as a format string. In theory this is a
+            // '0' flag and then an ill-formatted format string with just a '$'
+            // and no count, but this is better if we instead interpret this as
+            // no '0' flag and '0$' as the width instead.
+            if let Some(end) = self.consume_pos('$') {
+                spec.width = CountIsParam(0);
+                spec.width_span = Some(self.span(end - 1, end + 1));
+                havewidth = true;
+            } else {
+                spec.zero_pad = true;
+            }
+        }
+
+        if !havewidth {
+            let start = self.current_pos();
+            spec.width = self.count(start);
+            if spec.width != CountImplied {
+                let end = self.current_pos();
+                spec.width_span = Some(self.span(start, end));
+            }
+        }
+
+        if let Some(start) = self.consume_pos('.') {
+            if self.consume('*') {
+                // Resolve `CountIsNextParam`.
+                // We can do this immediately as `position` is resolved later.
+                let i = self.curarg;
+                self.curarg += 1;
+                spec.precision = CountIsStar(i);
+            } else {
+                spec.precision = self.count(start + 1);
+            }
+            let end = self.current_pos();
+            spec.precision_span = Some(self.span(start, end));
+        }
+
+        let ty_span_start = self.current_pos();
+        // Optional radix followed by the actual format specifier
+        if self.consume('x') {
+            if self.consume('?') {
+                spec.debug_hex = Some(DebugHex::Lower);
+                spec.ty = "?";
+            } else {
+                spec.ty = "x";
+            }
+        } else if self.consume('X') {
+            if self.consume('?') {
+                spec.debug_hex = Some(DebugHex::Upper);
+                spec.ty = "?";
+            } else {
+                spec.ty = "X";
+            }
+        } else if self.consume('?') {
+            spec.ty = "?";
+        } else {
+            spec.ty = self.word();
+            if !spec.ty.is_empty() {
+                let ty_span_end = self.current_pos();
+                spec.ty_span = Some(self.span(ty_span_start, ty_span_end));
+            }
+        }
+        spec
+    }
+
+    /// Parses an inline assembly template modifier at the current position, returning the modifier
+    /// in the `ty` field of the `FormatSpec` struct.
+    fn inline_asm(&mut self) -> FormatSpec<'a> {
+        let mut spec = FormatSpec {
+            fill: None,
+            fill_span: None,
+            align: AlignUnknown,
+            sign: None,
+            alternate: false,
+            zero_pad: false,
+            debug_hex: None,
+            precision: CountImplied,
+            precision_span: None,
+            width: CountImplied,
+            width_span: None,
+            ty: &self.input[..0],
+            ty_span: None,
+        };
+        if !self.consume(':') {
+            return spec;
+        }
+
+        let ty_span_start = self.current_pos();
+        spec.ty = self.word();
+        if !spec.ty.is_empty() {
+            let ty_span_end = self.current_pos();
+            spec.ty_span = Some(self.span(ty_span_start, ty_span_end));
+        }
+
+        spec
+    }
+
+    /// Parses a `Count` parameter at the current position. This does not check
+    /// for 'CountIsNextParam' because that is only used in precision, not
+    /// width.
+    fn count(&mut self, start: usize) -> Count<'a> {
+        if let Some(i) = self.integer() {
+            if self.consume('$') {
+                CountIsParam(i)
+            } else {
+                CountIs(i)
+            }
+        } else {
+            let tmp = self.cur.clone();
+            let word = self.word();
+            if word.is_empty() {
+                self.cur = tmp;
+                CountImplied
+            } else if let Some(end) = self.consume_pos('$') {
+                let name_span = self.span(start, end);
+                CountIsName(word, name_span)
+            } else {
+                self.cur = tmp;
+                CountImplied
+            }
+        }
+    }
+
+    /// Parses a word starting at the current position. A word is the same as
+    /// Rust identifier, except that it can't start with `_` character.
