interpolation: support '' (rounding like in C)

vlib/v/ast/str.v

@@ -116,7 +116,7 @@ pub fn (x &InfixExpr) str() string {
 pub fn (lit &StringInterLiteral) get_fspec_braces(i int) (string, bool) {
 	mut res := []string{}
 	needs_fspec := lit.need_fmts[i] || lit.pluss[i] ||
-		(lit.fills[i] && lit.fwidths[i] >= 0) || lit.fwidths[i] != 0 || lit.precisions[i] != 0
+		(lit.fills[i] && lit.fwidths[i] >= 0) || lit.fwidths[i] != 0 || lit.precisions[i] != 987698
 	mut needs_braces := needs_fspec
 	if !needs_braces {
 		if i + 1 < lit.vals.len && lit.vals[i + 1].len > 0 {
@@ -164,7 +164,7 @@ pub fn (lit &StringInterLiteral) get_fspec_braces(i int) (string, bool) {
 		if lit.fwidths[i] != 0 {
 			res << '${lit.fwidths[i]}'
 		}
-		if lit.precisions[i] != 0 {
+		if lit.precisions[i] != 987698 {
 			res << '.${lit.precisions[i]}'
 		}
 		if lit.need_fmts[i] {

vlib/v/checker/check_types.v

@@ -357,7 +357,7 @@ pub fn (mut c Checker) string_inter_lit(mut node ast.StringInterLiteral) table.T
 				node.need_fmts[i] = false
 			}
 		} else { // check if given format specifier is valid for type
-			if node.precisions[i] != 0 && !typ.is_float() {
+			if node.precisions[i] != 987698 && !typ.is_float() {
 				c.error('precision specification only valid for float types', node.fmt_poss[i])
 			}
 			if node.pluss[i] && !typ.is_number() {

vlib/v/fmt/tests/expressions_expected.vv

@@ -15,7 +15,7 @@ fn string_inter_lit(mut c checker.Checker, mut node ast.StringInterLiteral) tabl
 			[`E`, `F`, `G`, `e`, `f`, `g`, `d`, `u`, `x`, `X`, `o`, `c`, `s`, `p`, `_`] {
 			c.error('unknown format specifier `${fmt:c}`', node.fmt_poss[i])
 		}
-		if node.precisions[i] != 0 && !typ.is_float() {
+		if node.precisions[i] != 987698 && !typ.is_float() {
 			c.error('precision specification only valid for float types', node.fmt_poss[i])
 		}
 		if node.pluss[i] && !typ.is_number() {

vlib/v/fmt/tests/expressions_input.vv

@@ -16,7 +16,7 @@ fn string_inter_lit(mut c checker.Checker, mut node ast.StringInterLiteral) tabl
 				c.error('unknown format specifier `${fmt:c}`',
 					node.fmt_poss[i])
 		}
-			if node.precisions[i] != 0 &&
+			if node.precisions[i] != 987698 &&
 				!typ.is_float() {
 				c.error('precision specification only valid for float types',
 					node.fmt_poss[i])

vlib/v/gen/js/js.v

@@ -1443,7 +1443,7 @@ fn (mut g JsGen) gen_string_inter_literal(it ast.StringInterLiteral) {
 		fwidth := it.fwidths[i]
 		precision := it.precisions[i]
 		g.write('\${')
-		if fmt != `_` || fwidth != 0 || precision != 0 {
+		if fmt != `_` || fwidth != 0 || precision != 987698 {
 			// TODO: Handle formatting
 			g.expr(expr)
 		} else {

vlib/v/gen/str.v

@@ -333,7 +333,7 @@ fn (mut g Gen) string_inter_literal(node ast.StringInterLiteral) {
 		if node.fwidths[i] != 0 {
 			fmt = '$fmt${node.fwidths[i]}'
 		}
-		if node.precisions[i] != 0 {
+		if node.precisions[i] != 987698 {
 			fmt = '${fmt}.${node.precisions[i]}'
 		}
 		if fspec == `s` {

vlib/v/parser/parser.v

@@ -1346,7 +1346,8 @@ fn (mut p Parser) string_expr() ast.Expr {
 		mut has_fmt := false
 		mut fwidth := 0
 		mut fwidthneg := false
-		mut precision := 0
+		// 987698 is a magic default value, unlikely to be present in user input. NB: 0 is valid precision
+		mut precision := 987698 
 		mut visible_plus := false
 		mut fill := false
 		mut fmt := `_` // placeholder

vlib/v/tests/string_interpolation_floats_test.v

@@ -0,0 +1,29 @@
+fn test_f32_widths_and_precision() {
+	x := f32(200.43)
+	assert '|${x:10.4f}|' == '|  200.4300|'
+	assert '|${x:-10.4f}|' == '|200.4300  |'
+	assert '|${x:10.0f}|' == '|       200|'
+	assert '|${x:-10.0f}|' == '|200       |'
+	//
+	assert '|${x:0.4f}|' == '|200.4300|'
+	assert '|${x:.3f}|' == '|200.430|'
+	assert '|${x:.0f}|' == '|200|'
+	//
+	y := f32(200.90)
+	assert '|${y:.0f}|' == '|201|'
+}
+
+fn test_f64_widths_and_precision() {
+	x := f64(200.43)
+	assert '|${x:10.4f}|' == '|  200.4300|'
+	assert '|${x:-10.4f}|' == '|200.4300  |'
+	assert '|${x:10.0f}|' == '|       200|'
+	assert '|${x:-10.0f}|' == '|200       |'
+	//
+	assert '|${x:0.4f}|' == '|200.4300|'
+	assert '|${x:.3f}|' == '|200.430|'
+	assert '|${x:.0f}|' == '|200|'
+	//
+	y := f64(200.90)
+	assert '|${y:.0f}|' == '|201|'
+}