Implement builtin#format_args, using rustc's format_args parser

Cargo.lock

@@ -541,6 +541,7 @@ dependencies = [
  "mbe",
  "once_cell",
  "profile",
+ "ra-ap-rustc_lexer",
  "rustc-hash",
  "smallvec",
  "stdx",

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

crates/hir-def/src/body/lower.rs

@@ -29,9 +29,13 @@
     db::DefDatabase,
     expander::Expander,
     hir::{
-        dummy_expr_id, Array, Binding, BindingAnnotation, BindingId, BindingProblems, CaptureBy,
+        dummy_expr_id,
-        ClosureKind, Expr, ExprId, InlineAsm, Label, LabelId, Literal, LiteralOrConst, MatchArm,
+        format_args::{
-        Movability, OffsetOf, Pat, PatId, RecordFieldPat, RecordLitField, Statement,
+            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 @@ fn maybe_collect_expr(&mut self, expr: ast::Expr) -> Option<ExprId> {
             }
             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()) });
+                let e = self.collect_expr_opt(e.expr());
-                self.alloc_expr(expr, syntax_ptr)
+                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,
+                )
+            }
         })
     }
 

crates/hir-def/src/body/pretty.rs

@@ -156,6 +156,11 @@ fn print_expr(&mut self, expr: ExprId) {
             Expr::Missing => w!(self, "<EFBFBD>"),
             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);

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 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 @@ pub fn walk_child_exprs(&self, mut f: impl FnMut(ExprId)) {
         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);

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),
+        }
+    }
+}

crates/hir-expand/src/name.rs

@@ -54,6 +54,12 @@ const fn new_text(text: SmolStr) -> 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))
     }

crates/hir-ty/src/infer/closure.rs

@@ -9,7 +9,10 @@
 };
 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 @@ fn walk_expr(&mut self, tgt_expr: ExprId) {
     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);

crates/hir-ty/src/infer/expr.rs

@@ -9,7 +9,8 @@
 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 @@ fn infer_expr_inner(&mut self, tgt_expr: ExprId, expected: &Expectation) -> Ty {
                 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);

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 @@ fn infer_mut_expr_without_adjust(&mut self, tgt_expr: ExprId, mutability: Mutabi
             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);

crates/hir-ty/src/mir/lower.rs

@@ -376,6 +376,9 @@ fn lower_expr_to_place_without_adjust(
             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());

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
+        "#]],
+    );
+}

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]) {

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,

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);
 }

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

crates/syntax/src/ast/generated/nodes.rs

@@ -931,6 +931,9 @@ pub fn format_args_token(&self) -> Option<SyntaxToken> {
         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![')']) }
 }
 
@@ -1163,6 +1166,16 @@ impl UnderscoreExpr {
     pub fn underscore_token(&self) -> Option<SyntaxToken> { support::token(&self.syntax, T![_]) }
 }
 
+#[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 @@ fn cast(syntax: SyntaxNode) -> Option<Self> {
     }
     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 @@ fn can_cast(kind: SyntaxKind) -> bool {
                 | VARIANT
                 | CONST_PARAM
                 | TYPE_PARAM
+                | FORMAT_ARGS_ARG
                 | IDENT_PAT
         )
     }
@@ -4860,6 +4885,11 @@ fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         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)

crates/syntax/src/tests/ast_src.rs

@@ -169,6 +169,7 @@ pub(crate) struct KindsSrc<'a> {
         "OFFSET_OF_EXPR",
         "ASM_EXPR",
         "FORMAT_ARGS_EXPR",
+        "FORMAT_ARGS_ARG",
         // postfix
         "CALL_EXPR",
         "INDEX_EXPR",