+    fn word(&mut self) -> &'a str {
+        let start = match self.cur.peek() {
+            Some(&(pos, c)) if rustc_lexer::is_id_start(c) => {
+                self.cur.next();
+                pos
+            }
+            _ => {
+                return "";
+            }
+        };
+        let mut end = None;
+        while let Some(&(pos, c)) = self.cur.peek() {
+            if rustc_lexer::is_id_continue(c) {
+                self.cur.next();
+            } else {
+                end = Some(pos);
+                break;
+            }
+        }
+        let end = end.unwrap_or(self.input.len());
+        let word = &self.input[start..end];
+        if word == "_" {
+            self.err_with_note(
+                "invalid argument name `_`",
+                "invalid argument name",
+                "argument name cannot be a single underscore",
+                self.span(start, end),
+            );
+        }
+        word
+    }
+
+    fn integer(&mut self) -> Option<usize> {
+        let mut cur: usize = 0;
+        let mut found = false;
+        let mut overflow = false;
+        let start = self.current_pos();
+        while let Some(&(_, c)) = self.cur.peek() {
+            if let Some(i) = c.to_digit(10) {
+                let (tmp, mul_overflow) = cur.overflowing_mul(10);
+                let (tmp, add_overflow) = tmp.overflowing_add(i as usize);
+                if mul_overflow || add_overflow {
+                    overflow = true;
+                }
+                cur = tmp;
+                found = true;
+                self.cur.next();
+            } else {
+                break;
+            }
+        }
+
+        if overflow {
+            let end = self.current_pos();
+            let overflowed_int = &self.input[start..end];
+            self.err(
+                format!(
+                    "integer `{}` does not fit into the type `usize` whose range is `0..={}`",
+                    overflowed_int,
+                    usize::MAX
+                ),
+                "integer out of range for `usize`",
+                self.span(start, end),
+            );
+        }
+
+        found.then_some(cur)
+    }
+
+    fn suggest_format(&mut self) {
+        if let (Some(pos), Some(_)) = (self.consume_pos('?'), self.consume_pos(':')) {
+            let word = self.word();
+            let _end = self.current_pos();
+            let pos = self.to_span_index(pos);
+            self.errors.insert(
+                0,
+                ParseError {
+                    description: "expected format parameter to occur after `:`".to_owned(),
+                    note: Some(format!("`?` comes after `:`, try `{}:{}` instead", word, "?")),
+                    label: "expected `?` to occur after `:`".to_owned(),
+                    span: pos.to(pos),
+                    secondary_label: None,
+                    should_be_replaced_with_positional_argument: false,
+                },
+            );
+        }
+    }
+
+    fn suggest_positional_arg_instead_of_captured_arg(&mut self, arg: Argument<'a>) {
+        if let Some(end) = self.consume_pos('.') {
+            let byte_pos = self.to_span_index(end);
+            let start = InnerOffset(byte_pos.0 + 1);
+            let field = self.argument(start);
+            // We can only parse `foo.bar` field access, any deeper nesting,
+            // or another type of expression, like method calls, are not supported
+            if !self.consume('}') {
+                return;
+            }
+            if let ArgumentNamed(_) = arg.position {
+                if let ArgumentNamed(_) = field.position {
+                    self.errors.insert(
+                        0,
+                        ParseError {
+                            description: "field access isn't supported".to_string(),
+                            note: None,
+                            label: "not supported".to_string(),
+                            span: InnerSpan::new(arg.position_span.start, field.position_span.end),
+                            secondary_label: None,
+                            should_be_replaced_with_positional_argument: true,
+                        },
+                    );
+                }
+            }
+        }
+    }
+}
+
+/// Finds the indices of all characters that have been processed and differ between the actual
+/// written code (code snippet) and the `InternedString` that gets processed in the `Parser`
+/// in order to properly synthesise the intra-string `Span`s for error diagnostics.
