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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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Accept a nullable string in a '%s' interpolation hole 
The hole was checked against 'string', so under --checknulls a nullable string
gave FS3261 where 'sprintf "%s"' does not. Share the specifier's type as
CheckFormatStrings.stringFormatTy and check the hole against that.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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@charlesroddie @claude
charlesroddie and claude committed Jul 30, 2026
commit 71638572259fd1cc6ecd9077b3a07f1f8414b16a
6 changes: 4 additions & 2 deletions src/Compiler/Checking/CheckFormatStrings.fs
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Expand Up @@ -37,6 +37,9 @@ let mkFlexibleDecimalFormatTypar (g: TcGlobals) m =
let mkFlexibleFloatFormatTypar (g: TcGlobals) m =
mkFlexibleFormatTypar g m [ g.float_ty; g.float32_ty; g.decimal_ty ] g.float_ty

let stringFormatTy (g: TcGlobals) =
if g.checkNullness && g.langFeatureNullness then g.string_ty_ambivalent else g.string_ty

type FormatInfoRegister =
{ mutable leftJustify : bool
mutable numPrefixIfPos : char option
Expand Down Expand Up @@ -448,8 +451,7 @@ let parseFormatStringInternal
checkOtherFlags ch
collectSpecifierLocation fragLine fragCol 1
let i = skipPossibleInterpolationHole (i+1)
let stringTy = if g.checkNullness && g.langFeatureNullness then g.string_ty_ambivalent else g.string_ty
parseLoop ((posi, stringTy) :: acc) (i, fragLine, fragCol+1) fragments
parseLoop ((posi, stringFormatTy g) :: acc) (i, fragLine, fragCol+1) fragments

| 'O' ->
checkOtherFlags ch
Expand Down
3 changes: 3 additions & 0 deletions src/Compiler/Checking/CheckFormatStrings.fsi
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Expand Up @@ -18,6 +18,9 @@ val mkFlexibleIntFormatTypar: g: TcGlobals -> m: range -> TType
/// A flexible type variable constrained to 'decimal', as accepted by the '%M' specifier.
val mkFlexibleDecimalFormatTypar: g: TcGlobals -> m: range -> TType

/// The type accepted by the '%s' specifier: ambivalent about nullness when nullness is checked.
val stringFormatTy: g: TcGlobals -> TType

val ParseFormatString:
m: range ->
fragmentRanges: range list ->
Expand Down
3 changes: 2 additions & 1 deletion src/Compiler/Checking/Expressions/CheckExpressions.fs
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Expand Up @@ -7672,8 +7672,9 @@ and TcInterpolatedStringViaConcat (cenv: cenv, overallTy: OverallTy, env: TcEnv,
| SynInterpolationFormatting.Printf (spec, _) ->
match spec with
// A bare '%s' requires a string; pass it through (it may be null) instead of formatting via 'sprintf'.
// Its type is the one 'sprintf "%s"' uses, so a nullable string is accepted here too.
| "%s" ->
let fill, tpenv = TcExpr cenv (MustEqual g.string_ty) env tpenv synFill
let fill, tpenv = TcExpr cenv (MustEqual (CheckFormatStrings.stringFormatTy g)) env tpenv synFill
(fill, true), tpenv
| "%c" -> convertViaString g.char_ty
| "%d" | "%i" -> convertViaString (CheckFormatStrings.mkFlexibleIntFormatTypar g m)
Expand Down
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