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Implement interpolated strings via String.Concat #19971

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Merged
T-Gro merged 17 commits into from
Aug 1, 2026
Merged

Implement interpolated strings via String.Concat #19971

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f571df9
Lower string-typed interpolation to String.Concat (reflection-free)
charlesroddie Jun 18, 2026
423d007
Add release notes for interpolated string String.Concat lowering
charlesroddie Jun 18, 2026
c7beb02
tidy
charlesroddie Jun 19, 2026
1951198
Lower string interpolation by type-checking each part in place
charlesroddie Jun 20, 2026
3d5bb3d
cleanup
charlesroddie Jun 20, 2026
5eae16c
Pass a bare %s through as a string instead of via sprintf
charlesroddie Jun 20, 2026
b278961
%s is AOT compatible now
charlesroddie Jun 20, 2026
e83ddfb
Type-check formatted interpolation holes only once
charlesroddie Jun 20, 2026
6112fd2
Lower bare %c/%d/%i/%M interpolation holes to String.Concat
charlesroddie Jun 20, 2026
7d2a246
Check NativeAOT interpolated-string output against expected values
charlesroddie Jun 22, 2026
e87b12f
Add EmittedIL tests for interpolation cases not optimized before
charlesroddie Jun 22, 2026
42e84bb
Merge remote-tracking branch 'upstream/main' into InterpolatedStringC…
charlesroddie Jul 30, 2026
c36ae55
Use the shared mkStringConcat helper for interpolation lowering
charlesroddie Jul 30, 2026
930d2ad
Keep parens around a dangling hole with an alignment
charlesroddie Jul 30, 2026
7163857
Accept a nullable string in a '%s' interpolation hole
charlesroddie Jul 30, 2026
55d2c6b
Format ParseHelpers.fs
charlesroddie Jul 30, 2026
df0ef58
Update expected trimmed sizes for the interpolation codegen change
charlesroddie Jul 30, 2026
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cleanup
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@charlesroddie
charlesroddie committed Jun 20, 2026
commit 3d5bb3d890412e64d7c50fe6af468ede3a90b357
39 changes: 17 additions & 22 deletions src/Compiler/Checking/Expressions/CheckExpressions.fs
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Original file line number Diff line number Diff line change
Expand Up @@ -7561,10 +7561,7 @@ and TcFormatStringExpr cenv (overallTy: OverallTy) env m tpenv (fmtString: strin
)

/// Lower a string-typed interpolated string to a reflection-free System.String.Concat of its parts,
/// type-checking each part in place. A literal becomes its text; a plain '{x}' hole is built straight in
/// the typed tree (passed through if already a string, else converted with the 'string' operator); an
/// aligned/formatted or printf hole is checked from a small synthesized 'String.Format'/'sprintf' so name
/// resolution applies.
/// type-checking each part in place.
and TcInterpolatedStringViaConcat (cenv: cenv, overallTy: OverallTy, env: TcEnv, m: range, tpenv: UnscopedTyparEnv, parts: SynInterpolatedStringPart list) =
let g = cenv.g
let mSynth = m.MakeSynthetic()
Expand All @@ -7586,9 +7583,8 @@ and TcInterpolatedStringViaConcat (cenv: cenv, overallTy: OverallTy, env: TcEnv,
let args = paren (SynExpr.Tuple(false, [ invariant; strLit netFormat; e ], [ range0; range0 ], mSynth))
mkSynApp1 (mkSynLidGet mSynth [ "System"; "String" ] "Format") args mSynth

// Type-check one hole and convert it to a typed string expression. A '%spec' hole is constrained by
// its specifier (a function there is a type error), so it goes straight to 'sprintf'. A plain or
// aligned/formatted hole is checked here first, so its value can be warned about if it is a function.
// Type-check one hole and convert it to a string. A printf hole goes straight to 'sprintf'; a plain
// or formatted hole is checked here first, so a function value can be warned about.
let convertHole (synFill: SynExpr, formatting: SynInterpolationFormatting, tpenv: UnscopedTyparEnv) =
match formatting with
| SynInterpolationFormatting.Printf (spec, _) -> TcExpr cenv (MustEqual g.string_ty) env tpenv (sprintfOp (spec, synFill))
Expand All @@ -7598,24 +7594,23 @@ and TcInterpolatedStringViaConcat (cenv: cenv, overallTy: OverallTy, env: TcEnv,
if g.langVersion.SupportsFeature LanguageFeature.WarnWhenFunctionValueUsedAsInterpolatedStringArg && (isFunTy g fillTy || isDelegateTy g fillTy) then
warning (Error(FSComp.SR.tcFunctionValueUsedAsInterpolatedStringArg (), synFill.Range))
match alignment, format with
// A plain hole reuses the checked expression; an aligned or formatted hole goes via String.Format.
| None, None -> (if isStringTy g fillTy then fill else mkCallStringOperator g m fillTy fill), tpenv
| _ -> TcExpr cenv (MustEqual g.string_ty) env tpenv (stringFormatOp (alignment, format, synFill))

// Build one string expression per part, type-checking each hole in place.
let rec build (parts: SynInterpolatedStringPart list, tpenv: UnscopedTyparEnv) =
match parts with
| [] -> [], tpenv
| SynInterpolatedStringPart.String ("", _) :: rest -> build (rest, tpenv)
| SynInterpolatedStringPart.String (s, _) :: rest ->
let args, tpenv = build (rest, tpenv)
mkString g m (s.Replace("%%", "%")) :: args, tpenv
| SynInterpolatedStringPart.FillExpr (synFill, formatting) :: rest ->
let argExpr, tpenv = convertHole (synFill, formatting, tpenv)
let args, tpenv = build (rest, tpenv)
argExpr :: args, tpenv

let argExprs, tpenv = build (parts, tpenv)
// One string expression per non-empty part; a builder (not map) since 'tpenv' threads through the holes.
let argExprs, tpenv =
let ra = ResizeArray()
let mutable tpenvAcc = tpenv
for part in parts do
match part with
| SynInterpolatedStringPart.String (s, _) ->
if s <> "" then
ra.Add(mkString g m (s.Replace("%%", "%")))
| SynInterpolatedStringPart.FillExpr (synFill, formatting) ->
let argExpr, tpenvAfter = convertHole (synFill, formatting, tpenvAcc)
ra.Add argExpr
tpenvAcc <- tpenvAfter
List.ofSeq ra, tpenvAcc

let resultExpr =
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match argExprs with
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