+fn find_width_map_from_snippet(
+    input: &str,
+    snippet: Option<string::String>,
+    str_style: Option<usize>,
+) -> InputStringKind {
+    let snippet = match snippet {
+        Some(ref s) if s.starts_with('"') || s.starts_with("r\"") || s.starts_with("r#") => s,
+        _ => return InputStringKind::NotALiteral,
+    };
+
+    if str_style.is_some() {
+        return InputStringKind::Literal { width_mappings: Vec::new() };
+    }
+
+    // Strip quotes.
+    let snippet = &snippet[1..snippet.len() - 1];
+
+    // Macros like `println` add a newline at the end. That technically doesn't make them "literals" anymore, but it's fine
+    // since we will never need to point our spans there, so we lie about it here by ignoring it.
+    // Since there might actually be newlines in the source code, we need to normalize away all trailing newlines.
+    // If we only trimmed it off the input, `format!("\n")` would cause a mismatch as here we they actually match up.
+    // Alternatively, we could just count the trailing newlines and only trim one from the input if they don't match up.
+    let input_no_nl = input.trim_end_matches('\n');
+    let Some(unescaped) = unescape_string(snippet) else {
+        return InputStringKind::NotALiteral;
+    };
+
+    let unescaped_no_nl = unescaped.trim_end_matches('\n');
+
+    if unescaped_no_nl != input_no_nl {
+        // The source string that we're pointing at isn't our input, so spans pointing at it will be incorrect.
+        // This can for example happen with proc macros that respan generated literals.
+        return InputStringKind::NotALiteral;
+    }
+
+    let mut s = snippet.char_indices();
+    let mut width_mappings = vec![];
+    while let Some((pos, c)) = s.next() {
+        match (c, s.clone().next()) {
+            // skip whitespace and empty lines ending in '\\'
+            ('\\', Some((_, '\n'))) => {
+                let _ = s.next();
+                let mut width = 2;
+
+                while let Some((_, c)) = s.clone().next() {
+                    if matches!(c, ' ' | '\n' | '\t') {
+                        width += 1;
+                        let _ = s.next();
+                    } else {
+                        break;
+                    }
+                }
+
+                width_mappings.push(InnerWidthMapping::new(pos, width, 0));
+            }
+            ('\\', Some((_, 'n' | 't' | 'r' | '0' | '\\' | '\'' | '\"'))) => {
+                width_mappings.push(InnerWidthMapping::new(pos, 2, 1));
+                let _ = s.next();
+            }
+            ('\\', Some((_, 'x'))) => {
+                // consume `\xAB` literal
+                s.nth(2);
+                width_mappings.push(InnerWidthMapping::new(pos, 4, 1));
+            }
+            ('\\', Some((_, 'u'))) => {
+                let mut width = 2;
+                let _ = s.next();
+
+                if let Some((_, next_c)) = s.next() {
+                    if next_c == '{' {
+                        // consume up to 6 hexanumeric chars
+                        let digits_len =
+                            s.clone().take(6).take_while(|(_, c)| c.is_digit(16)).count();
+
+                        let len_utf8 = s
+                            .as_str()
+                            .get(..digits_len)
+                            .and_then(|digits| u32::from_str_radix(digits, 16).ok())
+                            .and_then(char::from_u32)
+                            .map_or(1, char::len_utf8);
+
+                        // Skip the digits, for chars that encode to more than 1 utf-8 byte
+                        // exclude as many digits as it is greater than 1 byte
