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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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Type-check formatted interpolation holes only once 
The formatted/aligned hole path lowered via a synthesized String.Format that
re-type-checked the hole expression a second time, which could duplicate an
error in the hole and leak a spurious 'Format' overload-resolution error. Bind
the already-checked, boxed hole value to a temporary and reference that from the
synthesized String.Format instead.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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@charlesroddie @claude
charlesroddie and claude committed Jun 20, 2026
commit e83ddfb6f1636cedd5e49d813cd00687c4d32f29
12 changes: 10 additions & 2 deletions src/Compiler/Checking/Expressions/CheckExpressions.fs
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Expand Up @@ -7604,8 +7604,16 @@ and TcInterpolatedStringViaConcat (cenv: cenv, overallTy: OverallTy, env: TcEnv,
match alignment, format with
| None, None -> (if isStringTy g fillTy then (fill, true) else (mkCallStringOperator g m fillTy fill, false)), tpenv
| _ ->
let arg, tpenv = TcExpr cenv (MustEqual g.string_ty) env tpenv (stringFormatOp (alignment, format, synFill))
(arg, false), tpenv
// Format the already-checked hole via a synthesized 'String.Format', binding its boxed value
// to a temporary so the hole is not type-checked a second time. Re-checking 'synFill' would
// duplicate any error in it; boxing to 'obj' keeps the 'Format' overload unambiguous (so a
// hole that already failed to check doesn't also leak a confusing 'Format' overload error).
let boxedFill = mkCallBox g m fillTy fill
let tmpVal, _ = mkLocal mSynth "interpHole" (tyOfExpr g boxedFill)
let envInner = AddLocalVal g cenv.tcSink mSynth tmpVal env
let tmpRef = SynExpr.Ident(mkSynId mSynth tmpVal.LogicalName)
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let arg, tpenv = TcExpr cenv (MustEqual g.string_ty) envInner tpenv (stringFormatOp (alignment, format, tmpRef))
(mkCompGenLet mSynth tmpVal boxedFill arg, false), tpenv

// One (string expression, may-be-null) per non-empty part; a builder (not map) since 'tpenv' threads
// through the holes. Literals and conversions are never null; only a raw string passthrough may be.
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