+                        //
+                        // So for a 3 byte character, exclude 2 digits
+                        let required_skips = digits_len.saturating_sub(len_utf8.saturating_sub(1));
+
+                        // skip '{' and '}' also
+                        width += required_skips + 2;
+
+                        s.nth(digits_len);
+                    } else if next_c.is_digit(16) {
+                        width += 1;
+
+                        // We suggest adding `{` and `}` when appropriate, accept it here as if
+                        // it were correct
+                        let mut i = 0; // consume up to 6 hexanumeric chars
+                        while let (Some((_, c)), _) = (s.next(), i < 6) {
+                            if c.is_digit(16) {
+                                width += 1;
+                            } else {
+                                break;
+                            }
+                            i += 1;
+                        }
+                    }
+                }
+
+                width_mappings.push(InnerWidthMapping::new(pos, width, 1));
+            }
+            _ => {}
+        }
+    }
+
+    InputStringKind::Literal { width_mappings }
+}
+
+fn unescape_string(string: &str) -> Option<string::String> {
+    let mut buf = string::String::new();
+    let mut ok = true;
+    unescape::unescape_literal(string, unescape::Mode::Str, &mut |_, unescaped_char| {
+        match unescaped_char {
+            Ok(c) => buf.push(c),
+            Err(_) => ok = false,
+        }
+    });
+
+    ok.then_some(buf)
+}
diff --git a/crates/hir-expand/src/name.rs b/crates/hir-expand/src/name.rs
index 7c179c0cf95..5395b867c96 100644
--- a/crates/hir-expand/src/name.rs
+++ b/crates/hir-expand/src/name.rs
@@ -54,6 +54,12 @@ impl Name {
         Name(Repr::Text(text))
     }
 
+    // FIXME: See above, unfortunately some places really need this right now
+    #[doc(hidden)]
+    pub const fn new_text_dont_use(text: SmolStr) -> Name {
+        Name(Repr::Text(text))
+    }
+
     pub fn new_tuple_field(idx: usize) -> Name {
         Name(Repr::TupleField(idx))
     }
diff --git a/crates/hir-ty/src/infer/closure.rs b/crates/hir-ty/src/infer/closure.rs
index 1f040393f1f..9431599ac79 100644
--- a/crates/hir-ty/src/infer/closure.rs
+++ b/crates/hir-ty/src/infer/closure.rs
@@ -9,7 +9,10 @@ use chalk_ir::{
 };
 use hir_def::{
     data::adt::VariantData,
-    hir::{Array, BinaryOp, BindingId, CaptureBy, Expr, ExprId, Pat, PatId, Statement, UnaryOp},
+    hir::{
+        format_args::FormatArgumentKind, Array, BinaryOp, BindingId, CaptureBy, Expr, ExprId, Pat,
+        PatId, Statement, UnaryOp,
+    },
     lang_item::LangItem,
     resolver::{resolver_for_expr, ResolveValueResult, ValueNs},
     DefWithBodyId, FieldId, HasModule, VariantId,
@@ -453,6 +456,14 @@ impl InferenceContext<'_> {
     fn walk_expr_without_adjust(&mut self, tgt_expr: ExprId) {
         match &self.body[tgt_expr] {
             Expr::OffsetOf(_) => (),
+            Expr::FormatArgs(fa) => {
+                self.walk_expr_without_adjust(fa.template_expr);
+                fa.arguments
+                    .arguments
+                    .iter()
+                    .filter(|it| !matches!(it.kind, FormatArgumentKind::Captured(_)))
+                    .for_each(|it| self.walk_expr_without_adjust(it.expr));
+            }
             Expr::InlineAsm(e) => self.walk_expr_without_adjust(e.e),
             Expr::If { condition, then_branch, else_branch } => {
                 self.consume_expr(*condition);
diff --git a/crates/hir-ty/src/infer/expr.rs b/crates/hir-ty/src/infer/expr.rs
index 555a9fae48e..06742f7b36d 100644
--- a/crates/hir-ty/src/infer/expr.rs
+++ b/crates/hir-ty/src/infer/expr.rs
@@ -9,7 +9,8 @@ use chalk_ir::{cast::Cast, fold::Shift, DebruijnIndex, Mutability, TyVariableKin
 use hir_def::{
     generics::TypeOrConstParamData,
     hir::{
-        ArithOp, Array, BinaryOp, ClosureKind, Expr, ExprId, LabelId, Literal, Statement, UnaryOp,
+        format_args::FormatArgumentKind, ArithOp, Array, BinaryOp, ClosureKind, Expr, ExprId,
+        LabelId, Literal, Statement, UnaryOp,
     },
     lang_item::{LangItem, LangItemTarget},
     path::{GenericArg, GenericArgs},
@@ -848,6 +849,25 @@ impl InferenceContext<'_> {
                 self.infer_expr_no_expect(it.e);
                 self.result.standard_types.unit.clone()
             }
+            Expr::FormatArgs(fa) => {
+                fa.arguments
+                    .arguments
+                    .iter()
+                    .filter(|it| !matches!(it.kind, FormatArgumentKind::Captured(_)))
+                    .for_each(|it| _ = self.infer_expr_no_expect(it.expr));
+
+                match self
+                    .resolve_lang_item(LangItem::FormatArguments)
+                    .and_then(|it| it.as_struct())
+                {
+                    Some(s) => {
+                        // NOTE: This struct has a lifetime parameter, but we don't currently emit
+                        // those to chalk
+                        TyKind::Adt(AdtId(s.into()), Substitution::empty(Interner)).intern(Interner)
+                    }
+                    None => self.err_ty(),
+                }
+            }
         };
         // use a new type variable if we got unknown here
         let ty = self.insert_type_vars_shallow(ty);
diff --git a/crates/hir-ty/src/infer/mutability.rs b/crates/hir-ty/src/infer/mutability.rs
index b8a1af96fba..763f4ed2f96 100644
--- a/crates/hir-ty/src/infer/mutability.rs
+++ b/crates/hir-ty/src/infer/mutability.rs
@@ -3,7 +3,10 @@
 
 use chalk_ir::Mutability;
 use hir_def::{
-    hir::{Array, BinaryOp, BindingAnnotation, Expr, ExprId, PatId, Statement, UnaryOp},
+    hir::{
+        format_args::FormatArgumentKind, Array, BinaryOp, BindingAnnotation, Expr, ExprId, PatId,
+        Statement, UnaryOp,
+    },
     lang_item::LangItem,
 };
 use hir_expand::name;
@@ -37,6 +40,13 @@ impl InferenceContext<'_> {
             Expr::Missing => (),
             Expr::InlineAsm(e) => self.infer_mut_expr_without_adjust(e.e, Mutability::Not),
             Expr::OffsetOf(_) => (),
+            Expr::FormatArgs(fa) => {
+                fa.arguments
+                    .arguments
+                    .iter()
+                    .filter(|it| !matches!(it.kind, FormatArgumentKind::Captured(_)))
+                    .for_each(|arg| self.infer_mut_expr_without_adjust(arg.expr, Mutability::Not));
+            }
             &Expr::If { condition, then_branch, else_branch } => {
                 self.infer_mut_expr(condition, Mutability::Not);
                 self.infer_mut_expr(then_branch, Mutability::Not);
diff --git a/crates/hir-ty/src/mir/lower.rs b/crates/hir-ty/src/mir/lower.rs
index b6408cea502..9cc98684bf3 100644
--- a/crates/hir-ty/src/mir/lower.rs
+++ b/crates/hir-ty/src/mir/lower.rs
@@ -376,6 +376,9 @@ impl<'ctx> MirLowerCtx<'ctx> {
             Expr::InlineAsm(_) => {
                 not_supported!("builtin#asm")
             }
+            Expr::FormatArgs(_) => {
+                not_supported!("builtin#format_args")
+            }
             Expr::Missing => {
                 if let DefWithBodyId::FunctionId(f) = self.owner {
                     let assoc = f.lookup(self.db.upcast());
diff --git a/crates/hir-ty/src/tests/simple.rs b/crates/hir-ty/src/tests/simple.rs
index e7cb7cd4174..0c73370a686 100644
--- a/crates/hir-ty/src/tests/simple.rs
+++ b/crates/hir-ty/src/tests/simple.rs
@@ -3612,3 +3612,25 @@ fn main() {
 "#,
     );
 }
+
+#[test]
+fn builtin_format_args() {
+    check_infer(
+        r#"
+#[lang = "format_arguments"]
+pub struct Arguments<'a>;
+fn main() {
+    let are = "are";
+    builtin#format_args("hello {} friends, we {are} {0}{last}", "fancy", last = "!");
+}
+"#,
+        expect![[r#"
+            65..175 '{     ...!"); }': ()
+            75..78 'are': &str
+            81..86 '"are"': &str
+            92..172 'builti...= "!")': Arguments<'_>
+            152..159 '"fancy"': &str
+            168..171 '"!"': &str
+        "#]],
+    );
+}
diff --git a/crates/parser/src/grammar/expressions/atom.rs b/crates/parser/src/grammar/expressions/atom.rs
index e13284d1b7a..4197f248e0a 100644
--- a/crates/parser/src/grammar/expressions/atom.rs
+++ b/crates/parser/src/grammar/expressions/atom.rs
@@ -219,7 +219,7 @@ fn tuple_expr(p: &mut Parser<'_>) -> CompletedMarker {
 // test builtin_expr
 // fn foo() {
 //     builtin#asm(0);
-//     builtin#format_args(0);
+//     builtin#format_args("", 0, 1, a = 2 + 3, a + b);
 //     builtin#offset_of(Foo, bar.baz.0);
 // }
 fn builtin_expr(p: &mut Parser<'_>) -> Option<CompletedMarker> {
@@ -249,6 +249,24 @@ fn builtin_expr(p: &mut Parser<'_>) -> Option<CompletedMarker> {
         p.bump_remap(T![format_args]);
         p.expect(T!['(']);
         expr(p);
+        if p.eat(T![,]) {
+            while !p.at(EOF) && !p.at(T![')']) {
+                let m = p.start();
+                if p.at(IDENT) && p.nth_at(1, T![=]) {
+                    name(p);
+                    p.bump(T![=]);
+                }
+                if expr(p).is_none() {
+                    m.abandon(p);
+                    break;
+                }
+                m.complete(p, FORMAT_ARGS_ARG);
+
+                if !p.at(T![')']) {
+                    p.expect(T![,]);
+                }
+            }
+        }
         p.expect(T![')']);
         Some(m.complete(p, FORMAT_ARGS_EXPR))
     } else if p.at_contextual_kw(T![asm]) {
diff --git a/crates/parser/src/syntax_kind/generated.rs b/crates/parser/src/syntax_kind/generated.rs
index 3e31e4628bd..db5278f89d5 100644
--- a/crates/parser/src/syntax_kind/generated.rs
+++ b/crates/parser/src/syntax_kind/generated.rs
@@ -210,6 +210,7 @@ pub enum SyntaxKind {
     OFFSET_OF_EXPR,
     ASM_EXPR,
     FORMAT_ARGS_EXPR,
+    FORMAT_ARGS_ARG,
     CALL_EXPR,
     INDEX_EXPR,
     METHOD_CALL_EXPR,
diff --git a/crates/parser/test_data/parser/inline/ok/0207_builtin_expr.rs b/crates/parser/test_data/parser/inline/ok/0207_builtin_expr.rs
index dbad0a91df9..14431b0210e 100644
--- a/crates/parser/test_data/parser/inline/ok/0207_builtin_expr.rs
+++ b/crates/parser/test_data/parser/inline/ok/0207_builtin_expr.rs
@@ -1,5 +1,5 @@
 fn foo() {
     builtin#asm(0);
-    builtin#format_args(0);
+    builtin#format_args("", 0, 1, a = 2 + 3, a + b);
     builtin#offset_of(Foo, bar.baz.0);
 }
diff --git a/crates/syntax/rust.ungram b/crates/syntax/rust.ungram
index 2ce609b97a6..3603560d35c 100644
--- a/crates/syntax/rust.ungram
+++ b/crates/syntax/rust.ungram
@@ -382,7 +382,13 @@ AsmExpr =
   Attr* 'builtin' '#' 'asm' '(' Expr ')'
 
 FormatArgsExpr =
-  Attr* 'builtin' '#' 'format_args' '(' ')'
+  Attr* 'builtin' '#' 'format_args' '('
+  template:Expr
+  (',' args:(FormatArgsArg (',' FormatArgsArg)* ','?)? )?
+  ')'
+
+FormatArgsArg =
+  (Name '=')? Expr
 
 MacroExpr =
   MacroCall
diff --git a/crates/syntax/src/ast/generated/nodes.rs b/crates/syntax/src/ast/generated/nodes.rs
index 1c5e2282ecb..7ba0d4dc656 100644
--- a/crates/syntax/src/ast/generated/nodes.rs
+++ b/crates/syntax/src/ast/generated/nodes.rs
@@ -931,6 +931,9 @@ impl FormatArgsExpr {
         support::token(&self.syntax, T![format_args])
     }
     pub fn l_paren_token(&self) -> Option<SyntaxToken> { support::token(&self.syntax, T!['(']) }
+    pub fn template(&self) -> Option<Expr> { support::child(&self.syntax) }
+    pub fn comma_token(&self) -> Option<SyntaxToken> { support::token(&self.syntax, T![,]) }
+    pub fn args(&self) -> AstChildren<FormatArgsArg> { support::children(&self.syntax) }
     pub fn r_paren_token(&self) -> Option<SyntaxToken> { support::token(&self.syntax, T![')']) }
 }
 
@@ -1164,6 +1167,16 @@ impl UnderscoreExpr {
 }
 
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub struct FormatArgsArg {
+    pub(crate) syntax: SyntaxNode,
+}
+impl ast::HasName for FormatArgsArg {}
+impl FormatArgsArg {
+    pub fn eq_token(&self) -> Option<SyntaxToken> { support::token(&self.syntax, T![=]) }
+    pub fn expr(&self) -> Option<Expr> { support::child(&self.syntax) }
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub struct StmtList {
     pub(crate) syntax: SyntaxNode,
 }
@@ -2855,6 +2868,17 @@ impl AstNode for UnderscoreExpr {
     }
     fn syntax(&self) -> &SyntaxNode { &self.syntax }
 }
+impl AstNode for FormatArgsArg {
+    fn can_cast(kind: SyntaxKind) -> bool { kind == FORMAT_ARGS_ARG }
+    fn cast(syntax: SyntaxNode) -> Option<Self> {
+        if Self::can_cast(syntax.kind()) {
+            Some(Self { syntax })
+        } else {
+            None
+        }
+    }
+    fn syntax(&self) -> &SyntaxNode { &self.syntax }
+}
 impl AstNode for StmtList {
     fn can_cast(kind: SyntaxKind) -> bool { kind == STMT_LIST }
     fn cast(syntax: SyntaxNode) -> Option<Self> {
@@ -4254,6 +4278,7 @@ impl AstNode for AnyHasName {
                 | VARIANT
                 | CONST_PARAM
                 | TYPE_PARAM
+                | FORMAT_ARGS_ARG
                 | IDENT_PAT
         )
     }
@@ -4860,6 +4885,11 @@ impl std::fmt::Display for UnderscoreExpr {
         std::fmt::Display::fmt(self.syntax(), f)
     }
 }
+impl std::fmt::Display for FormatArgsArg {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        std::fmt::Display::fmt(self.syntax(), f)
+    }
+}
 impl std::fmt::Display for StmtList {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         std::fmt::Display::fmt(self.syntax(), f)
diff --git a/crates/syntax/src/tests/ast_src.rs b/crates/syntax/src/tests/ast_src.rs
index 2c1d832d1e5..341bda892ba 100644
--- a/crates/syntax/src/tests/ast_src.rs
+++ b/crates/syntax/src/tests/ast_src.rs
@@ -169,6 +169,7 @@ pub(crate) const KINDS_SRC: KindsSrc<'_> = KindsSrc {
         "OFFSET_OF_EXPR",
         "ASM_EXPR",
         "FORMAT_ARGS_EXPR",
+        "FORMAT_ARGS_ARG",
         // postfix
         "CALL_EXPR",
         "INDEX_EXPR",